A plain-English guide to what a house extension costs — the typical build cost per m², the other costs to plan for, and how to get an accurate quote.
The cost of a house extension varies with its size, specification and how complex it is to build. Every project is different, but understanding the main factors — and typical ranges — helps you plan realistically. Orange Key works with homeowners across Berkshire and Oxfordshire on extensions from modest single-storey additions to larger multi-storey projects.
Build costs vary by location and specification, but as a broad guide many residential extensions currently fall within these ranges:
| Extension type | Typical build cost |
|---|---|
| Single-storey extension | £2,000–£3,000 / m² |
| Two-storey extension | £1,800–£2,800 / m² |
These figures are indicative only and vary with ground conditions, structural requirements, site access, the quality of materials and finishes, and the amount of glazing or structural openings. A contractor can price accurately once detailed drawings are prepared.
Beyond the build itself, a project usually involves:
Allowing for these early helps you set a realistic overall budget.
Builders can only price accurately from clear drawings. Planning drawings let the local authority assess the proposal, while building regulations drawings give contractors the technical detail they need. Coordinated drawings reduce uncertainty during construction and make it much easier to compare quotes on a like-for-like basis.
“Most people read a cost guide and realise the same thing — the next step is drawings. Once you have those, everything else becomes clearer.”
Orange Key — house extension designIf you're considering an extension, we can review your property and advise on a sensible way forward. Learn more about our house extension design service or get in touch.
As a broad guide, single-storey extensions currently cost around £2,000–£3,000 per m² to build, before design, planning and building control fees — depending on ground conditions, glazing, finishes and access.
Two-storey extensions often work out at around £1,800–£2,800 per m², as roof and foundation costs are spread over more floor area. Specification and structural complexity still make a real difference.
Design drawings, planning fees, building regulations drawings, structural engineering, building control fees and, where relevant, party wall surveyor fees.
Builders price accurately from clear drawings. Preparing coordinated planning and building regulations drawings first reduces uncertainty and makes quotes firm and comparable.
Tell us about your property in Berkshire or Oxfordshire and we'll help you plan the next step.