PART A · Structure
How deep should extension foundations be
There is no universal foundation depth that makes every extension compliant. The right depth and foundation type depend on the loads, ground conditions and surroundings. A neighbour's foundation detail is useful background, but it is not a design for your site.
What affects foundation depth
Clay, trees, made ground, nearby drains and changes in ground level can all affect the solution. Part A addresses structural loads and ground movement. A designer should consider those matters together rather than choosing a depth from the size of the extension alone.
An extension beside a mature tree may need a different approach from the same extension on stable ground without significant vegetation.
When is an engineer needed
Straightforward ground conditions may allow a conventional design within the limits of the guidance. Uncertain soil, existing movement, retaining structures or unusual loads may justify investigation and engineering design.
Ask what assumptions the design makes and how those assumptions will be checked during excavation. Trial holes or ground investigation can sometimes resolve uncertainty before a contractor prices the work.
What happens when the trench is opened
Arrange the required inspection before placing concrete. Excavation may reveal soft spots, old structures, water or services that were not visible beforehand. If conditions differ from the design assumptions, obtain an agreed revision rather than improvising on site.
Never enter an unsupported excavation to assess it. Safe excavation and temporary works require appropriate planning.
Do neighbouring foundations matter
Yes. Avoid undermining existing structures and identify their position where relevant. Nearby excavation may also fall within the Party Wall etc. Act 1996, depending on distance and depth.
Keep the foundation design, inspection records and any revised details. They provide a clearer record than a statement that the trench was simply dug to a customary depth.