NHBC Chapter 5.1 (substructure and ground-bearing floors), BS 8102

Basement / lower ground Inspection Checklist

The NHBC basement key stage — retaining structure, waterproofing system, drainage and cavity management, checked before anything is concealed or backfilled.

28 checks · Reviewed 23 August 2026 by the ProPlot site team · How we write these guides

In short

The basement inspection is a named NHBC key stage on any plot with habitable or usable space below ground. The inspector checks the retaining structure against the engineer's design, the waterproofing system and its grade against BS 8102, the drainage and sump arrangement, service penetrations, and the ventilation and escape provision — all before backfill or internal finishes conceal them.

Use it on site

A basement relying on combined protection: the water-resisting structure, a maintainable cavity drain membrane behind the finishes, and a sump with a serviceable access cover. ProPlot original drawing, prepared from the Building Regulations Approved Documents and British Standards. Indicative only — build to the approved drawings for the plot.

Basements are treated separately by NHBC for one reason: nothing else on a housing site carries the same cost of failure. Water entering a finished lower ground floor is not a snag, it is a strip-out. That is why the basement visit is a named key stage in its own right and why the inspector is looking at design intent as hard as workmanship.

The governing document is BS 8102, which asks the designer to state the grade of internal environment the space has to deliver — from a plant room that can tolerate seepage up to habitable accommodation that cannot. From that grade flows the choice of waterproofing: a water-resisting concrete structure (Type B), an external or sandwiched barrier membrane (Type A), an internal cavity drain system (Type C), or a combination. On habitable space the safe answer is nearly always a combination, because each type has a different failure mode.

What actually gets raised at the visit is rarely the choice of system. It is the junctions: the kicker joint, the wall-to-slab corner, the penetration for the incoming main, the change of level at the door threshold, and whether anyone can reach the sump once the plasterboard is on. Detail those on paper before the trade arrives and the visit is straightforward.

What happens at this visit

When it happens

Two moments matter. Before the retaining structure is poured, so reinforcement, cover and joint details can be seen; and after the waterproofing is installed but before it is covered by finishes or backfill. On most plots the inspector will want both, so plan the programme around two visits rather than one.

Who inspects

The warranty inspector for the key stage, plus building control for structure and escape. On a designed waterproofing system the specialist designer should also issue their own installation sign-off, which the inspector will ask to see.

If it fails

A failed basement visit stops backfill and stops the frame going up above it, because covering the work would conceal the defect. Remedial waterproofing after occupation runs into tens of thousands per plot and is the single most common source of large warranty claims below ground.

How this visit is booked and recorded

Inspection type: Key Stage Inspection (basement). Almost always paired with a Risk Based Inspection because below-ground waterproofing is one of the highest damage-potential elements on a site.
Notice required: Minimum 24 hours’ notice. Book it against the waterproofing programme, not the concrete pour — the inspector needs to see the system before it is covered.
What it signs off: Feeds the NHBC key stage record and Buildmark cover for the below-ground structure. A failure here is one of the most expensive defects to remedy after occupation, so the record matters as much as the visit.

What the inspector will ask for

Structural engineer’s basement design and any temporary works drawings
Waterproofing design and the designer’s BS 8102 grade justification
Ground investigation report with groundwater levels
Manufacturer installation certificate for the membrane or additive system
Sump, pump and alarm specification with commissioning record
Concrete delivery tickets and cube test results for water-resisting concrete

Photograph these moments in time

Retaining wall reinforcement, laps and cover before the pour
Kicker, construction joint and water bar details
Membrane or cavity drain installation before overboarding
Every service penetration sealed, photographed individually
Sump chamber, pump, standby pump and high-level alarm installed
Land drainage and external ground level relationship before backfill

Concealed work cannot be re-photographed later. Take these before the element is covered and file them against the plot.

Reportable Item hotspots at this stage

A Reportable Item (RI) is a defect the inspector records against the plot. NHBC weigh each one by its Damage Potential, and the score feeds your site's Construction Quality Review. These are the ones raised most often here:

Single-form waterproofing with no combined protection on a Grade 3 space
Service penetrations sealed after the membrane rather than detailed into it
No maintainable access to the sump, pump or drainage channel
External ground level and land drainage not resolved before backfill

How Reportable Items and Quality Common Scoring work

The numbers for this stage

The measurable values most often argued about at the basement / lower ground visit, each traced to an openly published source. Take a tape to the ones that apply before you book.

Critical dimensions that apply at the basement / lower ground inspection
What	Value	Type
Building settlement limits on fillTotal settlement of a building foundation on engineered fill should generally not exceed 25mm over its 60-year design lifeReview the geotechnical report to confirm predicted settlement values are within acceptable limits for the proposed foundations.	25 mm	Maximum
Cavity Fill Below DPCWhere partial or full fill cavity insulation is used, it should terminate at least 150mm below the lowest damp-proof course (DPC)Check that any cavity fill material, particularly concrete or insulation, finishes at least 150mm below the DPC level.	150 mm	Minimum
Concrete cover to reinforcement cast against the groundReinforced foundations, ground beams and ground-bearing slabs.Check the spacers on the cage before the pour — cover cannot be corrected once concrete is in.	75 mm cast against soil; 50 mm cast against blinding	Minimum
Damp-proof course height above finished ground levelExternal masonry wall, adjoining finished external levelMeasure from finished external level to the underside of the DPC at several points along each elevation, including where paths, patios and driveways are later raised. Record the lowest reading.	150 mm	Minimum
Damp-proof membrane lap and sealGround-supported slab, membrane above or below the concreteCheck laps before concrete is poured or the slab is covered. Every lap should be taped or welded, and the membrane carried up and sealed into the DPC at the perimeter.	150 mm	Minimum
DPC and Cavity Tray LocationsDamp-proof courses (DPCs) must be installed at the base of walls, at least 150mm above the external ground level, and linked to the damp-proof membrane (DPM) in solid floors to prevent rising dampVerify DPCs are correctly positioned at least 150mm above ground level and linked to the DPM, and that cavity trays are installed above all cavity interruptions.	150 mm	Minimum
Foundation depth during cold weatherWhen construction occurs in cold weather, foundations should be at least 450mm below existing ground level, or adequate measures must be taken to prevent ground freezingCheck foundation depth relative to existing ground level, particularly during cold periods, and confirm any ground protection measures.	450 mm	Minimum
Low-Level Protection for Timber Frame and SIPsFor closed panel timber frame and structural insulated panels (SIPs), the lowest timber element must be positioned at a minimum of 150mm above the finished ground levelCheck that the lowest timber members are at the correct height above finished ground level and DPCs are correctly installed beneath sole plates.	150 mm	Minimum
Minimum depth of a strip foundationStrip and trench fill foundations on non-aggressive ground, subject to the ground conditions on site.Record the dug depth and the ground conditions with a photograph before the pour; the trench is the only evidence that exists.	450 mm minimum; 750 mm or more in shrinkable clay, deeper near trees	Minimum
Minimum foundation depth in frost-susceptible groundTo prevent damage caused by frost heave, foundations in frost-susceptible ground must extend to a minimum depth of 450mm below the finished ground level.Measure the depth of the foundation trench from finished ground level to ensure it meets the minimum 450mm requirement in frost-susceptible areas.	450 mm	Minimum
Minimum thickness of a strip foundationPlain concrete strip foundations to Approved Document A.Measure the projection each side and compare it with the poured depth before the trench is backfilled.	150 mm, and not less than the projection beyond the wall face	Minimum
Minimum thickness of strip and trench fill foundationsThe thickness of a strip foundation must be equal to its projection beyond the wall face or a minimum of 150mm, whichever is greater, typically ranging from 150mm to 500mmVerify the depth and thickness of both strip and trench fill foundations meet the minimum specified requirements.	150 mm	Minimum
Minimum Timber Clearance and Drainage HolesThe lowest timber components of a structure must be at least 150mm above finished ground level, or 75mm where conditions permitVerify the minimum clearance of timber from ground level and ensure that cavity drainage holes are present and clear.	150 mm	Minimum
Percolation test procedure for small soakawaysThe standard percolation test for small surface water soakaways involves boring a 150mm diameter hole, filling it with 300mm of water, and recording the time taken for it to soak awayConfirm that the percolation test steps, including hole diameter, water depth, and repeat measurements, were correctly followed and documented.	150 mm	Exact
Pipe support spacing and thermal movementHorizontal drainage pipes, typically 110mm diameter, require support at regular intervals, often around 900mm to 1000mm centres, or at every joint/socketConfirm that pipe supports are installed at the correct spacing along the run and that provisions for thermal movement are visible on long sections.	110 mm	Maximum
Ready-mixed concrete temperature and curingReady-mixed concrete delivered to site should have a minimum temperature of 5°C, as specified in BS EN 206Verify delivery tickets show concrete temperature is at least 5°C, and check that insulation or heating is applied to new pours, especially on cold substrates.	5 °C	Minimum
Timber Sole Plate Height Above GroundThe lowest timber elements of a building, such as sole plates, should be positioned a minimum of 150mm above the finished ground levelMeasure the distance from the lowest timber element to the finished ground level to ensure compliance with the minimum height requirements.	150 mm	Minimum
Traditional manhole constructionTraditional manholes feature a concrete base at least 100mm thick and walls constructed from brick, blockwork, or concrete appropriate to ground conditionsVerify base thickness, wall material and thickness relative to depth and loading, and confirm benching is smooth, rounded, and has the correct fall.	100 mm	Minimum
Waterproofing around retaining elementsWalls and/or floors require waterproofing where structural elements like retaining walls, buried podiums, or raised external ground levels greater than 150mm create a risk of contact with groundwaterVerify that waterproofing is correctly applied and continuous for all retaining elements, ensuring no pathways for water ingress.	150 mm	Exact
Waterproofing for specific structural configurationsSpecific architectural and structural configurations, such as stairs adjacent to the building, stepped floor slabs where retained ground exceeds 150mm, or raised external ground levels, necessitate careful consideration of waterproofingEnsure that the detailed design for these specific structural configurations includes adequate and continuous waterproofing measures.	150 mm	Exact

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The basement / lower ground checklist

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Excavation, temporary works and formation

Confirm the excavation support matches the temporary works design and is inspected by a competent person (CDM 2015)
CDM 2015
Check formation level and bearing stratum against the ground investigation and engineer’s design (NHBC 4.2)
NHBC 4.2
Record groundwater level observed in the dig and report any variance from the site investigation (BS 8102)
BS 8102
Verify no adjacent foundations, services or trees are undermined by the excavation profile (NHBC 4.2)
NHBC 4.2

Retaining structure

Check reinforcement size, spacing, laps and cover against the engineer’s bar bending schedule (BS EN 1992)
BS EN 1992
Confirm water bars or hydrophilic strips are fitted at every construction joint and kicker (NHBC 5.1)
NHBC 5.1
Verify the concrete mix is the specified water-resisting design where Type B protection is relied on (BS 8102 Type B)
BS 8102 Type B
Check the wall-to-slab junction detail is built as drawn, including any toe or upstand (NHBC 5.1)
NHBC 5.1
Confirm pour sequence, compaction and curing arrangements are agreed before concrete arrives (BS EN 13670)
BS EN 13670
Retain delivery tickets and cube results for every structural pour (NHBC 5.1)
NHBC 5.1

Waterproofing system

Confirm the designer has stated the BS 8102 grade required for the intended use of the space (BS 8102)
BS 8102
Check the installed system matches the specified type — A barrier, B structurally integral, C drained cavity, or a combination (BS 8102)
BS 8102
Verify combined protection is used where the space is habitable and there is no maintainable secondary defence (NHBC 5.1)
NHBC 5.1
Inspect membrane laps, terminations and internal and external corners for continuity (BS 8102)
BS 8102
Check the membrane is dressed correctly at the DPC and at the head of the retaining wall (NHBC 5.1)
NHBC 5.1
Confirm the installer is the manufacturer-approved contractor and holds the installation certificate (NHBC 5.1)
NHBC 5.1
Verify the system is protected from following trades before any board or backfill goes on (BS 8102)
BS 8102

Drainage, sump and penetrations

Check the cavity drainage channel falls to the sump and has rodding or flushing access (BS 8102)
BS 8102
Confirm the sump chamber has a duty and standby pump plus a high-level alarm on habitable space (BS 8102)
BS 8102
Verify the pump discharge has a non-return valve and a route that cannot surcharge back into the basement (AD H)
AD H
Check land drainage and external surface water are taken away from the retaining wall (AD C)
AD C
Confirm every service penetration is a designed, sealed detail rather than a site-formed core (NHBC 5.1)
NHBC 5.1
Check the access cover to the sump remains reachable after finishes (BS 8102)
BS 8102

Ventilation, escape and finishes readiness

Confirm ventilation provision for the below-ground rooms meets the approved scheme (AD F)
AD F
Verify means of escape from the basement, including protected routes and any escape window (AD B)
AD B
Check radon or ground gas protection is installed and continuous where the site requires it (BR 211)
BR 211
Confirm insulation and floor build-up match the SAP specification for the lower ground floor (AD L)
AD L
Check the dry lining or independent wall is isolated from the membrane as designed (BS 8102)
BS 8102

Above and beyond: what separates a dry basement from a lucky one

The compliance checks confirm the system is the specified one. These are the habits that decide whether it is still working in ten years. No inspector will fail the visit on these — they are the difference between a plot that passes and a plot that hands over clean.

Walk the waterproofing designer round the actual excavation before the first pour
Designs are drawn from survey information. Ground rarely matches it exactly, and the changes are cheapest to absorb before any concrete is placed.
When: As soon as the dig is to formation.
Set every service entry position and sleeve it before the wall is poured
A core drilled through a completed water-resisting wall is a deliberate hole in the only thing keeping water out, and it will never be as good as a cast-in detail.
When: At reinforcement fixing, agreed with the M&E co-ordinator.
Photograph every metre of membrane and every junction before it is covered
If a damp patch appears in year two, the photographic record is the only way to tell an installation defect from later damage. It also closes out the RI without opening anything up.
When: Immediately before overboarding or backfill.
Protect the membrane from the follow-on trades physically, not with a memo
Scaffold feet, reinforcement offcuts and dropped tools puncture membranes daily. A sacrificial board layer costs less than one repair.
When: The moment the membrane is complete.
Install the sump with a standby pump, a high-level alarm and a maintainable access cover
A single pump with no alarm is one power cut away from a flooded plot, and a sump you cannot reach cannot be serviced by the customer.
When: At first fix, before the floor screed.
Commission the pump under real water and record the run, not just the power-on
A pump that runs dry proves nothing. Filling the sump proves the float, the discharge, the non-return valve and the alarm together.
When: Before the pre-handover visit.
Resolve external levels, land drainage and the surface water route before backfill
Most below-ground water problems start above ground. A basement wall asked to hold back a saturated backfill is being asked the wrong question.
When: Before the first load of backfill goes in.
Leave the customer a written maintenance note for the drainage system
Cavity drain systems need periodic servicing. Handing that over in writing protects both the customer and your warranty position.
When: Handover pack.

Full inspection point register

259 inspection points that apply at the basement / lower ground stage, each written in our own words from openly published guidance, with how to check it on site and what counts as acceptable. Use the checklist above on the day and this register when you need to settle a detail.

Acceptable natural and recycled engineered fill materials
How to check
Confirm that all fill materials, whether natural or recycled, have supporting documentation demonstrating compliance with relevant British Standards and project specifications.
Acceptable when
A range of natural materials such as sands, gravels, specific clays, chalk, and crushed rock are generally suitable for engineered fill when compliant with classification criteria. Recycled aggregates like crushed concrete or brick can also be used, but crushed brick requires further strength assessment to confirm its durability.
Accuracy of foundation excavations
How to check
Verify that trench dimensions are accurate, especially width, to ensure walls can be constructed centrally without eccentric loading.
Acceptable when
Foundation excavations must be dug precisely to their specified design dimensions to ensure walls are centrally loaded. Inaccurate trenching can result in eccentric loading, potentially leading to foundation failure.
Accurate foundation setting out
How to check
Confirm that the foundation trenches are set out precisely according to the approved drawings for location and dimensions.
Acceptable when
Strip and trench fill foundations must be precisely set out on site in accordance with the approved design details. Accurate setting out is crucial to ensure the building's structural integrity and correct positioning.
Action for Non-Compliant Engineered Fill
How to check
Review all earthworks verification reports for any instances of non-compliant fill and confirm that appropriate design-approved actions have been taken and recorded.
Acceptable when
When testing reveals engineered fill that does not meet specification, the deviation must be thoroughly evaluated to determine if it is a marginal variance or indicative of fundamental issues with material, placement, or site conditions. The designer should be consulted for advice on any necessary remedial actions, which must be carefully implemented and documented.
Addressing localised effects in trench bottoms
How to check
Before concreting, inspect trench bottoms for stability, water, and localized defects, ensuring they are firm and conform to engineering advice.
Acceptable when
Trench bottoms affected by adverse conditions such as rain, groundwater, or drying must be re-bottomed to create a sound, stable surface before concrete is poured. Differences in bearing capacity or the presence of soft spots, hard spots, or roots require specific actions, including consultation with the engineer, local deepening, or removal.
Addressing localised ground conditions in excavations
How to check
Check for and properly address soft spots, hard spots, and tree roots at the base and sides of excavations as per design or engineer's instructions.
Acceptable when
During excavation, soft spots in the trench bottom must be deepened to a suitable bearing stratum or reinforced if necessary, while hard spots should be removed. Visible tree roots, particularly in cohesive soils, may necessitate deeper excavations or specific engineering measures.
Addressing non-compliant engineered fill
How to check
Ensure that any non-compliant fill is promptly identified, and agreed remedial actions are implemented and retested to confirm compliance.
Acceptable when
If testing reveals that compacted engineered fill does not meet specification requirements, appropriate corrective actions must be agreed upon immediately with relevant stakeholders, such as warranty providers. Potential remedies include further drying, additional compaction, excavation and replacement of the failed section, or undertaking performance testing.
Addressing Unstable Ground Conditions
How to check
Verify that any identified voids or areas of unstable ground have been professionally treated and certified as stable before proceeding with foundations or earthworks.
Acceptable when
Voids, such as dissolution features, mine workings, or old services, that could collapse or cause excessive settlement, must be thoroughly investigated and appropriately infilled or grouted before earthworks or foundation construction commences.
Adequate Concrete Cover to Reinforcement
How to check
Inspect to ensure that adequate spacers and chairs are used to achieve and maintain the specified concrete cover to all reinforcement before concrete pouring.
Acceptable when
It is critical to provide adequate concrete cover to all reinforcement, including main bars and stirrups, to protect the steel from corrosion and ensure the concrete's long-term durability and fire resistance. The specified cover must be maintained consistently throughout the element.
See the measured value
Admixture Application and Compliance
How to check
Check that admixtures are stored correctly, added to the mix water, dosed accurately, and align with the approved concrete mix design and manufacturer guidance.
Acceptable when
Admixtures must comply with BS EN 934-2 and be used strictly according to the manufacturer's instructions and BS EN 206. They should be added to the mix water for complete dispersal, correctly dosed, and only where permitted by the specification. Trial mixes are recommended to confirm performance, particularly for accelerators or plasticisers.
Approved Methods for Earthworks Filling
How to check
Verify that all earthworks filling operations use methods appropriate for the materials and site conditions as specified in the earthworks design.
Acceptable when
All earthworks filling, including engineered fill, must be executed using methods that are suitable for the specific materials, site conditions, and intended end-use of the land. Key considerations include the choice of conventional or non-standard compaction techniques, and potential use of preloading or surcharging to achieve desired ground properties.
Basic Geotechnical and Contamination Investigations
How to check
Ensure a basic ground investigation report has been completed and reviewed before commencing significant groundworks.
Acceptable when
Prior to development, a basic ground investigation is essential to understand site conditions, including geotechnical properties and potential contamination. This typically involves physical tests on soil samples, along with observations during trial pit excavations to identify hazards. Should this initial assessment indicate potential issues or fail to meet objectives, further detailed investigation is necessary.
Calculation of dead and imposed loads for foundations
How to check
Review structural engineer's calculations for dead and imposed loads to ensure compliance with relevant standards.
Acceptable when
The calculation of dead and imposed loads for foundation design is a critical step that must adhere to specific standards. These calculations ensure the foundations are adequately sized and configured to safely support the full weight of the building and its anticipated contents and occupants.
Cavity cleanliness and pipe insulation
How to check
Inspect cavities for cleanliness and ensure any pipework within the cavity is correctly spaced from masonry and fully insulated.
Acceptable when
Building cavities must be kept consistently clean and clear of mortar droppings or debris throughout construction to prevent cold bridging and dampness. Any services, such as water pipes, running within the cavity must be set off from masonry and adequately insulated to prevent freezing and heat loss.
Characteristics of Geotechnical Category 1 projects
How to check
Verify that all project parameters align with the criteria for Geo Cat 1, such as the absence of hazards and limited earthwork scope, to justify the simplified design approach.
Acceptable when
Geotechnical Category 1 projects typically involve greenfield sites with no known geotechnical hazards, uncomplicated ground conditions, and minimal preparatory works. These projects usually restrict engineered fill placement to infrastructure and external works, with gentle earthwork slopes, and utilise general fill, allowing for a simplified Geotechnical Design Statement.
Checking foundation excavation accuracy
How to check
Measure trench lengths, widths, and diagonals to confirm they match design drawings, and verify trench depths against benchmarks.
Acceptable when
During foundation excavation, the accuracy of setting out must be verified through control measurements of trench lengths, widths, and diagonal dimensions. Levels should also be checked against established benchmarks to ensure correct depth and fall.
Chloride content in concrete
How to check
Review delivery tickets and mix specifications to ensure chloride limits are met, especially in marine or road-adjacent environments.
Acceptable when
Chlorides, present in concrete materials, can increase the risk of steel reinforcement corrosion and reduce the concrete's chemical resistance. The chloride content in fresh concrete must be limited in accordance with BS EN 206. Specialist guidance is required for cured concrete that may be exposed to chlorides from the ground, sea spray, or de-icing agents to prevent deterioration.
Clarity of earthworks reports
How to check
Review the earthworks report for clear presentation, appropriate data summaries, and use of visual aids like graphs and charts.
Acceptable when
Earthworks verification reports must be presented in a clear and understandable format, with test results suitably summarised. The use of graphs and charts is essential for effective communication and interpretation of the data, ensuring that all findings are readily comprehensible to stakeholders.
Clay brick specification for manholes
How to check
Check brick specifications, confirming compliance with BS EN 771, a compressive strength of at least 48 N/mm², and suitability for underground conditions.
Acceptable when
Clay bricks used for manhole construction must comply with BS EN 771 and possess a minimum compressive strength of 48 N/mm². Additionally, they must have a low active soluble salt content to minimise the risk of efflorescence and other salt-related degradation.
Compaction requirements for engineered fill
How to check
Ensure that compaction testing results confirm the required MDD and air void percentages are met for each layer of engineered fill.
Acceptable when
For engineered fill supporting building foundations, a high level of compaction is required, typically achieving a minimum of 95% Maximum Dry Density (MDD) and less than 5% air voids. Different compaction efforts or plant may influence the precise MDD target, which is critical for ensuring adequate ground support.
Compatibility and homogeneity of fill materials
How to check
Visually inspect fill material stockpiles for homogeneity and confirm that geotextiles are correctly installed where different fill types interface.
Acceptable when
When different types of fill materials are placed in layers or distinct zones, their compatibility must be ensured, often requiring geotextile separators to prevent issues like fine migration. For materials sourced from stockpiles, it is essential to confirm their homogeneity and account for potential weathering or particle segregation that can affect grading.
Comprehensive ground appraisal for foundation design
How to check
Confirm that a detailed ground investigation report has been completed and that foundation designs align with its findings.
Acceptable when
A thorough site and ground appraisal is essential to gather all necessary information for the correct and proper design of foundations. Building over areas where ground characteristics change abruptly should generally be avoided to prevent structural issues.
Compressible materials for heave protection
How to check
Check that the specified compressible material is correctly installed, has the correct thickness, and is positioned to protect the foundation from potential ground heave.
Acceptable when
When constructing foundations in ground susceptible to heave, particularly from expansive clays or tree roots, compressible materials must be incorporated. These materials are designed to absorb upward pressure and protect the foundation from damaging forces.
Concrete Admixture Considerations
How to check
Verify that any specified admixtures are correctly identified in mix designs and comply with project requirements.
Acceptable when
When concrete admixtures are specified, their selection should be based on a thorough understanding of their intended effects, such as improved workability, accelerated strength gain, waterproofing properties, or set retardation. The specific properties of the admixture must align with the performance requirements of the concrete mix and the project's structural or durability needs.
Concrete and calcium silicate bricks for manholes
How to check
Confirm the use of correct brick types, checking compliance with BS EN 771, specified crushing strength, and appropriate cement content for concrete bricks in foul drainage.
Acceptable when
Concrete bricks for manholes must comply with BS EN 771 and achieve a minimum crushing strength of 48 N/mm², with a minimum cement content of 350kg/m³ for foul drainage applications. Calcium silicate bricks for foul drainage should be at least strength class 20.
Concrete and Reinforcement Compliance
How to check
Check that all concrete deliveries have accompanying documentation confirming compliance with specifications.
Acceptable when
All concrete and its reinforcement used in construction must conform to the specified technical requirements and standards. This ensures the structural integrity and longevity of the building elements.
Concrete compliance for foundations
How to check
Check that concrete materials and placement procedures comply with specifications and relevant standards, including batching and mix design.
Acceptable when
The procurement and execution of concrete works, including reinforcement, for foundations must conform to relevant British and European Standards. Specifically, BS EN 13670 provides requirements for the execution of concrete structures, ensuring quality and structural integrity.
Concrete Cube Testing and Documentation
How to check
Confirm that concrete test cubes are taken correctly, clearly marked, properly cured, and that records of all tests are readily available.
Acceptable when
When required, concrete testing must be performed according to British Standards by UKAS accredited laboratories. Test cubes should be carefully prepared, marked, cured, and stored as specified, typically by the engineer, until their designated testing date. Comprehensive documentation, including test reports and certificates, must be maintained and made available for quality assurance purposes.
Concrete Curing for Strength Development
How to check
Verify that concrete is being properly cured immediately after placement and finishing, as per specified methods.
Acceptable when
Concrete must be adequately cured to ensure it achieves its full designed strength and durability. Proper curing is essential for the hydration process, which develops the concrete's mechanical properties. This process helps prevent premature drying and cracking, leading to a more robust final product.
Concrete Curing Specification
How to check
Confirm that the specified concrete curing methods and durations are being implemented correctly on site, especially for critical elements.
Acceptable when
The performance and long-term durability of concrete are critically dependent on effective curing. Project specifications or structural designs should clearly stipulate the curing regime required, including any specific methods or durations. These requirements must be communicated and adhered to on site to ensure the concrete performs as intended.
Concrete Design and Specification Compliance
How to check
Verify that concrete mix designs and specifications are in accordance with project requirements and relevant British Standards.
Acceptable when
The design and specification of concrete, including mix design, must comply with relevant British Standards and established industry practices. This applies to both plain and reinforced concrete, whether precast or in-situ, ensuring adequate strength and durability.
Concrete in aggressive ground conditions
How to check
Check that ground investigation reports confirm the chemical class of the ground and that the concrete mix specified is suitable for this classification.
Acceptable when
Where concrete is to be placed in aggressive ground conditions, the mix specification must conform to BS 8500. This is particularly important for conditions presenting lower range 'chemical aggressiveness', with specialist advice being mandatory for more severe scenarios to ensure the concrete's long-term integrity.
Concrete mix approval and placement procedure
How to check
Before placing concrete, confirm approvals from the structural engineer and building control are obtained, and verify that the excavation is clean and ready, and reinforcement correctly installed.
Acceptable when
Foundation concrete requires a mix design capable of achieving specified strength and resistance to environmental factors like frost and chemical attack. Before placement, excavations and reinforcement may require approval from the engineer or building control. Concreting should ideally be completed in one continuous operation, considering weather conditions, and placed promptly after preparatory work.
Concrete mix design for durability
How to check
Check that the site is maintaining correct water addition, ensuring full compaction, and implementing effective curing procedures.
Acceptable when
The selection of concrete mixes must consider the geographical location's environmental conditions and the specific structural element's exposure. Higher grade concrete provides enhanced resistance to chemical attack and mechanical wear. Achieving durability also depends on correct water-cement ratios, thorough compaction, and proper curing post-placement.
Concrete Mix Design for Strength and Durability
How to check
Confirm that the specified concrete mix design adequately addresses both the structural strength and anticipated exposure conditions for durability.
Acceptable when
Concrete mix design must specifically account for both required strength and long-term durability, adhering to British Standards. This principle applies universally to all concrete applications, whether plain, reinforced, precast, or cast in-situ.
Concrete mix specification and types
How to check
Verify that the concrete delivery tickets match the specified mix design for the element being cast.
Acceptable when
Concrete specified for UK construction projects must be suitable for its intended use and is typically defined using either designated mixes, supplied ready-mixed, or standardised prescribed mixes for site-batched concrete. These specifications ensure the concrete meets performance requirements for various applications.
Concrete quality for foundations
How to check
Confirm concrete mix aligns with specification, check for correct placement and compaction, and ensure appropriate curing methods are applied.
Acceptable when
Concrete used for foundations must be of a suitable mix for its intended purpose and designed to be durable against environmental factors like chemical attack or frost. It must be correctly mixed, placed without segregation, and properly cured to achieve its specified strength and durability.
Concrete Specification Considerations
How to check
Confirm that the specified concrete mix design adequately addresses all site-specific exposure conditions and potential risks.
Acceptable when
Concrete must be correctly specified, considering various factors to ensure adequate strength and durability for its intended application and exposure conditions. Key considerations include exposure to climatic, atmospheric, and aggressive ground conditions (such as sulfates and acids), potential for chlorides, effects of alkali-silica reaction, and the properties of aggregates.
Construction of raft, pile, pier, and beam foundations
How to check
Check that all foundation elements are constructed precisely to the design drawings and specifications.
Acceptable when
Raft, pile, pier, and ground beam foundations must be constructed strictly in accordance with their design. Key aspects include accurate setting out, proper excavation, correct installation of piles, piers, and beams, and verification of pile load capacity where required.
Content of earthworks verification reports
How to check
Verify that the earthworks report includes all specified details, such as contractor information, material sources, methods, and as-built levels.
Acceptable when
An earthworks verification report must detail the contractors and personnel involved, initial site preparation, references to the earthworks specification and method statements, and sources and classification of fill materials. It should also include any material treatment, plant and methods used, and as-built survey records, including final formation levels.

Most common reasons this visit fails

Single-form waterproofing on habitable space

A water-resisting concrete box on its own, with no cavity drain and no maintainable secondary defence, leaves nothing to fall back on if one joint fails. On habitable accommodation the inspector expects combined protection.

High damage potential — this is the classic below-ground warranty claim.

Penetrations formed after the event

A duct or main cored through finished waterproofing is a designed defect. Set positions at reinforcement stage and cast in the sleeves with the correct collars.

High — repairs are difficult to prove and difficult to guarantee.

Sump inaccessible once the room is finished

Cavity drain systems are maintenance systems. If the sump is behind fixed dry lining or under a fitted floor, it cannot be serviced and the design assumption fails.

Medium to high — usually raised as a Reportable Item at pre-handover too.

Backfilled before external drainage was resolved

Saturated backfill against the wall loads the waterproofing far beyond the design case. Land drainage, granular fill and the surface water route need to be in before the dig closes.

High — reopening the excavation is the only remedy.

No commissioning record for the pump and alarm

Powering the pump up is not commissioning. The inspector wants a record of a wet test showing float operation, discharge and the high-level alarm sounding.

Medium — but it holds up the final certificate.

Basement / lower ground inspection FAQs

Yes. Basement is one of NHBC’s named key stages alongside excavation, superstructure, pre-plaster and pre-handover. On a plot with below-ground accommodation it is inspected in its own right rather than as part of the substructure visit.

BS 8102 asks the designer to state the internal environment grade. Habitable accommodation is normally Grade 3 — a dry environment with no visible water penetration or damp — which in practice means combined protection rather than a single system.

Not automatically, but on habitable space most designers combine a barrier or water-resisting structure with a drained cavity, because a drained system is maintainable and a barrier is not. Whatever the combination, it has to come from the waterproofing designer, not from site.

Plan for two visits: one before the retaining structure is poured, so reinforcement and joint details can be seen, and one after the waterproofing is installed but before it is covered. Give a minimum of 24 hours’ notice for each.

On habitable below-ground space, yes in practice. A duty and standby arrangement with a high-level alarm is what a warranty inspector expects, because a single pump failure during a power cut floods the finished space.

Avoid it. Every retrospective core is a hole through the waterproofing that has to be sealed with a proprietary detail and can never be inspected again from outside. Set and sleeve service positions before the pour.

The engineer’s design, the waterproofing designer’s BS 8102 grade justification and system design, the ground investigation, the approved installer’s certificate, concrete tickets and cube results, and the sump and pump commissioning record.

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Where this sits in the build

The key stage inspection sequence on a typical UK housing plot.

Other inspection stage checklists

Excavation & foundations

21 checks →

Drainage

20 checks →

Ground Floor & Slab

19 checks →

Superstructure

22 checks →

Timber Frame Erection

21 checks →

Roof

22 checks →

Windows and External Doors

24 checks →

Insulation & Airtightness

23 checks →

First Fix

28 checks →

Fire Stopping

22 checks →

Pre-Plaster

21 checks →

Second Fix

31 checks →

External Works & Pre-Handover

23 checks →

Pre-handover

22 checks →

General Quality

20 checks →

All key stage checklists

Related guides

Snag lists with GPS photo evidence

How to record and close out defects with evidence that stands up at handover.

NHBC stage inspections on site

Running key stage inspections and keeping the paperwork straight.

Build programme scheduling

Generating and maintaining plot-by-plot build programmes.

BREL report photo requirements

What Part L photographic evidence has to show, and when.

Go deeper: Quality Intelligence

Our free, cited reference library — critical dimensions, build stage guidance, common defects and plain-English guides.

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