UK Home Extension Cost Calculator - Updated 2026
Get an instant estimate for your home extension project. Answer a few questions and see costs update in real-time.
How Much Does a Home Extension Cost in 2026?
The cost of a home extension in the UK varies significantly based on size, location, and specification. On average, expect to pay between £1,800 and £2,500 per square metre for a single-storey extension, with London and the South East typically 20-35% higher than the national average.
Key Cost Factors
- Location: Labour costs vary by up to 40% across UK regions
- Size: Larger extensions benefit from economies of scale
- Specification: Standard vs premium finishes can double costs
- Structural work: Opening up to existing rooms adds complexity
- Glazing: Bi-fold doors and roof lanterns are popular but premium priced
Average Extension Costs (2026)
Small single storey (15m²)
£30,000 - £45,000
Medium single storey (25m²)
£50,000 - £75,000
Large single storey (40m²)
£80,000 - £120,000
Double storey (40m² per floor)
£120,000 - £180,000
Loft conversion (dormer)
£40,000 - £60,000
Extension Cost by Region
Location is one of the biggest factors affecting extension costs. Labour rates, material delivery costs, and local demand all vary across the UK. London and the South East command the highest prices, while regions like the North East and Wales tend to be the most affordable.
Our calculator automatically adjusts costs based on your postcode, using up-to-date regional multipliers derived from trade labour rates and local material pricing. This ensures your estimate reflects real costs in your area rather than a generic national average.
Regional Price Multipliers (vs National Average)
London
+25% to +35%
South East
+15% to +20%
South West & East of England
+5% to +10%
Midlands & North West
Average
North East & Yorkshire
-5% to -10%
Wales & Scotland
-5% to -15%
Extension Cost per m² by Type
Not every extension costs the same per square metre. The shape of the build, how it meets the existing house and how much of the roof has to be reworked all move the rate. These are the build rates our calculator uses, before regional labour rates and your chosen specification are applied.
Single storey rear extension cost
£1,800 - £2,400 per m²
The most common extension in the UK, usually built across the back of the house to open up a kitchen or dining space. It is the cheapest single storey option per square metre because the footprint is simple and the roof is straightforward, most often a flat roof with a lantern or a shallow pitch.
Side return extension cost
£1,900 - £2,500 per m²
A side return fills in the narrow alley alongside a terraced or semi-detached house. It costs slightly more per square metre than a rear extension because the strip is narrow, access for materials is tight, and the work usually involves underpinning or rebuilding a boundary wall.
Wraparound extension cost
£2,000 - £2,600 per m²
A wraparound combines a rear extension and a side return into a single L-shaped build. It carries the highest single storey rate per square metre because the roof structure is more complex and the extension meets the existing house on two elevations, which means more structural steelwork.
Double storey rear extension cost
£1,500 - £2,000 per m², per floor
Adding two floors is cheaper per square metre than a single storey build. The foundations, roof and scaffolding are shared across twice the floor area, so the fixed costs are spread further. Budget for the full figure on both floors, and expect a party wall agreement if you are attached to a neighbour.
Double storey side extension cost
£1,600 - £2,100 per m², per floor
A two storey side extension typically adds a room downstairs and a bedroom above. It costs a little more per square metre than a rear equivalent because it usually needs a new gable or a reworked roof line to tie into the existing house.
Loft Conversion Cost by Type
A loft conversion is usually the cheapest way to add a bedroom, because the roof, walls and foundations already exist. What you pay depends on how much of the roof structure has to be rebuilt to get usable head height.
Velux loft conversion cost
£1,150 - £1,350 per m²
The cheapest loft conversion, because the roof structure is left alone. Rooflights are fitted into the existing slope, so there is no dormer to build and usually no planning application. It only works if you already have enough head height, generally 2.2m or more at the ridge.
Dormer loft conversion cost
£1,500 - £2,000 per m²
A box dormer is built out from the rear roof slope to create full standing headroom and a flat ceiling. This is the most popular conversion in the UK and usually falls under permitted development at the rear, which keeps the approval process simple.
Hip to gable loft conversion cost
£1,800 - £2,300 per m²
Used on semi-detached and end-terrace houses with a sloping hip end. The hip is rebuilt as a vertical gable wall, which squares off the loft and adds usable floor area. It is more expensive because it is a structural rebuild of part of the roof, and it is often combined with a rear dormer.
Mansard loft conversion cost
£2,200 - £2,800 per m²
The most expensive conversion and the one that adds the most space. The rear roof slope is rebuilt at a near vertical angle, creating close to a full extra storey. Mansards almost always need full planning permission, particularly in conservation areas.
Professional Fees and Approvals
These are the costs people most often leave out of an extension budget. They land before a single brick is laid, and on a typical project they add up to several thousand pounds on top of the build cost.
Architectural drawings
£2,000 - £5,000
Measured survey, design drawings and the drawing pack submitted with your planning application. Larger or more bespoke schemes sit at the top of this range.
Structural engineer
£800 - £2,000
Calculations and specifications for the steel beams, lintels and foundations. Required as soon as you remove any load bearing wall.
Building control
£800 - £1,500
The building regulations application plus the site inspections through the build, ending in your completion certificate. Needed whether or not you require planning permission.
Planning application fee
£258
The statutory householder application fee paid to your local authority. Not payable if the work falls under permitted development.
Party wall surveyor
£700 - £2,000
Applies when you build against or near a shared boundary, which covers most terraced and semi-detached extensions. The cost rises if each neighbour appoints their own surveyor.
What Affects Extension Costs?
Beyond location and size, several factors can significantly influence the final cost of your home extension. Understanding these helps you make informed decisions about where to invest and where to save.
Extension type and storey count are the primary cost drivers. A double-storey extension costs roughly 50-60% more than a single storey of the same footprint, not double, because the foundations and roof are shared. Wraparound extensions that combine rear and side elements offer excellent value per square metre due to shared structural walls.
Construction method matters too. Traditional brick and block remains the most common choice, but timber frame construction can be 5-10% cheaper and faster to erect. Structural insulated panels (SIPs) offer superior insulation but at a premium price.
Specification and finishes can swing costs dramatically. A standard kitchen extension with vinyl flooring and basic units might cost £1,800/m², while the same space with underfloor heating, bi-fold doors, a roof lantern, and a high-end kitchen could reach £3,000/m² or more.
Planning permission and building regulations add both cost and time. If your extension falls within Permitted Development rights, you'll save on planning fees and potentially months of waiting. Building regulations approval is always required and typically costs £500-£1,500 depending on the project size and your local authority.
Site access and conditions can catch homeowners off guard. Restricted access for deliveries, sloping ground, poor soil requiring deeper foundations, or the need to reroute drains and services can all add £2,000-£10,000 to the budget.
How Our Calculator Works
Our extension cost calculator combines three data sources to produce accurate, personalised estimates. First, we use current UK trade rates for labour. Bricklayers, electricians, plumbers, plasterers, and other trades, updated quarterly from industry surveys and builder feedback.
Second, we apply regional pricing multipliers based on your postcode. These account for the well-documented variation in labour costs and material delivery charges across different parts of the UK. A £50,000 extension in Manchester might cost £65,000 in London due to these regional differences.
Third, wherever possible we use live material pricing from major UK builders' merchants. This means your estimate reflects actual current prices for bricks, blocks, timber, insulation, roofing materials, and other key items rather than outdated averages. You can see how these prices have been moving on our UK material price trends page. As you answer each question, the calculator refines the estimate in real-time. You can see costs update as you make choices about specification, finishes, and features.
UK Building Material Price Trends
Building material prices are currently trending down, with an overall decrease of 33.4% over the past six months. We track 277 materials across 12 categories, with 4 trending down, 7 trending up, and 1 stable.
These price movements directly affect extension costs. Our calculator uses live pricing data to ensure your estimate reflects current market conditions.
Frequently Asked Questions
Common questions about home extension costs, planning, and how to save money on your build.
Builders typically mark up materials by 15-25% to cover their time sourcing and managing supplies, trade account administration, delivery coordination, and the risk of wastage or damage on site. This markup also compensates for their expertise in selecting the right materials and their trade relationships that can secure better availability. By self-managing your build and sourcing materials directly, you can avoid this markup entirely - which is why our calculator shows both self-managed and contractor pricing.
A single-storey extension typically costs £1,800-£2,500 per square metre in the UK. A 20m² kitchen extension might cost £40,000-£60,000, while a 40m² double-storey extension could cost £120,000-£180,000. Costs vary significantly by location, with London 25-35% higher than national averages. Our calculator adjusts for your specific region using live regional pricing data.
Single-storey rear extensions with flat roofs are typically the most cost-effective option, starting from around £1,800 per m². Using timber frame construction can reduce costs by 5-10% compared to traditional brick and block. Minimising structural alterations to the existing house (such as avoiding removing load-bearing walls) also keeps costs down. A simple 15m² rear extension can cost as little as £30,000.
Yes, self-sourcing materials can save 15-25% on material costs compared to having a builder supply them. Our calculator shows both self-managed and contractor pricing so you can see the difference. By comparing prices across multiple suppliers using PriceNailer, you can find the best deals on building materials and potentially save thousands on your extension project.
Many single-storey rear extensions qualify as Permitted Development and do not need planning permission, provided they meet size limits (3m for attached houses, 4m for detached, or 6m/8m under prior approval). Double-storey extensions almost always require planning permission. You’ll also need Building Regulations approval regardless of whether planning permission is needed. Check with your local planning authority for specific requirements.
A typical single-storey extension takes 8-12 weeks to build once work begins. Larger or double-storey extensions may take 12-16 weeks. Add 8-12 weeks beforehand for planning permission (if needed) and building regulations approval. Our calculator provides a project timeline breakdown showing each construction phase and estimated duration.
Kitchen extensions typically add the most value, often recouping 50-70% of their cost in added property value. Additional bedrooms and bathrooms are also strong value-adders. Open-plan living spaces with bi-fold doors, quality finishes, and good natural light (via skylights or roof lanterns) enhance both value and liveability. The key is ensuring the extension is proportionate to the property and neighbourhood.
Our calculator uses current UK trade rates, regional labour multipliers, and live material pricing data from major suppliers. Estimates are typically within 10-20% of actual costs for standard builds. Accuracy increases as you answer more questions (shown by the confidence percentage). For the most accurate estimate, we recommend getting 3 quotes from local builders and comparing them against our breakdown.
Related Calculators
Planning the finer details of your extension? Use our specialist material calculators to get accurate quantities and costs.