What affects the cost?
Extension budgets vary mainly by floor area, storeys, structural complexity, glazing, kitchens or bathrooms, finish level and location. Use the calculator for a planning estimate, then compare it with itemised quotes.
- Floor area and number of storeys
- London and regional labour rates
- Steelwork and structural changes
- Kitchen or bathroom installation
- Glazing and door specification
- Finish level and contingency
Typical cost breakdown
Regional planning examples
The examples below apply our published modelling factor to the same national benchmark. They are not claimed as observed city-wide average quotes.
Sources used for this guide
House Extension: Cost Breakdown 2026
Published / checked: June 2026
Used for: Extension cost-per-m² benchmarks and common add-on costs
Construction output price indices
Published / checked: 13 August 2026
Used for: Construction price trend and update checks
Frequently asked questions
How much should I allow for contingency?
A planning contingency of around 10% is a sensible starting point for many projects, with more allowance where the scope or existing structure is uncertain.
Are London extensions more expensive?
Often yes. Labour, access, logistics and contractor overheads can push costs above many other UK regions.
Related costs
House renovation
Typical UK whole-house renovation costs, contingency, labour and the main factors that change a refurbishment budget.
£43,530–£110,350 for a 3-bed houseView guide →Cost guideBuild a house
UK new-build and self-build costs per square metre, by house size, specification and project complexity.
£1,400–£3,000 per m²View guide →Cost guideLoft conversion
Typical UK loft conversion costs for rooflight, dormer, hip-to-gable and more complex projects.
£35,000–£85,000View guide →