Trial pit and foundation inspection report
Excavation of a trial pit beside an existing wall to record its foundations and the soil beneath.
About this service
A trial pit is a hole dug against the side of an existing wall so you can see what holds it up. It shows how deep the foundation goes, how wide it is, what it is built from, and what kind of soil it bears on. Old London houses were often built on shallow brick corbelled footings or thin concrete strips, and the only reliable way to know which you have is to dig down and look.
This simple piece of fieldwork answers several common questions. Can a new extension be built alongside the existing wall without underpinning? Is the existing foundation deep enough to cope with clay shrinkage from a nearby tree? Has the house moved because the footing is too shallow, or because a drain has washed the soil away? A short visit and an accurate record can save a lot of guesswork in the design.
We inspect the open pit, measure and record the foundation, describe the soil and take photographs and sketches. The pit can be dug by your builder to our instructions or arranged as part of the service. The result is a concise factual report your structural engineer, Building Control or insurer can rely on. Fees start from £600 + VAT, with the scope and pit locations confirmed in writing beforehand.
When you need it
- You need to know the depth and type of existing foundations
- A building has moved and the foundations need checking
- You're designing an extension or underpinning next to existing walls
What's included
- Supervision or inspection of the trial pit
- Record of foundation depth, width and construction
- Description of the soil
- Photographs and sketches
How it works
- Choose the locationWe pick the pit position based on the question being asked, such as the corner where a new extension meets the house, the area of worst cracking, or the wall closest to a tree.
- Excavate safelyThe pit is dug by hand alongside the wall, kept short in length so the foundation is not undermined, with buried services checked first and the excavation supported if it goes deep.
- Inspect and recordWe measure foundation depth, projection and thickness, note the materials, describe the soil layers and check for roots, softening, voids or water, with photographs and sketches throughout.
- Backfill and reportThe pit is backfilled and compacted, and you receive a factual report with the measurements, soil descriptions and photos, ready for the next stage of design or investigation.
What you receive
What a trial pit can tell you about older London foundations
Many Victorian and Edwardian homes sit on footings less than a metre deep, which was normal at the time but is shallower than current guidance on clay near trees. A trial pit shows whether your foundation is affected by roots, and whether fine roots continue below the footing into the clay. That evidence is central to a subsidence report for insurers or a decision about whether underpinning is needed.
For a new extension, the pit shows whether the new foundations can safely sit beside or below the old ones. Where the new footing will go deeper than the existing wall, the old foundation may need local underpinning to avoid undercutting it, and the Party Wall etc. Act 1996 may apply if the wall is shared. The findings feed straight into the extension design, explained further in our rear extension guide.
Frequently asked questions
How do I find out how deep my house foundations are?
The most reliable way is a trial pit dug against the wall to expose the footing. Old drawings rarely exist for period houses, and even when they do, what was built can differ. A small hand-dug pit, usually about a metre square at the top, lets an engineer measure the depth, width and construction directly.
Can my builder dig the trial pit?
Yes, and this often keeps the cost down. We give the builder clear instructions on position, size and safety, including checking for buried services and not undermining the foundation. We then attend to inspect and record while the pit is open. It should be backfilled promptly afterwards and not left open in wet weather.
How deep are foundations on a Victorian terrace?
They vary, but many are shallow, often a stepped brick footing well under a metre deep, sometimes on a thin lime concrete layer. Some houses had deeper footings where the ground was poor. The only way to be sure for your property is to look, which is exactly what a trial pit does.
Will a trial pit damage my foundations?
Not if it is dug properly. The pit is kept short in length along the wall and is not taken far below the footing, so the foundation stays supported. It is backfilled and compacted soon after the inspection. Problems arise when pits are dug too long, left open for weeks or allowed to fill with water.