Foundation Depth Assessment: When It’s Needed and What’s Required

Not every project needs a bespoke foundation depth assessment — many residential extensions can be designed to standard depths using the rules in Approved Document A. But certain site conditions mean a more detailed assessment is unavoidable.

When a Depth Assessment Becomes Necessary

A site-specific foundation depth assessment is typically required when one or more of the following applies:

Trees within 20 metres. Tree roots on clay soils cause seasonal shrinkage and heave — foundations must be deep enough to avoid the zone of influence. The required depth depends on the species, height and distance of each tree.

Clay soils. Clay is susceptible to volume change with moisture. Even without trees, clay sites often need deeper foundations than sand or gravel to reach stable, stable ground.

Building Control requirement. The BCO may specifically request that foundation depths are calculated and justified rather than assumed from standard tables.

Ground investigation results. If a soil survey reveals variable or soft ground, standard depths may be insufficient and need to be reviewed against the actual soil profile.

What Information Is Needed

To carry out a foundation depth assessment, your structural engineer needs:

1. Tree survey data — species, height (actual or estimated), trunk distance from the proposed foundation, and whether trees are to be retained or removed
2. Soil type — ideally confirmed by a ground investigation, though the national soil map can give an indication if a full survey is not available
3. Structural layout — the proposed floor plan and the loads the foundations need to carry
4. Site plan — showing the position of all trees relative to the new building footprint

If the project is in clay and there are trees nearby, the LABC Foundation Depth Calculator can give a useful indicative depth before formal design work begins.

How the Assessment Is Used

Once the required foundation depth is established, it feeds directly into the structural drawings. The drawings will specify:

– Minimum formation depth below external ground level
– Foundation width (derived from the wall loading and Approved Document A tables)
– Concrete specification (typically C35 for residential pad and strip foundations)
– Whether heave protection is needed (required on clay at depth)

This information is issued to the builder and to Building Control. The BCO then confirms on site that actual ground conditions match the design assumptions before concrete is poured.

Book a Foundation Assessment

PorthouseDean carries out foundation depth assessments as part of our structural design service for extensions and new builds across England and Wales. Get a quote to start the process.