How Much Does a House Extension Cost in the UK?
House extension costs in the UK typically fall between £1,800 and £3,000 per square metre, depending on the finish, structure and location. That means a modest 20m2 extension can range from about £36,000 to £60,000+, excluding premium kitchens, bathrooms and bespoke glazing.
Every property and project is different, so use the figures below as a starting guide, then request tailored quotes from builders who have seen your plans and site.
Average UK Extension Cost Per m2 (2025)
| Specification | Typical Cost per m² | Includes |
|---|---|---|
| Basic build | £1,800 – £2,400 | Standard materials, basic finishes, minimal structural changes |
| Mid-range | £2,400 – £3,000 | Better glazing, higher standard of finishes, more design input |
| High-end | £3,000 – £3,800+ | Premium materials, large glazing, complex structure, bespoke details |
Note: The cost per m² normally covers the main structure and basic finishes. It may not include fitted kitchens, bathrooms, professional fees, or landscaping.
House Extension Cost by Type
The type of extension you build has a big impact on your final cost. Below are realistic 2025 UK price ranges for the most common projects.
Single Storey Extension Cost
A single storey extension is the most popular way to create extra living space, usually at the rear or side of the property.
- Typical size: 15–30m2
- Guide cost: £35,000 – £75,000+
- Cost per m²: around £1,900 – £2,800
This is ideal for a larger kitchen-diner, a family room, or a home office. Adding rooflights or bi-fold doors will increase the budget but can dramatically improve light and usability.
Double Storey Extension Cost
Adding a second storey can be cost-effective when you need more bedrooms and bathrooms as well as living space.
- Typical size: 30–60m2 over two floors
- Guide cost: £60,000 – £140,000+
- Additional cost for the second floor: often 50–70% of the ground floor cost
Because the foundations and roof are already being built for the ground floor, the extra cost per m² of the second storey is often lower than the first.
Rear Extension Cost
A rear extension typically creates an open-plan kitchen-diner or larger living space opening onto the garden.
- Guide cost: £40,000 – £90,000+
- Popular additions: sliding or bi-fold doors, roof lanterns, underfloor heating, landscaping
Costs increase when you include structural openings into existing rooms, steel beams, and high-spec glazing.
Side Return Extension Cost
Victorian and Edwardian terraces often have an unused side alley. A side return extension fills this in, transforming narrow kitchens into bright, usable spaces.
- Guide cost: £35,000 – £70,000+
- Typical size: 10–20m2
- Challenges: access, drainage, and working close to party walls
Wrap-Around Extension Cost
A wrap-around combines a rear and side extension, wrapping the new structure around the existing house.
- Guide cost: £70,000 – £160,000+
- Best for: creating large open-plan living spaces and re-configuring the entire ground floor
Typical House Extension Cost Examples
Below are simplified examples to help you compare potential budgets.
Example 1: 20m2 Single Storey Rear Extension (Mid-Range)
- Size: 20m2
- Build cost (shell & basic finishes): ~£46,000 (at £2,300/m²)
- Kitchen supply & fit: £8,000 – £15,000
- Professional fees & permissions: £3,000 – £6,000
- Contingency (10–15%): £5,000 – £8,000
Likely total project cost: £62,000 – £75,000
Example 2: 30m2 Double Storey Side Extension
- Ground floor (20m² @ £2,300/m²): ~£46,000
- Second floor (20m² @ £1,600/m²): ~£32,000
- Bathrooms & fittings: £6,000 – £12,000
- Professional fees & permissions: £5,000 – £9,000
- Contingency (10–15%): £9,000 – £13,000
Likely total project cost: £98,000 – £112,000
What Factors Affect House Extension Cost?
Your final extension cost depends on far more than just size. These are the main drivers:
- Location: London and the South East are typically 10–25% more expensive than the UK average.
- Access and site conditions: Restricted access, sloping sites or poor ground conditions increase labour and engineering costs.
- Design complexity: Unusual shapes, large spans, vaulted ceilings and complex roof forms require more structural work.
- Specification and finishes: Premium kitchens, bathrooms, flooring and glazing can quickly add tens of thousands of pounds.
- Structural alterations: Removing load-bearing walls and inserting steel beams adds cost but can transform the space.
- Services and upgrades: Upgraded electrics, plumbing, heating systems and underfloor heating all affect the budget.
- Planning and regulations: Planning conditions, party wall agreements and building control approvals may add professional and legal fees.
House Extension Costs by Region
Construction costs vary across the UK. Below is a broad guide to how pricing can differ by region.
| Region | Relative Cost | Typical Range per m² |
|---|---|---|
| London | Highest | £2,400 – £3,800+ |
| South East & Home Counties | High | £2,200 – £3,400 |
| South West & Midlands | Medium | £2,000 – £3,000 |
| North of England | Medium-Low | £1,800 – £2,700 |
| Scotland, Wales & Northern Ireland | Medium-Low | £1,800 – £2,600 |
Breaking Down Your House Extension Budget
Many homeowners focus on builder quotes alone, but a full project budget should include all of the following:
- Construction cost: usually 60–75% of the overall budget.
- Professional fees: architect or designer, structural engineer, surveyors – often 8–15% of construction cost.
- Planning fees: planning application, lawful development certificate, and building control inspections.
- Party wall costs: if you share walls or boundaries with neighbours, you may need party wall surveyors.
- Fittings & finishes: kitchen, bathrooms, flooring, lighting, built-in storage, decoration.
- External works: patios, steps, garden walls, drainage changes, landscaping.
- Contingency: always allow at least 10–15% for unexpected costs or design changes.
How to Reduce Your House Extension Cost
You don’t have to compromise on quality to keep control of your budget. Consider the following:
- Keep the footprint simple: rectangular plans and simple roofs are cheaper to build.
- Minimise structural alterations: work with existing walls where possible.
- Choose cost-effective glazing: standard-size windows and doors are far cheaper than fully bespoke designs.
- Phase your project: prioritise structure and services now, and upgrade finishes later if needed.
- Get multiple quotes: compare like-for-like specifications from several vetted builders.
- Decide details early: last-minute changes are a common cause of overspends.
Planning Permission and Building Regulations
Understanding the rules early will help you avoid redesign costs and delays:
- Permitted development: many modest single-storey rear extensions fall under permitted development, meaning you may not need a full planning application.
- Full planning permission: usually required for larger extensions, double storey work, or projects in conservation areas and on listed buildings.
- Building regulations: all extensions must comply with current regulations for structure, fire safety, energy performance, drainage and more.
- Party Wall etc. Act 1996: if you share a wall or are building near a boundary, you may need to serve notices and appoint surveyors.
Professional advice from an architect or experienced builder is highly recommended at the planning stage.
Do House Extensions Add Value?
Well-planned extensions can significantly increase both the value and usability of your home. Typical estate agent estimates suggest:
- A quality extension can add 10–25%+ to the value of many UK homes, depending on location and design.
- Adding a bedroom and bathroom is often more valuable than simply increasing living space.
- Finishing to a good standard, in keeping with the property and neighbourhood, helps maximise resale value.
Ultimately, the best projects deliver both lifestyle benefits now and strong resale potential in the future.
Get Accurate House Extension Cost Quotes
Online guides are helpful, but nothing replaces a detailed, written quote from a reputable builder who has visited your property and reviewed your plans.
Best Builders connects you with vetted, insured, and experienced UK builders who specialise in house extensions. Submit your project details once, and we’ll match you with suitable trades who are genuinely interested in your job.
- No obligation to hire
- Compare multiple quotes
- Save time and avoid chasing unreliable contractors
House Extension Cost FAQs
How much does a 20m² extension cost in the UK?
For a 20m² single storey extension, expect £36,000 – £60,000+ depending on finish and location. Remember to add the cost of kitchens/bathrooms, professional fees and contingency on top of the basic build price.
Is it cheaper to extend up or out?
Extending out on one storey is usually simpler and cheaper in absolute terms. However, adding a second storey can deliver more space per pound spent because you are spreading the cost of foundations and the roof over two floors.
How long does a house extension take?
A straightforward single storey extension usually takes 10–16 weeks on site once work starts. Larger or more complex double storey projects can take 20–30 weeks or more, especially if major structural changes are involved.
When is the best time of year to build an extension?
Extensions can be built year-round, but spring and summer often offer more reliable weather and longer daylight hours. Good builders book up quickly, so it’s wise to start design and planning several months before you hope to begin work.
Can I live in my house during the extension?
Many homeowners remain in the property during construction, particularly for single storey projects. Expect noise, dust and some disruption. For major extensions and reconfigurations, temporary accommodation may be more comfortable and can sometimes speed up the build.