Foundations and concrete bases for extensions, outbuildings and structures
Foundations and Concrete Bases
Foundations are the below-ground concrete structure that transfers the load of a building safely into the ground, with the type — strip, trench-fill or raft — and depth determined by ground conditions, nearby trees and the load of the structure above.
Definition
What Foundations covers
Foundation and concrete base work covers the excavation, formwork and concrete needed to support a new structure, whether an extension, outbuilding or garden room. The appropriate foundation type and depth depends on soil conditions, proximity to trees and drains, and the loads involved, and is confirmed by a site investigation and, for anything beyond a simple base, an engineer's design submitted to Building Control.
Suitable for
- New extensions, annexes and outbuildings needing purpose-designed foundations
- Garden rooms and offices needing a concrete or reinforced base
- Sites with straightforward ground conditions suited to strip or trench-fill foundations
- Projects where an existing foundation needs extending or underpinning is being considered separately
Not suitable for
- Sites with known ground contamination or made ground without a prior site investigation
- Structural underpinning of an existing building's foundations, which is a specialist scope in its own right
Process
How the work is carried out
- 01
Site investigation
Checking ground conditions, trial pits where needed, and proximity to trees, drains and existing foundations.
- 02
Design and approval
Agreeing the foundation type and depth, with Building Control approval before excavation for most projects.
- 03
Excavation and formwork
Digging to the approved depth and forming edges ready for concrete.
- 04
Concrete pour and inspection
Pouring concrete to the specified mix and depth, with a Building Control inspection of the open trench beforehand.
Typical timescale
Foundations for a typical extension or outbuilding base take a few days to a week or so on site, depending on ground conditions and the depth required. Concrete curing time is factored into the programme before construction continues.
Cost guidance
Foundations and concrete bases typically cost £120–£250 per m² in our area, with ground conditions, depth required and access for machinery being the main factors. We provide an itemised quotation after a site investigation rather than a fixed per-square-metre price.
Limitations and exclusions
- Foundation depth and type must be agreed with Building Control before work starts and may be revised if ground conditions differ from what was expected on inspection
- Nearby trees, particularly on clay soils, can require deeper foundations than a standard site, assessed as part of the site investigation
- Existing drains, services or previous foundations found during excavation may require design changes
Areas we cover
Foundations across Sussex, Surrey, Kent and South London
Local property styles, planning authorities and neighbourhood coverage for foundations in each town we work in.
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FAQs
Foundations FAQs
How deep do foundations need to be?
Depth depends on ground conditions, the load of the structure and proximity to trees or drains — a straightforward strip foundation might be around 1m deep, but clay soils near mature trees can require significantly deeper trench-fill foundations. This is confirmed by site investigation and agreed with Building Control.
Does Building Control need to inspect the foundations before concrete is poured?
Yes. An open-trench inspection is a standard Building Control stage, carried out once excavation is complete and before concrete is poured, to confirm the ground conditions and depth match what was approved.
What's the difference between strip, trench-fill and raft foundations?
Strip foundations are a shallow concrete strip with blockwork built up to ground level; trench-fill foundations fill the trench almost entirely with concrete, which suits deeper foundations or restricted access; raft foundations spread the load over a wider concrete slab and suit poor or variable ground. Which is appropriate depends on the site investigation.
Discuss your foundations project
Tell us about the property and what you have in mind, and we'll arrange a site visit.
Last reviewed 2026-07-28.