How much does a house extension cost?
The average house extension cost in the UK can range from £25,000 to £130,000+. The average cost of an extension depends on size, type of extension, and location. How much an extension will cost depends on a combination of these factors. The overall cost can shift significantly between projects of similar size. A small single-storey extension in London and the South East will cost considerably more. The same build elsewhere in the UK is much cheaper. The size and complexity of the design affects the total cost too.
This cost guide walks through extension cost examples by type. It covers the additional costs that affect your overall cost.
How much does a house extension cost in 2025?
Labour and material costs have risen over recent years. In 2025, budget £1,800–£3,000 per square metre. That applies in London and elsewhere in the UK. That covers a standard extension build. That covers construction costs only. It excludes professional fees, VAT, and fittings. Extension costs can vary significantly within this range. Type of extension, location, and specification all play a part. Treat it as a guide rather than a fixed figure.
Cost of a house extension by type
The type of extension is one of the biggest variables. It directly affects what a UK house extension will cost. Here are extension cost examples by build type. Size and complexity determine where your build sits within these ranges.
Cost of extension shell only
A shell-only extension build typically costs £1,200–£1,800 per square metre. This covers foundations, walls, roof, windows, and a weathertight finish. Internal finishes, plastering, electrics, plumbing, and any fit-out are additional costs on top.
Cost of full house extension per square metre
A fully fitted extension runs £1,800–£3,000 per square metre for mid-range specification. High-end finishes or significant structural complexity can push this above £3,000 per square metre. A larger extension build is usually cheaper per square metre. Fixed construction costs spread across more floor area.
House extension prices per project
Single storey house extension cost
A single-storey extension is the most common extension type in the UK. Expect to pay:
- 15–20 m²: £27,000 – £45,000
- 20–30 m²: £45,000 – £70,000
- 30–40 m²: £70,000 – £95,000
These are full build costs including a standard finish. A single-storey extension in London may cost 20–30% more than elsewhere in the UK. Higher labour rates are the main reason.
Cost of two storey house extension
A two storey extension costs more overall but offers better value per square metre. You get roughly 1.5x the space. The total cost is around 1.3–1.4x that of a single storey build. Typical costs:
- Small (20–25 m² per floor): £65,000 – £90,000
- Medium (25–35 m² per floor): £90,000 – £130,000
- Large or complex: £130,000+
Two storey double storey extensions always require full planning permission. Detailed house extension drawings are needed before any extension work begins.
First-floor extension cost
A first-floor-only extension adds space above an existing structure such as a garage. It typically costs £1,600–£2,600 per square metre, depending on the structural work required beneath. Expect to pay £30,000–£65,000 for most first-floor extension projects.
Semi-detached house extension costs
A rear single storey extension on a semi typically costs £30,000–£60,000. A side extension adding a bedroom and bathroom above can reach £70,000–£100,000. Extending toward the shared boundary means you’ll need a party wall agreement. Budget an additional £800–£2,000 per surveyor.
What does a mid terrace house extension to the rear cost?
A single storey rear extension on a mid terrace usually costs £25,000–£55,000. Under permitted development, terraced houses can extend up to 6 metres to the rear. No planning permission is needed, which saves both time and cost. Use our permitted development checker to confirm if your project qualifies.
Basement extension costs
Basement extensions are the most expensive extension build type. Construction costs start at £3,000 per square metre. Deep excavation and waterproofing can push this to £5,000 per square metre. A typical basement extension project runs £100,000–£250,000.
Loft conversion costs
A loft conversion is often considered alongside extension ideas for gaining living space. A basic Velux conversion costs from £25,000. See our full loft conversion cost guide for a detailed breakdown. A full mansard runs £65,000+. Most loft conversions don’t require planning permission under permitted development.
Cost of garage extension
Converting or extending a garage into living space costs £15,000–£40,000. The price depends on whether the structure needs rebuilding and the finish required.
Kitchen extension costs
A kitchen extension combines construction costs with fit-out costs. A new kitchen adds £8,000–£30,000 depending on specification. All-in costs for a typical kitchen extension project run £35,000–£75,000.
Factors affecting the cost of a house extension
Size of extension
Larger extensions cost more in total. But the cost per square metre often falls as the new extension gets bigger. Fixed construction costs spread across more floor area. This makes larger extension builds more cost effective per square metre.
Location of home
Location is one of the biggest factors in how much does an extension cost. London and the South East carry a significant labour premium. Expect to pay 20–30% more than the national average. How much a UK house extension will cost in rural areas can be lower. But access difficulties often offset those savings.
Ease of access
Difficult sites increase construction costs. Narrow access or nearby structures all affect your extension cost.
Quality
Specification is one of the biggest factors that affect home extension costs. Feature glazing or specialist finishes push the cost per square metre up considerably.
Finishings and fittings
Flooring, plastering, and decoration are rarely included in headline extension build costs. Budget separately for these based on the rooms you’re creating.
Windows and doors
Standard double-glazed windows and doors are included in most extension build quotes. Bifold doors or roof lanterns can add £5,000–£15,000 to your extension cost.
Architect fees
Extension plans for a single storey project typically cost £1,500–£4,000. Read our full architect fees for extension guide for a detailed breakdown. Building regulation drawings cost £800–£2,500. AC Design Solution offers fixed-fee packages for both. Fees are typically 50% less than traditional architect rates.
Planning permission fees
A householder planning application in England currently costs £258. Many single-storey extension projects qualify under permitted development and avoid this additional cost entirely.
Building regulations
Building regulations approval is required for all extension work. This applies even without planning permission. See our guide to single storey extension building regulations for more detail. Fees are typically £200–£900 depending on the size of the extension build.
Structural engineer costs
Removing load-bearing walls in your existing house requires a structural engineer. The same applies for new openings. Structural calculations for a standard extension project cost £500–£1,500.
Party wall agreement
Extensions within 3 metres of a neighbour’s foundation trigger the Party Wall Act. Act 1996 applies. Party wall surveyors in the UK typically charge £800–£2,000 each. This is one of the more variable additional costs in an extension project.
How to reduce house extension costs
Get multiple quotes
Always get three quotes from builders using the same extension plans. Proper house extension drawings give every builder the same information to price from. It’s the most cost effective step before extension work starts.
Choose cheaper options
Roof type, glazing, and internal fittings can vary extension costs significantly. None of these affect the structural quality of the build. A simpler design can cut the cost of your extension considerably. It won’t compromise the living space it creates.
House extension costs as a percentage
Professional fees typically represent 10–15% of the overall build cost. This covers design, planning, building regulations, and structural engineering. Hold a contingency of a further 10–15% for any house extension project. On older properties, hidden issues may only appear once extension work begins. Ground conditions are a common surprise.
Frequently asked questions
How much do house side extensions cost?
A side extension typically costs £30,000–£70,000. Price depends on size and whether it’s single or double storey. Side return extensions on Victorian terraces tend to cost £25,000–£45,000.
How much do semi-detached extensions cost?
A rear extension on a semi-detached house costs £30,000–£60,000 for a mid-range finish. A two storey addition will cost £70,000–£100,000+. House extension costs in London for the same project would typically sit 20–30% higher.
Are two-storey extensions cheaper?
Not in total. But they’re cheaper per square metre than building single storey twice. The additional cost of going to two storeys is relatively small. The extra space makes them cost effective for larger projects.
Does a house extension add value to my house?
A well-designed extension typically adds 10–20% to a property’s value. Location and quality of extension work both play a part. In London, the return on a house extension project can be higher still.
How long will my extension take?
A single storey extension takes 10–16 weeks to build once work starts on site. Factor in 8–16 weeks before that for extension plans, planning, and building regulations. A two storey extension typically takes 16–24 weeks on site.
Why add on a home extension?
Common extension ideas include a kitchen-diner, an extra bedroom, or a home office. Building a house extension is often more cost effective than moving. Stamp duty and moving costs add up fast.
Is 30k enough for an extension?
At £30,000, you’re at the lower end of what’s achievable. A small 12–15 m² build may be feasible with a basic finish. This only applies in lower-cost areas of the UK. A single-storey extension in London or the South East will cost more. Once fees, VAT, and fit-out are added, £30,000 won’t go far.
How much is a 20 square metre extension?
A 20 m² extension at mid-range specification costs approximately £36,000–£55,000. Add professional fees, VAT, and fit-out costs on top as additional costs.
How much does a 3m by 4m extension cost?
A 3m × 4m extension is 12 m². The cost to build this extension is typically £22,000–£36,000. House extension costs in London for the same project rise to £28,000–£45,000. This covers a compact kitchen extension or small utility room on an existing house.
Can I build an extension for £20,000?
It’s possible for a very small, basic-finish build elsewhere in the UK. But VAT, fees, and fit-out eat through £20,000 fast in 2025. It’s more realistic for a small porch than a full extension project.
What is the most expensive part of a house extension?
Foundations, walls, and roof account for the largest share of construction costs. Glazing and kitchen or bathroom fit-outs are typically the next most significant additional costs. Hidden issues and poor ground conditions can become the most expensive element. This is common in older properties. These are often only discovered once extension work begins.
AC Design Solution are MCIAT Chartered Architectural Technologists based in London. We provide architectural services for house extensions across London and the Home Counties. Get a quick quote or get in touch for a fixed-fee quote on extension drawings or building regulation drawings.
