Builder stages for your extension project
Please note that these are rough guides only. To get a clear cost breakdown of each stage for your extension project, please post your project and get up to 5 free quotes
Contract & Pre-Start Administration
Formalise agreements and prepare for a smooth project start
Site Mobilisation & Protection
Set up the site safely and protect existing areas
Temporary Works, Demolition & Strip-Out
Safely remove existing elements and prepare the structure
Groundworks & Foundations
Form the project's structural base
Structural Frame & Superstructure
Build the main structure of the project
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Builder step-by-step cost guide for your extension project.
(Please note that these are rough guides only; to get an extension cost breakdown of each stage, please post your project and get up to 5 free quotes.)
Builder Cost Breakdown – Typical Home Extension Project
All figures are rough guides only and vary by location, design complexity, and specification. Based on 2025 averages, excluding VAT.
Typical overall cost: £2,200 – £3,500+ per m²
Example: 25 m² single-storey extension ≈ £55,000 – £87,500
Pre-Start & Site Setup
2–5%
Approx. cost: £1,000 – £4,000
Groundworks & Foundations
10–15%
Approx. cost: £7,000 – £15,000+
Structural Frame & Superstructure
20–25%
Approx. cost: £12,000 – £25,000+
Roofing, Windows & External Finishes
10–15%
Approx. cost: £7,000 – £15,000+
First Fix Services (Mechanical, Electrical & Plumbing)
8–12%
Approx. cost: £5,000 – £10,000+
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