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What is a strip foundation and when is one used?

A strip foundation is a continuous strip of concrete laid in a trench under a load-bearing wall, spreading the wall's weight into firm ground. It is common under house extensions and garden walls, and its size and depth are set by the ground and the design.

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By the QuoteZila editorial team. Editorial policy.

What is a strip foundation?

A strip foundation is the most familiar kind of foundation for a small building. A trench is dug along the line of each load-bearing wall, down to ground firm enough to carry it, and the bottom of the trench is filled with a band of concrete. The wall is then built up from that concrete. The strip spreads the weight of the wall over a wider area of ground, so it does not sink or crack.

A close cousin is the trench fill foundation, where the trench is filled with concrete almost to the surface instead of having a thin strip with masonry built up from it. It uses more concrete but less bricklaying below ground. Your designer or builder will choose between them.

QuoteZila is starting in Cheshire, with Crewe, Nantwich and Sandbach among the first towns, and anyone in the UK can post a job. This page explains what strip foundations are so you can follow what your builder and building control are talking about. It does not give a depth, a width or a method to follow.

Where are strip foundations used?

You are most likely to meet them in:

  • Single and two-storey extensions, under the new outside walls.
  • Garages and outbuildings with masonry walls.
  • Garden walls, especially taller brick or block walls.
  • Porches and small additions, depending on the design.

Larger or unusual buildings, poor ground, or sites with big trees nearby may need a different type, such as a raft or piles. That is a decision for a structural engineer or building control, not something to settle on price alone.

What decides the size and depth?

Several things shape a strip foundation, and they vary from site to site:

  • The ground. Firm gravel behaves differently from soft clay or made-up ground. Clay can shrink and swell with the seasons.
  • Trees and hedges. Roots draw water from clay soils, so foundations near trees may need to go deeper.
  • Drains. A trench that crosses or runs close to a drain may need special detailing.
  • The load. A two-storey wall carries more than a single-storey one.
  • Neighbouring buildings. Digging close to next door's foundations brings extra care and, sometimes, legal steps.

This page does not print depths or widths, because they depend on all of the above. The figures on your drawings, and building control's view on site, are what count.

There are no DIY steps here. Digging and pouring foundations is structural work, and trenches can collapse. Leave it to a builder working to approved drawings.

Building control and inspections

Foundations are one of the stages building control will want to see. GOV.UK says you must check whether your work needs building regulations approval before you start [S3]. An extension usually does.

Once the trench is dug, the builder tells building control, and an inspector checks the ground at the bottom before any concrete goes in. Leeds City Council says this foundations inspection checks the ground conditions so the foundations can carry the load, and that it must take place before the foundation concrete is poured [S1]. Your own council or approved inspector will have its own arrangements, so ask your builder how they will book it.

If the inspector finds softer ground than expected, they may ask for the trench to go deeper. That can change the price, so agree with your builder in writing how extra depth will be priced before work starts.

GOV.UK: find your local council for building control. QuoteZila is not affiliated with this organisation.

You can read GOV.UK's guidance on building regulations approval before you start.

Neighbours and the Party Wall Act

If your trench is close to a neighbour's house or garage, the Party Wall etc. Act 1996 may apply. Section 6 of the Act deals with some excavations near a neighbouring building [S2]. Where it applies, a party wall notice is served on the neighbour before work starts, and GOV.UK's guidance explains the process and that the Act is separate from planning permission and building regulations [S4].

Talk to your neighbours early. Ask your builder or a party wall surveyor whether the Act applies to your trench, and keep copies of anything you send or receive.

What to ask a builder

When you ask builders to price an extension or wall with strip foundations, ask each one:

  1. Is the price based on the approved drawings, and what foundation depth has been allowed?
  2. How will extra depth be priced if building control asks for it?
  3. How will spoil from the trench be removed?
  4. Are drains, trees or neighbouring foundations close enough to change the design?
  5. Who books the building control inspections?
  6. Does the Party Wall Act apply, and who deals with it?

Jobs you might post

These are examples, not taken from real job posts.

  • "Single-storey rear extension with plans approved. Need a builder for the groundworks and foundations."
  • "Want a brick garden wall along the side boundary. Would like advice on the foundation needed."
  • "Detached garage to be built at the end of the drive. Large tree about ten metres away."

Get quotes from builders

Describe the job, share any drawings and give your postcode. Up to five matched local businesses can send you a quote [QZ2]. QuoteZila is free for customers and local businesses [QZ1].

Honest status: QuoteZila is new. We're starting in Cheshire, beginning with Crewe, Nantwich and Sandbach, and growing area by area. Anyone in the UK can still post a job or register a business. How fast you get quotes depends on how many local businesses have joined near you. Businesses can register and receive jobs straight away. We don't check insurance, qualifications or registrations, so please check these yourself before you hire.

Posting a job is not open yet

See our builders page. For the roof end of an extension, see what changes the cost of roof trusses, and for the floor, what changes the cost of a floor screed.

Questions about strip foundations

What is the difference between a strip foundation and a trench fill foundation?

A traditional strip foundation is a band of concrete at the bottom of the trench, with brick or block built up from it to ground level. A trench fill foundation fills most of the trench with concrete instead. Both sit under the wall; the designer or building control will say which suits the ground.

How deep should a strip foundation be?

This page does not print a depth, because it depends on the ground, nearby trees and drains, and the design. Leeds City Council says its foundations inspection must take place before the foundation concrete is poured.

Do I need to tell my neighbour before digging foundations?

You might. Section 6 of the Party Wall etc. Act 1996 covers some excavations close to a neighbour's building. If your trench is near next door, ask your builder or a surveyor whether you need to serve a party wall notice before work starts.

Does QuoteZila check the businesses?

No. We don't check insurance, qualifications or registrations, so please check these yourself before you hire. You can report a quote or a business from your job page.

This page is general information, not professional or legal advice. QuoteZila does not check Gas Safe, WaterSafe, NFRC, NICEIC or other registrations against the register on your behalf: look them up yourself.

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Sources

  1. QZ1: QuoteZila product fact. QuoteZila home and jobs-for-tradesmen copy.
  2. QZ2: QuoteZila product fact. QuoteZila FAQ and app README.
  3. S1: Site inspections, Leeds City Council. Accessed 2026-10-27. https://www.leeds.gov.uk/planning/building-regulations-and-approval/building-regulations/site-inspections
  4. S2: Party Wall etc. Act 1996, section 6, legislation.gov.uk. Accessed 2026-10-27. https://www.legislation.gov.uk/ukpga/1996/40/section/6
  5. S3: Building regulations approval, GOV.UK. Accessed 2026-10-27. https://www.gov.uk/building-regulations-approval
  6. S4: Party Wall etc Act 1996 guidance, GOV.UK. Accessed 2026-10-27. https://www.gov.uk/government/publications/preventing-and-resolving-disputes-in-relation-to-party-walls