For a typical house extension, the trial pit needs to tell you what is actually supporting the proposed foundation, not just what is sitting near the surface.
As a practical starting point, take the pit at least 1m below the proposed foundation level. That is a starting point, not a magic number. Clay, trees, groundwater and made ground can all mean you need to go deeper.
Start with the proposed foundation level
The important measurement is not simply “2m deep from ground level”.
If the proposed extension foundation is 1m below existing ground level, a trial pit taken 1m below the foundation formation would need to reach about 2m below existing ground level.
The purpose is to establish the ground beneath the proposed foundation. You want to see the soil profile, identify changes in strata, and establish whether the foundation is bearing on competent natural ground.
A shallow pit that stops at the same level as the proposed foundation tells you very little about what lies underneath it.
This is consistent with the approach in BS 5930, which sets out good practice for ground investigations, while foundation design is covered by BS 8004 and Eurocode 7.
Clay soils can mean deeper investigation
Clay is where the simple “1m below foundation” rule can start to fall apart.
Shrinkable clay changes volume as its moisture content changes. That movement can cause both subsidence and heave. The risk depends on the soil’s volume change potential, as well as what is happening around it.
NHBC guidance classifies shrinkable soils by volume change potential and sets different foundation depth requirements. High volume change potential clay, for example, can require deeper foundations than lower volume change potential material.
The trial pit therefore needs to establish the clay profile properly. Record changes in material, strength, structure and moisture condition. If the ground is still clearly shrinkable clay at the bottom of the pit, do not assume you have investigated enough simply because you have reached an arbitrary depth.
The foundation designer needs enough information to decide whether the proposed depth is suitable.
Trees can change the answer completely
A tree near an extension is not just a landscaping issue.
Roots can alter moisture content in shrinkable clay. The resulting shrinkage can affect shallow foundations, particularly during dry periods. NHBC guidance requires the effects of tree species, water demand, mature height, distance from the foundation, and soil volume change potential to be considered.
This means a trial pit beside a large oak, willow or poplar needs more thought than a pit in an open lawn.
Do not just record “tree 5m away”. Record the species if known, approximate height, trunk location and distance from the proposed foundation. The foundation designer can then assess the relevant zone of influence.
Record water when you strike it
Groundwater is useful information, so record it properly.
If water enters the pit, note the depth below ground level, where it entered, and whether it was a slow seep or a sustained inflow. Ideally record the conditions and time of the observation as well.
Water can affect foundation design, excavation stability and how the excavation can safely be worked. It can also tell you something about drainage and the likely conditions during construction.
Do not write “water at 1.5m” and leave it at that. A decent trial pit record should make it clear what was actually observed.
Safety comes before the extra 500mm
There is a common site saying that excavations need shoring once they reach 1.2m.
Be careful with that one.
CDM 2015 does not contain a simple rule saying every excavation deeper than 1.2m must be shored. Regulation 22 requires suitable measures to prevent collapse, including supports or battering where necessary. The correct control depends on the excavation, ground conditions and risks involved.
HSE guidance is equally clear that excavations must be made safe, with safe access and suitable precautions against collapse. A competent person should inspect an excavation at the start of each shift.
So if your investigation needs a 2m or 2.5m pit, plan the excavation properly. Do not send someone into a vertical-sided hole because “it’s only a trial pit”.
Underground services must also be considered before digging. HSE’s HSG47 guidance covers planning, locating services and safe excavation.
So, how deep should you go?
For a straightforward extension on reasonable ground, 1m below the proposed foundation level is a sensible minimum starting point for a trial pit.
Go deeper where the ground conditions demand it. Shrinkable clay, trees, made ground, variable strata or groundwater can all change the investigation requirements.
The aim is simple. Give the designer enough information to understand what the foundation will actually sit on, and what lies beneath it.
That is where a properly recorded Trial Trench Survey can save a lot of guesswork later. A good survey records the strata, foundation depths, water, and site observations in a format the engineer can actually use.
What have you found at the bottom of a trial pit that completely changed the foundation design?
And how often do you see water or trees properly recorded on domestic extension investigations?