Cost of a House Extension in the UK (2025)
House extensions cost £35,000–£130,000+ depending on size, type, and specification. Single-storey rear extensions are the most common and cost £35,000–£75,000.
Single-storey vs double-storey extensions
A single-storey rear extension adds ground floor space — typically used to extend a kitchen or create an open-plan kitchen-diner. A double-storey extension adds space on both floors, making it better value per square metre but requiring planning permission in almost all cases. Side return extensions fill the narrow alley alongside terrace and semi-detached houses to widen the kitchen.
What is included in an extension cost?
Extension costs include groundworks and foundations, structural steel if required, blockwork walls, roof structure and covering, windows and doors (including bifolds or sliding doors if specified), internal plastering, electrics first and second fix, plumbing if extending to include bathroom or utility, underfloor heating if specified, and flooring. Architectural drawings are separate and typically cost £1,500–£4,000.
How long does an extension take?
A single-storey extension (groundworks to completion) typically takes 3–5 months. A double-storey extension takes 5–8 months. These timelines assume planning permission is already granted — if permission is needed, allow an additional 8–13 weeks for the application process.
Cost Breakdown
All prices are indicative UK averages for 2025. Actual costs vary by location, specification, and contractor. London and South East rates are typically 20–35% higher.
| Item | Low | High |
|---|---|---|
| Single-storey rear (3m x 4m) | £35,000 | £55,000 |
| Single-storey rear (4m x 6m) | £55,000 | £85,000 |
| Double-storey rear extension | £70,000 | £130,000 |
| Side return extension | £30,000 | £60,000 |
| Garage conversion | £15,000 | £30,000 |
| Architect drawings | £1,500 | £4,000 |
What Affects the Cost?
- ✓Size in square metres — biggest single cost driver
- ✓Specification: standard finish vs high-end materials
- ✓Presence of bifold/sliding doors (adds £3,000–£8,000)
- ✓Underfloor heating (adds £3,000–£7,000)
- ✓Planning permission requirement and complexity
- ✓Site access and ground conditions
- ✓Location — London and South East significantly higher
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Post a Job — It's FreeFrequently Asked Questions
Do I need planning permission for a single-storey extension?
Not always — many single-storey rear extensions fall under Permitted Development rights, subject to size and location restrictions. Your builder or architect will check whether your extension qualifies or needs a formal application.
Does an extension add value?
Generally yes — a well-designed extension adds 5–15% to property value. Open-plan kitchen extensions are particularly popular with buyers. The return depends heavily on location and the quality of the build.
Do I need a structural engineer for an extension?
Yes — any extension with loadbearing walls, steel beams, or changes to the foundations requires structural engineer calculations. Your builder will typically coordinate this. Engineer fees are usually £500–£1,500.
Can I use my house during an extension build?
In most cases yes, though the rear of the house will be disrupted. The most disruptive phase is when the rear wall is opened up — your builder should weather-proof the opening quickly and discuss temporary arrangements.