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A guide to house foundations.
Strip, trench-fill, raft, pile-and-beam — which suits your soil, building and budget.
✓ Updated May 2026✓ Free quotes✓ Vetted pros
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Costs, timelines, pitfalls and the right questions to ask before any groundworks starts on house Foundations.
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Inadequate depth
Cheap foundations don't reach firm soil. Settlement and subsidence.
Wrong type for soil
Strip on bad clay = differential settlement. Soil survey essential.
Concrete spec wrong
20N concrete may need 30N. Match to load.
Tree roots vs foundation
Tree roots cause settlement. Deeper foundations needed.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Strip foundation per metre1m deep, standard
£50–£150
Raft foundation per m²Continuous slab
£100–£250/m²
Pile foundation per pileMini-piles 5-15m
£200–£600
Soil investigation reportGeotechnical survey
£500–£2.5k
Standard extension foundationsPer house
£3.5k–£8k
Difficult soil foundationsRaft or piles
£5k–£15k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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Groundworks explained
Foundations support buildings and resist subsidence. UK foundation types: strip (common), raft (poor soil), pile (deep). Expect to pay £3,500–£15,000+ for UK foundations on a typical home extension.
What Does the Work Involve?
Structural engineer designs foundations. Soil survey identifies ground conditions. Strip foundation: concrete trench around perimeter. Standard for normal soils. Raft foundation: continuous slab. Poor soils. Pile foundation: deep concrete piles. Very poor soils. Building inspector approves design.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Strip foundation per metre | 50 | 150 | 1m deep, standard |
| Raft foundation per m² | 100 | 250 | Continuous slab |
| Pile foundation per pile | 200 | 600 | Mini-piles 5-15m |
| Soil investigation report | 500 | 2,500 | Geotechnical survey |
| Standard extension foundations | 3,500 | 8,000 | Per house |
| Difficult soil foundations | 5,000 | 15,000+ | Raft or piles |
| Underpinning existing | 1,000 | 5,000+ | Per metre |
How Long Does It Take?
Standard strip: 3-5 days plus concrete cure (7 days). Raft: 5-10 days plus cure. Piles: 7-14 days. Soil investigation: 2 weeks lead.
DIY or Professional?
Specialist groundworker only. Building Regs notification mandatory. Structural engineer for design.
Choosing the Right Tradesperson
NFB or FMB member groundworker. Structural engineer for design. Building inspector for approval.
UK Regulations
Building Regs Part A (structure). Building inspector approval at foundation depth. Soil investigation typically required.
Common Problems
- Inadequate depth: Cheap foundations don't reach firm soil. Settlement and subsidence.
- Wrong type for soil: Strip on bad clay = differential settlement. Soil survey essential.
- Concrete spec wrong: 20N concrete may need 30N. Match to load.
- Tree roots vs foundation: Tree roots cause settlement. Deeper foundations needed.
Frequently Asked Questions
How deep should foundations be?
UK minimum: 1m below ground level. 1.5m+ in clay soils. 2m+ if trees nearby. Soil survey confirms.
Strip vs raft?
Strip: standard, cheaper, for stable soils. Raft: for soft/poor soils, more expensive, spreads load. Soil determines choice.
Piles when needed?
Very deep firm soil layers, soft surface soils, narrow access where excavation impractical. Premium.
Foundation cost for typical extension?
£3,500-£8,000 standard. £5,000-£15,000+ for difficult soils or large extensions.
Subsidence — fixable?
Yes — underpinning. £1,000-£5,000 per metre. Major work.
Will builder include foundation in extension quote?
Yes — extension quotes typically include foundations. Specialist may sub-contract groundwork.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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