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Pour extension foundations.
Strip, trench-fill or raft — designed to soil report, sized to extension footprint and structural load.
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Trees affecting foundations
Trees within 2-3 times their mature height affect foundations. Deeper or piled foundations needed.
Existing foundations shallow
Old houses have shallow foundations. New must connect properly without undermining.
Soil investigation skipped
Clay or made-ground revealed during excavation. Cost overrun.
Party wall surveyor
Adjoining property = Party Wall Award needed. £400-£1,500 fee.
Costs & timeline
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Indicative UK ranges and what affects price.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Extension foundation excavationPer extension
£800–£2.5k
Strip foundation concreteIncluding reinforcement
£1.2k–£3.5k
Connection to existing foundationsTying in
£500–£2k
DPC and brickwork to floor levelCavity wall
£1.5k–£4k
Insulated floor slabPart L
£1.2k–£3.5k
Standard extension foundations20-30m² extension
£3.5k–£8.5k
!
Quote spread is typically ± 18% — always get 3 quotes.
At a glance
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Groundworks explained
Extension foundations support new built-on additions to existing homes. Connect to existing structure carefully. Expect to pay £3,500–£12,000 for UK extension foundations depending on size and soil.
What Does the Work Involve?
Structural engineer designs foundations. Connection to existing foundations critical. Strip foundations (typical) or piles if poor soil. Excavation. Concrete pour. Reinforcement. DPC and wall up to floor level. Floor slab on insulation.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Extension foundation excavation | 800 | 2,500 | Per extension |
| Strip foundation concrete | 1,200 | 3,500 | Including reinforcement |
| Connection to existing foundations | 500 | 2,000 | Tying in |
| DPC and brickwork to floor level | 1,500 | 4,000 | Cavity wall |
| Insulated floor slab | 1,200 | 3,500 | Part L |
| Standard extension foundations | 3,500 | 8,500 | 20-30m² extension |
| Large extension foundations | 5,500 | 15,000+ | 40m²+ extension |
How Long Does It Take?
Extension foundation: 2-4 weeks including curing. Plus design and Building Regs (4-8 weeks).
DIY or Professional?
Specialist groundworker. Structural engineer for design. Building inspector for approval.
Choosing the Right Tradesperson
NFB or FMB member. Specialist extension builder. Structural engineer.
UK Regulations
Full Building Regs for extension. Part A (structure), Part L (thermal), Part C (moisture), Part H (drainage). Party Wall Act if affecting boundary.
Common Problems
- Trees affecting foundations: Trees within 2-3 times their mature height affect foundations. Deeper or piled foundations needed.
- Existing foundations shallow: Old houses have shallow foundations. New must connect properly without undermining.
- Soil investigation skipped: Clay or made-ground revealed during excavation. Cost overrun.
- Party wall surveyor: Adjoining property = Party Wall Award needed. £400-£1,500 fee.
Frequently Asked Questions
Match existing foundation depth?
Yes — should be at same level or deeper. Stepped foundations possible. Engineer designs.
Party Wall Act issue?
Yes if extension within 6m of neighbour and excavating below their foundations. Party Wall Award required.
Cost as % of extension?
Foundations: 10-15% of extension build cost. £30,000 extension: £3,000-£5,000 foundation.
How long is foundation phase?
2-4 weeks including concrete cure. Part of overall 3-6 month extension.
Soil survey worth it?
For extensions in clay or unknown ground: yes. £500-£3,000 well spent.
Building inspector visits?
Yes — at excavation depth (before concrete), at DPC, before floor cover. 2-3 visits minimum.
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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