Depth is only one dimension of the design
Approved Document A addresses loading and reasonably foreseeable ground movement, rather than prescribing one depth regardless of circumstances. [2]
Ask the engineer to explain the chosen width and foundation type as well as depth. The useful answer identifies the ground being relied upon and the evidence supporting that choice.
Ask the engineer what ground the foundation is intended to bear on and what evidence supports that assumption. A note saying “to suitable ground” should lead to a clear plan for confirming suitability, not leave the entire design decision until a trench is open.
Trees and shrinkable clay
LABC's foundation-depth guidance explains why tree species, size, distance and soil characteristics can affect the required solution. The relevant assessment is not limited to whether a tree stands inside your boundary. [1]
BGS advises taking specialist advice before major work in areas with shrink–swell potential and designing foundations for the actual clay conditions. Planting, removing trees or changing impermeable surfaces can affect the moisture conditions that the design needs to consider. [3]
Give the team available information about current trees and previous removals. Do not ask the contractor to reduce the depth merely because no roots are visible at the bottom of the first excavation.
Existing foundations and drains
Planning Portal identifies nearby drains and existing foundations among the matters affecting an extension foundation. An excavation must not undermine adjoining construction or disregard how existing drainage is supported and protected. [4]
Ask for the relationship to neighbouring footings and services to be shown where relevant. The level of a neighbour's foundation can inform the assessment, but it does not automatically establish a safe depth or type for your additional loads.
Use investigation to reduce uncertainty
A trial pit can expose an existing foundation and permit examination and sampling of adjacent ground. [5]
Agree the locations and the information required with the engineer before arranging work. Ask how the resulting observations will inform the design, and what further investigation might be needed if the findings differ.
Request a record of what was found and how it affected the design. Where the answer remains uncertain, ask what further investigation is proportionate before committing to the main construction method.
What happens when excavation differs from the design?
Ask the contractor to stop the affected operation and obtain advice if the ground, water or nearby construction differs materially from the assumptions. HSE warns that excavation can undermine adjacent structures and requires suitable precautions; a deeper trench is not automatically a safer one. [6]
Agree who makes the revised design decision and who arranges the necessary checks before concrete is placed. Keep the final foundation detail and inspection record so the depth actually built is not lost once the work is covered.