NHBC Chapter 4.1–4.5

Excavation & foundations Inspection Checklist

The checks a site manager should complete before the foundation inspection — trench dimensions, formation level, reinforcement, concrete spec, DPM and service entries.

21 checks · Reviewed 20 August 2026 by the ProPlot site team · How we write these guides

In short

The foundation inspection is a pre-pour visit. The inspector checks the trench depth, width and formation against the engineer's design, the ground conditions and any tree or drain influence, reinforcement and cover, plus the concrete mix and pour arrangements. It has to happen while the excavation is open and safe to view.

Use it on site

Section through a strip foundation at the pre-pour visit: trench depth to a suitable bearing stratum, trench width to the engineer’s design, and a clean, undisturbed formation. ProPlot original drawing, prepared from the Building Regulations Approved Documents and British Standards. Indicative only — build to the approved drawings for the plot.

The foundation visit is the first key stage inspection on most plots and the one that costs the most to get wrong. Once concrete is in the ground, a shallow trench, a missed step or a reinforcement cage in the wrong place stops being a snag and becomes a breaking-out job. The inspector is looking at an open excavation, so the window to fix anything is the few hours before the wagon arrives.

In practice the visit is short. The inspector walks the trenches with the structural engineer's drawings, checks depth and width against the design, looks at the formation for soft spots, standing water and root activity, and confirms that anything cast into the pour — starter bars, holding-down straps, sleeves for services — is in the right position. On sites with trees, made ground, mining history or shrinkable clay they will pay particular attention to depth and to whether heave precautions match the design.

Use the list below as your own pre-inspection walk. Everything on it is something that gets written up as a Reportable Item somewhere in the UK most weeks. Tick it off yourself, photograph the concealed items, and the inspector's visit becomes a formality rather than a gamble.

What happens at this visit

When it happens

After excavation is complete and any reinforcement is fixed, but before concrete is placed. Most warranty providers want at least 24–48 hours' notice, and the trenches must still be open when they arrive.

Who inspects

Your warranty provider's inspector (NHBC, LABC Warranty, Premier Guarantee) and, on many sites, building control. The structural engineer may also attend where the design is piled, rafted or on made ground.

If it fails

A failed foundation visit stops the pour. Concrete already ordered is wasted, the plot loses days, and if the pour has gone ahead the remedy is usually breaking out or an engineer-designed strengthening detail — both of which need re-inspection before you can build off it.

How this visit is booked and recorded

Inspection type: Key Stage Inspection (excavation). Often paired with a Risk Based Inspection where the site has trees, made ground, mining history or a piled design.
Notice required: NHBC ask for a minimum of 24 hours’ notice. Book the visit when the dig is programmed, not when the wagon is ordered, and re-book if the pour slips.
What it signs off: Feeds the Buildmark warranty record for the plot and, where NHBC also holds Building Control, the foundation stage approval.

What the inspector will ask for

Structural engineer’s foundation design and setting-out drawings
Ground investigation report and any contamination assessment
Tree survey and the NHBC Chapter 4.2 depth calculation for shrinkable soils
Concrete mix design and, after the pour, delivery tickets
Piling records and pile driving logs where the plot is piled

Photograph these moments in time

Open trench with a staff or tape showing depth against the design
Formation before blinding, showing it is clean, dry and undisturbed
Reinforcement cage with spacers in position and cover visible
Service sleeves, starter bars and holding-down straps before the pour
Compressible material and slip membrane where heave precautions apply

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:

Depth insufficient for tree or drain influence
Soft, wet or frost-affected formation left unprotected
Reinforcement cover below the engineer’s figure

How Reportable Items and Quality Common Scoring work

The numbers for this stage

The measurable values most often argued about at the excavation & foundations 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 excavation & foundations 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

Browse the full critical dimensions library

The excavation & foundations checklist

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Excavation and formation

Check trench dimensions match structural engineer drawings (depth, width, steps) (NHBC 4.4)
NHBC 4.4
Confirm formation level is clean, firm and free of loose material or standing water (NHBC 4.4)
NHBC 4.4
Inspect step positions, depths and overlap against NHBC Chapter 4.4 guidance (NHBC 4.4)
NHBC 4.4
Confirm depth allows for nearby trees, hedgerows and shrinkable clay per the site investigation (NHBC 4.2)
NHBC 4.2
Check any engineered fill or compaction test certificates are available on site (NHBC 4.6)
NHBC 4.6

Reinforcement and concrete

Verify steel reinforcement placement, laps, cover and spacer positions (NHBC 4.3)
NHBC 4.3
Check concrete specification (mix, slump, sulphate class) matches design requirements (BS 8500-1)
BS 8500-1
Check piling mat or raft slab reinforcement layout matches engineer specification (NHBC 4.2)
NHBC 4.2
Inspect ground beam dimensions, reinforcement and bearing onto pile caps (NHBC 4.2)
NHBC 4.2
Confirm trench fill concrete level is within 150mm of finished ground level (or as designed) (NHBC 4.4)
NHBC 4.4
Check foundation bolts / starter bars / holding-down straps positioned per design (NHBC 4.4)
NHBC 4.4

Damp, gas and substructure

Confirm radon barrier / DPM installed correctly with sealed laps and no punctures (BR 211)
BR 211
Inspect DPM overlaps are correctly taped and dressed up inner leaf to meet floor DPC (NHBC 5.1)
NHBC 5.1
Verify cavity tray / DPC at base of cavity is correctly positioned and lapped (NHBC 5.1)
NHBC 5.1
Check substructure blockwork mortar joints are full, perpends filled and coursing level (NHBC 6.1)
NHBC 6.1
Confirm weep holes are present at base of external leaf above DPC (NHBC 6.1)
NHBC 6.1
Confirm any retaining wall design and waterproofing matches structural engineer details (BS 8102)
BS 8102

Services, levels and sign-off

Verify drainage pipes passing through foundations are correctly sleeved with flexible joints (AD H)
AD H
Verify service entries (gas, electric, water) are sleeved and sealed through foundation walls (NHBC 8.1)
NHBC 8.1
Verify ground levels and falls away from the building for surface water drainage (AD C)
AD C
Confirm building control / warranty provider sign-off of excavation before concrete pour (NHBC 1.4)
NHBC 1.4

Above and beyond: protecting the dig and setting the plot up

None of this is on the inspector's list. It is what stops the excavation collapsing, the concrete being wasted and the next trade standing about. No inspector will fail the visit on these — they are the difference between a plot that passes and a plot that hands over clean.

Keep the excavation clean and the spoil back from the edge
Loose spoil piled at the edge surcharges the trench, causes collapse and drops soil into the pour, which is a genuinely weak foundation.
When: From the moment the machine starts, not the morning of the visit.
Cover open trenches overnight and in rain
A soaked or frozen bottom has to be dug out and re-inspected. A sheet and a couple of boards is a fraction of the cost of a return visit.
When: End of every shift until the pour.
Cap and plug every drainage or service duct left in the ground
Silt, concrete washings and rubble in a run you cannot see is a jetting job later, or a failed air test.
When: The day the pipe or duct is laid.
Mark and record service entry positions before backfill
Once it is backfilled, the incoming water, electric and telecoms positions become guesswork and the slab gets cored.
When: Photograph with a tape in shot before covering.
Protect the concrete in hot and cold weather
Green concrete that freezes or dries too fast loses strength and cracks. Sheet, cure and record the temperature.
When: First 48 hours after the pour.
Photograph the trench bottom, depth and any step details
This is the only chance. It answers the inspector's question later and it settles arguments about depth near trees.
When: Before concrete goes in.

Full inspection point register

259 inspection points that apply at the excavation & foundations 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

Trench depth taken from the wrong datum

Depths are set out from a reduced level that has since been re-graded, so the trench is nominally correct and physically too shallow. Re-check against the site datum, not the surrounding ground.

High Damage Potential — structural, and expensive to remedy after the pour.

Water and slurry left in the bottom

Overnight rain fills the trench and the pour goes ahead on a softened formation. Pump out and remove the softened material rather than displacing it with concrete.

High — affects the whole plot and usually needs an engineer’s remedy.

Steps too long or too short

Step overlap not matching the guidance is one of the most-raised foundation RIs. Set steps out on the drawing, not by eye in the excavation.

Medium — recorded as an RI and re-checked at the next visit.

Sleeves and starter bars added after the pour

Drilling and grouting after the event needs an engineer's sign-off. Mark every penetration and cast-in item before the wagon arrives.

Medium — but drives durability claims years later.

Excavation & foundations inspection FAQs

Yes. A soaked or frozen trench bottom usually has to be dug out and re-inspected, which costs a day and a second visit. Boards and a sheet overnight avoid it.

As soon as the excavation is complete and any reinforcement is fixed, and before you order concrete. Most providers ask for 24–48 hours' notice, so book it the day before you intend to pour and keep the trenches open until it has been signed off.

On shrinkable clay the minimum is 900mm, but the depth is set by the site investigation and the proximity of trees — it can be well over 2m near mature species. The structural engineer's drawing is the authority; the checklist confirms you have built what it says.

The warranty provider can refuse to accept the foundation without evidence. You will normally need trial holes, photographs, or an engineer's certificate, and in the worst case breaking out. Photograph the open trench, the reinforcement and the depth rod every time, so you always have evidence.

Both have an interest. Building control checks compliance with the Building Regulations; the warranty inspector checks compliance with the warranty standards. They are separate visits and one does not substitute for the other.

Step details and formation condition dominate. Steps that do not overlap correctly, and pours onto soft or waterlogged formations, account for a large share of foundation-stage RIs.

Next visitDrainage inspection checklist

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ProPlot's audit tool includes these excavation & foundations checks as interactive helpers — tick items off, attach GPS-stamped photos, and share the report with your inspector in one tap. Free while in beta.

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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

Drainage

20 checks →

Basement / lower ground

28 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.

Build stage quality library Critical dimensions Common defects All guides Ask a question

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