Minimum depth of a strip foundation
450 mm minimum; 750 mm or more in shrinkable clay, deeper near trees
Strip and trench fill foundations on non-aggressive ground, subject to the ground conditions on site.
A strip or trench fill foundation should normally be taken at least 450 mm below finished ground level to be clear of frost action, and considerably deeper — commonly 750 mm or more, increasing near trees — in shrinkable clay.
Basis in regulation
How to check it on site
Record the dug depth and the ground conditions with a photograph before the pour; the trench is the only evidence that exists.
Common failure
Foundations dug to the drawing depth without allowing for trees or the actual clay found on site.
Last updated 23 August 2026. This is our editorial summary of publicly available regulation, not building control approval or a warranty determination.