Understanding Foundation Depth and Party Wall Requirements

If your project involves building close to a party wall, you may receive separate requests about foundation depth from two different sources: Building Control and your neighbour’s party wall surveyor. These are different processes with different purposes, and both need to be handled correctly.

What Your Structural Drawings Show

PorthouseDean produces structural drawings and calculations for Building Regulations compliance. The foundation depth on your drawings is the minimum depth required for structural adequacy, based on your site’s ground conditions and load requirements.

Your drawings typically include wording along the lines of: “Foundation depth to be agreed on site with Building Control Officer. Minimum depth as shown.” This means the BCO will inspect the excavation and confirm the formation depth is acceptable on site.

What Party Wall Surveyors Want

Party wall surveyors are not primarily concerned with Building Regulations compliance — they are focused on ensuring your works do not damage your neighbour’s property. When your foundation is within 3 metres of a neighbouring wall or its foundations, the Party Wall etc. Act 1996 can apply, and your neighbour’s surveyor may want to know exactly where your new foundation sits relative to the neighbour’s existing one.

Two approaches are commonly accepted:

Option 1: Assumed depth note on drawings. In many cases, a note on the structural drawings stating an assumed foundation depth (typically 1 metre) is sufficient for a party wall notice. The surveyor accepts the assumption and agrees that this will be confirmed on site.

Option 2: Trial pit. Where the party wall surveyor needs more certainty, a trial pit can be dug adjacent to the neighbour’s foundation to measure its actual depth, projection and condition. This produces hard data — the existing foundation base depth, the concrete thickness, whether it is strip or trench fill — which is then used to confirm the safe depth for the new foundation without risk of undermining.

The BCO Has Final Say

Whichever documentation route you take for the party wall process, the BCO retains authority over the actual foundation depth on site. If ground conditions at formation level are not as expected, the BCO can require deeper excavation regardless of what the drawings say.

Getting Both Right

If your project triggers party wall obligations, co-ordinate between your structural engineer (for Building Regulations drawings) and your party wall surveyor (for the award) early. Delays in one can hold up the other.

PorthouseDean produces the structural documentation. For party wall appointments, you will need a separate surveyor. Get in touch if you need the structural side of this sorted.