How Deep Should Foundations Be in the UK?

Why a standard depth is not a design

A foundation must transfer the building’s loads into suitable ground. A dimension that worked next door may not suit your proposed extension, the ground exposed on your plot or nearby trees. Greater depth is not automatically the answer either: the appropriate foundation type also matters.

The government’s Approved Document A: Structure covers foundations, loading and ground movement in England. It is guidance for compliance, not a substitute for assessing an individual site. Requirements and guidance differ across the UK’s nations; this article focuses on projects in England.

What affects the depth of extension foundations?

• Ground conditions: the bearing ground, fill, soft areas and water conditions need to be understood.
• Trees and vegetation: existing or removed trees can matter, particularly where soils are susceptible to moisture-related movement.
• The proposed structure: loads, wall positions and structural openings affect the design.
• Existing buildings and boundaries: the work must take account of neighbouring foundations and other structures.
• Drains and buried services: routes, depths, ownership and access can constrain the work.

These factors are reasons to investigate and design the foundations properly. They are not a checklist from which a homeowner can safely calculate a trench depth.

Strip, trench-fill, raft or piled foundations?

Different foundation arrangements deal with different designs and ground conditions. A shallow strip, trench-fill foundation, raft or piled solution is not chosen on price alone. Your structural designer should specify the suitable arrangement, dimensions and materials, with the relevant checks through Building Control.

For an overview of the options, read which foundations an extension may need. Do not use an online comparison to change a foundation specification already issued for your project.

What should happen before concrete is poured?

1. Confirm the current drawings, design information and responsibility for decisions on unexpected conditions.
2. Identify services and plan excavation, access, spoil removal and temporary safety measures.
3. Arrange the necessary inspection before the foundation is covered or concrete is poured.
4. If the exposed ground or existing structure differs from the assumptions, refer it back for assessment rather than improvising a new detail.
5. Record agreed changes, inspections and relevant handover information.

HSE warns that excavation work needs precautions against collapse, undermining nearby structures and damage to underground services. Read its excavation safety guidance. Do not enter a trench or dig a trial pit yourself to check a foundation.

Planning permission and Building Regulations are different

An extension’s planning status does not settle its structural requirements. Planning Portal explains that most extensions need Building Regulations approval. Check the approval and inspection route for your proposal before work starts, including any separate drainage or neighbouring-property matters.

What to include in a Surrey foundation enquiry

Send the site postcode, the proposed use or extension type, drawings and structural details if available, photographs of access and any information you have about nearby drains, trees or previous ground problems. Tell the contractor whether you need foundations only, or drainage, slab preparation and a wider building package as well.

Ask whether the price includes setting out, excavation, spoil removal, concrete, access arrangements and reinstatement. Any allowance for unknown ground should be explained rather than hidden behind a single total.

A Jones Contractors carries out groundworks and foundation work across Surrey, including Guildford and Woking. See the house extensions service if you need a wider package.

Send your project details or call 07414 577346. A contractor’s quote does not replace the structural design or required approvals.

Frequently asked questions

Are one-metre-deep foundations always enough?

No. There is no universal depth that can be confirmed without the design and site conditions. Ask your designer and Building Control what is required for your project.

Can foundations be priced before the ground is opened up?

A quote can be based on available drawings, investigations and stated assumptions. Ask what happens if the ground differs, who will assess it and how any revised work will be agreed.

Can the foundation depth change during the dig?

Unexpected conditions can require a review. The correct response is to obtain the appropriate design advice and inspection, not simply to dig deeper without agreement.