Find the Right Specialist for Your Extension Project
How much will your extension cost? Receive stage-by-stage breakdowns and get free quotes from a tailored list of extension specialists in your area.
How Much Does a House Extension Cost in 2025?
Please Note
The figures presented are for guidance only and are rough estimates. Since every extension is unique, its final cost is subject to major variables, including your location, design complexity, and the type of structure (e.g., single-storey vs. wrap-around).
| Project Type | Typical Cost per m² (excl. VAT) | Example Size | Estimated Build Cost |
|---|---|---|---|
| Single Storey / Rear Extension | £2,200 – £3,300 | 30 m² | £66,000 – £99,000 |
| Double Storey Extension | £1,650 – £2,500 | 40 m² | £66,000 – £100,000 |
| Wrap-Around / L-Shaped Extension | £2,200 – £3,300+ | 50 m² | £110,000 – £165,000+ |
| Side Return Extension | £1,800 – £2,600 | 12–15 m² | £21,600 – £39,000 |
| Kitchen Extension | £2,400 – £3,500 | 25 m² | £60,000 – £87,500 |
| Loft Conversion | £1,800 – £2,800 | 30 m² | £54,000 – £84,000 |
| Garage Conversion | £1,000 – £2,000 | 18 m² | £18,000 – £36,000 |
| Basement / Cellar Conversion | £2,500 – £4,000+ | 30 m² | £75,000 – £120,000+ |
Additional Costs to Consider
| Item | Allow for ~ |
|---|---|
| VAT | +20% |
| Architect, Engineer, Surveyor | +10–15% |
| Kitchen / Bathroom Fit-Out | Varies |
| Contingency | +10–15% |
Example: A 30 m² single-storey rear extension at £2,750/m² ≈ £82,500 build cost (around £110k total with fees & VAT).
To receive a clear, stage-by-stage cost breakdown tailored to your extension, simply post your project today and get up to 5 free quotes from local specialists.
The Extension Journey — Step-by-Step
| Stage | What Happens | Who’s Involved | Typical Time |
|---|---|---|---|
1Initial Design | Measured survey and concept drawings to shape your brief and budget. | 2–3 weeks | |
2Planning / Permitted Development | Planning application or lawful development certificate prepared and submitted. | ArchitectLocal Council | 8–10 weeks |
3Technical Design | Building Regulations drawings, structural calculations, and specifications prepared. | 2–4 weeks | |
4Tender & Builder Selection | Tender packs issued, quotes compared, contracts agreed with preferred builder. | 2–3 weeks | |
5Construction | Groundworks, structure, roof, installations, and finishes delivered on site. | 3–6 months | |
6Snagging & Handover | Final snagging, testing, certification, and handover completed. | BuilderClient | 1–2 weeks |
Initial Design
What Happens
Measured survey and concept drawings to shape your brief and budget.
Who’s Involved
Typical Time
2–3 weeks
Planning / Permitted Development
What Happens
Planning application or lawful development certificate prepared and submitted.
Who’s Involved
Typical Time
8–10 weeks
Technical Design
What Happens
Building Regulations drawings, structural calculations, and specifications prepared.
Who’s Involved
Typical Time
2–4 weeks
Tender & Builder Selection
What Happens
Tender packs issued, quotes compared, contracts agreed with preferred builder.
Who’s Involved
Typical Time
2–3 weeks
Construction
What Happens
Groundworks, structure, roof, installations, and finishes delivered on site.
Who’s Involved
Typical Time
3–6 months
Snagging & Handover
What Happens
Final snagging, testing, certification, and handover completed.
Who’s Involved
Typical Time
1–2 weeks
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Extension Project FAQs
Get Answers to All Your Extension Project Questions
Please Note: The figures & stages presented are for guidance only and are rough estimates. Since every extension is unique, its final cost is subject to major variables, including your location, design complexity, and the type of structure (e.g., single-storey vs. wrap-around).
To get a clear extension cost breakdown of each stage please and get up to 5 free quotes in your area.
What are Architect Fees for an Extension?+
Typical Architect Fees
Architects usually charge 7–12% of the total build cost, depending on scope and project complexity. For example, on an £80,000 extension, fees could range between £5,600 – £9,600. Some offer fixed fees for smaller jobs like single-storey extensions.
Read more →What’s Included
- Measured survey & existing drawings
- Concept and planning designs
- Planning application submission
- Technical drawings for Building Control
- Tender and builder selection support
- Site visits during construction
How to Save
You can reduce costs by agreeing a fixed fee or hiring for specific stages only (e.g. planning design only). Prioritise the stages where an architect adds the most value for your project.
Read more →What Does an Extension Architect Do?+
Design & Planning
Architects turn your ideas into a design that fits your space, budget, and planning policy. They also handle planning drawings and council submissions.
Read more →Technical & Compliance Drawings
Architects prepare Building Regulations drawings and work with structural engineers to ensure your design is safe and compliant.
Read more →Tendering & Builder Coordination
They help you find and compare builders, issue tender packs, and review quotes so you appoint the right team.
Read more →Site Support During Construction
Some architects offer site monitoring — checking quality, progress, and assisting with design queries on-site.
Read more →What are Extension Builder Costs?+
Typical Build Costs
| Extension Type | Typical Cost per m² (excl. VAT) | Example Size | Estimated Build Cost |
|---|---|---|---|
| Single Storey (Rear/Side) | £2,200 – £3,300 | 30 m² | £66,000 – £99,000 |
| Double Storey | £1,650 – £2,500 | 40 m² | £66,000 – £100,000 |
| Wrap-Around / L-Shaped | £2,200 – £3,300+ | 50 m² | £110,000 – £165,000+ |
| Side Return | £1,500 – £2,400 | 12 m² | £18,000 – £28,800 |
| Kitchen Extension | £2,200 – £3,500 | 25 m² | £55,000 – £87,500 |
Extension Type
Single Storey (Rear/Side)
Typical cost per m² (excl. VAT)
£2,200 – £3,300
Example size
30 m²
Estimated build cost
£66,000 – £99,000
Extension Type
Double Storey
Typical cost per m² (excl. VAT)
£1,650 – £2,500
Example size
40 m²
Estimated build cost
£66,000 – £100,000
Extension Type
Wrap-Around / L-Shaped
Typical cost per m² (excl. VAT)
£2,200 – £3,300+
Example size
50 m²
Estimated build cost
£110,000 – £165,000+
Extension Type
Side Return
Typical cost per m² (excl. VAT)
£1,500 – £2,400
Example size
12 m²
Estimated build cost
£18,000 – £28,800
Extension Type
Kitchen Extension
Typical cost per m² (excl. VAT)
£2,200 – £3,500
Example size
25 m²
Estimated build cost
£55,000 – £87,500
What’s Usually Included
- Labour and materials
- Foundations, structural shell, and roof
- Plumbing, electrics, plastering, and first-fix services
- Basic kitchen/bathroom fittings
- Decoration and finishes (depending on quote)
What’s Not Usually Included
- Design or planning fees
- Structural engineering
- High-end finishes or landscaping
What is the Builder's Role on an Extension?+
Pre-Construction
Your builder reviews all drawings, arranges site setup and safety measures, and prepares a detailed programme of works before construction begins.
Read more →Construction Stages
- Groundworks & foundations
- Structure & roof
- First fix (wiring, plumbing)
- Second fix (finishes, fittings)
- Decoration & external works
- Snagging & handover
Quality & Safety
Builders ensure all works meet Building Regulations, structural safety, and your agreed specifications.
Read more →Client Communication
Builders provide regular progress updates, agree payment milestones, and handle snagging before completion.
Read more →
