Copying a neighbour's foundation depth does not establish what is suitable for your proposed work. [1] [2]
What determines the foundation design?
The foundation needs to transfer the extension's loads to suitable ground. The engineer considers the ground's characteristics, the proposed structure and the influence of adjoining construction. The necessary solution may be a conventional foundation or a more specialised arrangement, depending on the findings. [2]
A preliminary drawing should identify assumptions clearly. Ask whether the design relies on an unconfirmed soil type, an assumed existing foundation or a drainage route that has not yet been surveyed. This makes the remaining risks visible before the construction price is agreed.
Trees and clay need particular attention
Planning Portal's foundation guidance explains that shrinkable clay can change with moisture conditions and that trees can influence the required foundation depth. The relevant assessment is site-specific: tree presence, soil and the proposed building need to be considered together. [1]
An engineer may recommend arboricultural or ground-investigation input. Do not assume that removing a nearby tree automatically removes the foundation issue or creates permission to undertake tree work. Record trees close to the proposed building, including those outside your own boundary, for the design team to assess. [2] [1]
What investigations may be needed?
Trial pits can help establish existing foundation profiles and local conditions. A more detailed ground investigation may be necessary where the engineer needs information that trial pits alone cannot provide. IStructE's domestic-client guidance distinguishes this investigation work from the structural engineer's own design appointment. [2]
Ask who will arrange the investigation, what it is intended to establish and when the results are needed. Do not instruct excavation beside an existing structure without a competent method and a check of the relevant neighbour procedures. [3] [4]
Coordinate drains and boundaries
A public sewer can affect the foundation layout and require a build-over or build-near agreement. Thames Water's requirements should be considered before concrete is ordered, not after a trench has exposed a conflict. [5]
At a boundary, there may not be space for a conventional centrally loaded footing in the assumed position. An offset foundation changes the structural behaviour and needs appropriate design. Neighbouring foundations can also bring the Party Wall Act's excavation provisions into play. [6] [4]
What happens when excavation starts?
Building control may inspect the exposed excavation to assess the ground, nearby drains, trees and other conditions. Arrange that visit before the foundation is concealed. [7]
If the excavation reveals different conditions, stop at that decision point and obtain the necessary revised advice. Record the agreed foundation detail and any resulting change to cost or programme. A provisional depth in a quotation should not become an instruction to proceed regardless of what is found.