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Property Surveying Blog
May 23, 2016

Party Wall Trial Pits

Written by Beau Davies

Party Wall Trial Pits come in all shapes and sizes, however there are a few key points that will always remain the same, these are as follows:

  • The depth of the trial pit will be to base of the foundation that is being investigated.
  • The width of the trial pit will be to the width of the foundation that is being investigated.
  • The trial pit will always be alongside the structure that is being investigated.
  • The outcome of the trial pit will most likely determine the foundation design that the building owner’s contractor adheres to.
  • The purpose of a Party Wall trial pit is to confirm the depth and profile of the foundation in question.
  • A trial pit will always be required when the building owner’s drawings illustrating the proposed works are based off assumed foundation depths, as opposed to actual foundation depths.

So why do proposed construction works drawings have assumed foundation depths?

Party Wall trial pitA lot of the time at the planning application stage the building owner’s proposed construction design will not be based off any detailed or in depth site investigation.

This is usually because at the panning stage the building owner will not have put the project out to contractor tender and therefore most likely doesn’t have a contractor to actually dig the pit.

The building owner will likely also not want to incur the cost of detailed site investigation before planning permission is granted, as there is always the risk that the permission could be refused.Party Wall Trial Pit


As Party Wall Surveyors our aim is to ensure that the building owner’s proposed
construction work poses the least amount as risk possible to the adjoining owner’s property.

To agree a Party Wall Award with assumed foundation depths would most likely see the contractor design a foundation solution while on site.

While this wouldn’t be an issue for an experienced contractor, for a less experienced contractor this could result in a solution that detrimentally effects the structural integrity of the adjoining owner’s structure.

Party Wall investigation Trial pits also help fully inform all parties as to what is going on beneath the ground and in many cases the outcome of the trial pit can result in design changes such as underpinning, sectional hit and miss foundation bays, eccentric foundation design or even trimming back the adjoining owner’s foundations.
A trial pit is a key part in the Party Wall Agreement procedure as it ensures that all parties are ‘on the same page’ and have a full understanding of what the proposed works are.

A lot of the time we find that building owners will be a little agreevied with having to undertake the trial pit, as there is usually cost involved with the contractor digging the pit and the architect or engineer designing the proposed foundation based off the pit.

Party Wall Trial Pit

Our take on it here at Berry Lodge is that although there is a small cost associated with the trial pit, that cost will be far less than
the cost having to make good the adjoining owner’s property
as a result of the damage caused by a poorly designed foundation!
The trial pit also avoids any unplanned delay on site as technically speaking the proposed design is based off the ‘lay of the land’ and therefore there shouldn’t be any unexpected changes

We have included a number of different Trial Pit photographs that we have taken over the years to hopefully help fully inform you as to what a trial pit is and what to expect when a Party Wall Surveyor requests one.

If you have any questions about Trial Pits we would recommend speaking to our experienced Party Wall team now who will be happy to assist you.