How to Use the Home Extension Cost Calculator
- Choose your units. UK mode prices in pounds per m²; USA mode prices in dollars per sq ft.
- Pick the extension type, such as a single-storey rear extension, a double-storey extension or a side return (UK), or a room addition, primary suite or second story (USA).
- Enter the internal floor area you're adding. For a double-storey extension, enter the total of both floors.
- Choose a finish level: basic, standard or premium.
- Choose your region (UK) or a local cost factor (USA). Building costs in London and big US cities are far higher than the national average.
- Set fees, contingency and VAT. The defaults are sensible starting points, and you can change them.
- Add fit-out items such as a new kitchen or bathroom, which are usually priced separately.
The result shows the build cost range, the all-in budget, and the cost per m² or sq ft.
How Much Does an Extension Cost in the UK?
Typical 2026 build costs for a standard specification, before regional adjustment:
| Extension type | Build cost per m² |
|---|---|
| Single-storey rear | £2,200–£2,700 |
| Side return | £2,500–£3,100 |
| Wraparound | £2,300–£2,900 |
| Double-storey rear | £1,950–£2,400 |
| Over-garage | £2,100–£2,600 |
| Orangery or garden room | £2,100–£2,800 |
Why is double-storey cheaper per m²? Foundations, groundworks, scaffolding and the roof are the expensive parts, and a second floor shares them across twice the floor area. The total is still higher, but you get more space for each pound.
Regional differences
| Region | Adjustment vs national average |
|---|---|
| London | +20% to +35% |
| South East | +10% to +20% |
| South West and East of England | 0% to +10% |
| Midlands | −5% to +5% |
| North of England and Wales | −10% to −5% |
What the build cost doesn't include
Build rates usually exclude:
- Professional fees, commonly 8–15% of the build cost: architect or designer, structural engineer, building control, planning application, party wall surveyor
- Kitchens, bathrooms and premium glazing such as large bifold or sliding doors
- Landscaping and making good the garden
- Unknown ground conditions: drains under the footprint, poor soil, trees nearby
- VAT at 20%, if your builder is VAT-registered and their quote is ex-VAT
Always add a contingency of 10–15%. Extensions to older houses almost always reveal something unexpected.
Worked example (UK): London kitchen extension
A 20 m² single-storey rear extension in London at standard specification:
- Build cost: 20 × £2,200–£2,700 × 1.275 (London, mid-point) = £56,100–£68,850
- Plus 12% fees and 10% contingency: about £68,400–£84,000
- Add a new kitchen, and many London kitchen extensions end up with an all-in budget of £75,000–£105,000
The same extension in the Midlands would cost about £44,000–£54,000 to build.
Do I Need Planning Permission? (England)
Many single-storey rear extensions can be built under permitted development, without a planning application, if they meet these limits:
- Depth: up to 3 m beyond the original rear wall for semi-detached and terraced houses, or 4 m for detached houses
- Larger home extensions: up to 6 m (semi or terrace) or 8 m (detached), if you apply for prior approval and neighbours don't raise valid objections. The fee is £249.
- Height: no more than 4 m, and no more than 3 m at the eaves within 2 m of a boundary
- Coverage: extensions and outbuildings can't cover more than half the land around the original house
- Side extensions: single storey, no more than 4 m high, and no wider than half the original house
- Two-storey extensions: no more than 3 m deep, at least 7 m from the rear boundary, with matching roof pitch
Permitted development rights don't apply to flats and maisonettes, are restricted in conservation areas and other designated land, and may have been removed by an Article 4 direction. If you need full planning permission, the householder application fee is £548 (from April 2026). A Lawful Development Certificate (£274) proves your extension is permitted development and is worth having when you sell.
Whatever the planning position, extensions need Building Regulations approval. New walls, floors and roofs must meet Part L insulation standards (U-values of 0.18 for walls and floors and 0.15 for roofs), and new windows 1.4. If glazing is more than 25% of the extension's floor area, you'll need extra calculations. Building next to a shared wall or within 3–6 m of a neighbour's foundations may need a Party Wall Act notice.
How Much Does a Home Addition Cost in the USA?
Typical 2026 costs for permitted, professionally built additions:
| Addition type | Cost per sq ft |
|---|---|
| Basic room addition | $150–$350 |
| Family room | $150–$400 |
| Sunroom | $150–$550 |
| Primary suite | $250–$500 |
| Second story | $200–$500+ |
| Bathroom addition | $250–$600+ |
| Kitchen addition | $300–$750+ |
Across all types, additions typically run $200–$600 per sq ft. Kitchens and bathrooms cost the most because of plumbing, electrical work, cabinets and fixtures. A bump-out (a small addition of a few feet) often costs more per sq ft than a full room, because foundations, roofing and permits don't shrink much with size. Garage conversions are usually cheaper than building new space, because the walls, roof and slab already exist.
Costs usually exclude:
| Item | Typical cost |
|---|---|
| Design and architecture | $2,000–$20,000+ |
| Structural engineering | $1,000–$5,000+ |
| Survey or site plan | $500–$2,500 |
| Permits and inspections | $1,000–$10,000+ |
| Contingency | 10–20% of budget |
Worked example (USA): primary suite addition
A 400 sq ft primary suite at standard finish (the middle third of the $250–$500 range):
- Build cost: 400 × $333–$417 = $133,000–$167,000
- Plus 10% fees and 10% contingency: about $160,000–$200,000
In a high-cost metro area, a local cost factor of 1.2–1.4 would push this up further. In lower-cost regions, it may be 10–20% less.
Permits and zoning in the USA
Almost every addition needs a building permit, with plans and inspections. Local zoning sets setbacks from property lines, maximum lot coverage and height. Homeowners associations may have their own rules, and homes on septic systems may need an expanded system for new bedrooms. Check with your building department before paying for detailed designs.
How Long Does an Extension Take?
Allow time for design and approvals as well as the build itself:
- Design and drawings: several weeks to a few months, depending on how quickly you settle the layout.
- Planning: councils in England aim to decide householder applications within 8 weeks. Prior approval for a larger rear extension has a 42-day decision period.
- Party wall notices: neighbours have 14 days to respond, and a dispute can add weeks.
- Building: a straightforward single-storey extension commonly takes around 3–4 months on site; double-storey and complex projects take longer.
In the USA, permit review times vary widely by city, from a few days for simple projects to several months in busy jurisdictions. Ask your building department for current timescales before setting a start date with your builder.
How to Keep Extension Costs Down
- Keep the shape simple. Rectangles with a simple roof cost less than complex shapes, vaulted ceilings and large spans needing steel.
- Go up rather than out if you need lots of space; per m² or sq ft, two storeys are better value.
- Fix the design before you start. Changes during the build are expensive.
- Get three detailed, itemised quotes for the same drawings and specification.
- Agree a payment schedule tied to stages of work, and never pay large sums up front.
- Think about the alternatives. A loft or attic conversion can add a bedroom for less: see the Loft Conversion Cost Calculator.
For the windows and insulation in your extension, the Window Replacement Cost Calculator and Insulation Calculator can help. The Concrete Calculator and Steel Beam Calculator cover foundations and openings.
Frequently Asked Questions
How much does a single-storey extension cost per m² in the UK?
About £2,200–£2,700 per m² at standard specification, before fees and VAT, and 20–35% more in London.
How much does a 4m × 5m extension cost?
A 20 m² single-storey extension typically costs about £44,000–£54,000 to build outside London and £56,000–£69,000 in London, plus fees, contingency and any kitchen.
Is a double-storey extension worth it?
Usually, yes, if you need the space. It costs less per m² than single storey because the foundations and roof are shared across two floors.
How much are architect fees for an extension?
Total professional fees are commonly 8–15% of the build cost in the UK. In the USA, design fees range from about $2,000 to $20,000+.
Can I build an extension without planning permission?
Often, under permitted development: up to 3 m deep (4 m for detached houses) for a single-storey rear extension, or 6–8 m with prior approval. Building Regulations still apply.
How much does a room addition cost in the USA?
Typically $200–$600 per sq ft overall. A basic room starts around $150 per sq ft, while kitchens and bathrooms can exceed $600.
How much contingency should I add?
10–15% in the UK and 10–20% in the USA. Older homes and complex sites need the higher end.
Get quotes from local builders and architects
Compare at least three detailed quotes for the same drawings before you commit.