The direct answer
Start with the complete system, not one product or headline rate.
How clay, nearby trees, made ground, drains and existing foundations can influence an extension foundation strategy in Surrey. A dependable plan connects the site evidence, approvals, access, installation sequence and finished reinstatement.
Start with evidence, not a standard depth
A nominal trench depth is only a starting assumption. The approved design needs to respond to bearing ground, shrinkable clay, tree species and distance, groundwater, existing construction and the loads from the proposed extension.
Trees can affect the design beyond the canopy
On shrinkable clay, existing, removed and proposed trees may influence foundation depth and heave precautions. Record species, approximate height and distance early so the designer and Building Control can assess the same site information.
Made ground and previous alterations matter
Filled ponds, demolished structures, old drains and terracing can produce variable ground. Trial holes or a suitable site investigation can reveal conditions that are not visible from the surface.
Existing foundations need to be understood
Where new work meets the house, opening-up can help establish foundation depth, construction and condition. The junction detail should allow for the old and new structures rather than assuming they will behave identically.
Drainage and excavation must be coordinated
Deep trenches beside active drains or neighbouring structures create sequencing and temporary-support questions. Drain routes, inspection points and safe excavation methods should be planned before plant arrives.
Keep an inspection record
Photographs, dimensions, ground observations, concrete tickets and Building Control inspection records provide a useful construction record before the foundation disappears below ground.
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