A house extension in the UK typically costs between £1,500 and £3,000 or more per square metre, depending on materials, location and complexity. The average cost of a house extension sits around £1,500 to £2,500 per square metre for standard masonry builds, whilst premium options push higher.
Key Points
- House extension costs in the UK typically range from £1,500 to £3,000+ per m², with London and the South East commanding a 15–25% premium over other regions.
- Double-storey extensions offer better value per m² than single-storey builds, delivering roughly 200% more space for only 50–75% additional budget.
- Headline build costs represent only part of the total spend—factor in professional fees (15–20%), VAT at 20%, and a contingency fund for unexpected expenses.
- Extension type, finish quality and site-specific factors such as ground conditions and access constraints have the greatest impact on your final cost per m².
House Extension Cost Per m²: Quick Answer
Your final price depends heavily on what you’re building. A basic brick rear extension costs less per square metre than a bespoke oak-frame sunroom, which can reach £7,000 or more per square metre due to specialist joinery. Steel-integrated designs and timber-frame structures fall somewhere between, with timber offering faster construction but requiring greater upfront design investment.
London and the South East typically add a 15–25% premium to these figures, so always gather local quotes before setting your budget.
Extension Costs by Type
Extension type determines your baseline budget. Bolt-on timber-frame builds—often weather-tight within three to four days—typically cost £2,300–£2,500 per m². Complex integrations, such as adding an upper storey to a bungalow, require structural surveys and carry higher costs. A single-storey rear extension can be cost-effective when carrying out a house extension in the St Albans area. Finish levels also shift the final price considerably.
- Extension type determines baseline cost
- Detailed breakdowns follow for each category
- Distinction between ‘Bolt-on’ extensions vs. ‘Complex Integrations’ (e.g., roof removals)
- Impact of finish levels (Basic, Mid-Range, Premium) on final assembly costs
- Prefabricated kits vs. traditional site-built timber frames
Single-Storey Extension Costs
A single-storey extension is one of the most popular ways to add usable living space without the complexity of building upwards. Costs typically fall between £1,800 and £3,000 per square metre in 2026, though the final price depends heavily on your project’s size, finish level and structural requirements.
Understanding the cost of a house extension early on helps you budget realistically before committing to plans.
Standard-quality builds generally sit around £2,200 to £2,500 per square metre, while high-end finishes with premium materials push costs towards £3,000 or beyond. Smaller footprints—particularly those under 10 square metres—tend to cost more per square metre because fixed expenses like groundworks, building regulations fees and professional charges do not scale down proportionally. A compact 3×3 metre extension will therefore prove considerably pricier per square metre than an 18 square metre project.
Side-return extensions, common on Victorian terraced properties, often start lower at roughly £1,500 per square metre since they utilise existing ‘dead’ alleyway space. Rear lean-to projects of around 18 square metres typically range from £25,000 to £40,000 total.
Extensions exceeding six metres in depth trigger additional structural requirements and mandatory neighbour consultations under Party Wall legislation, adding both time and cost.
Double-Storey Extension Costs
Double-storey extension costs offer better value per square metre than single-storey builds. The main reason is simple: expensive elements like excavation, foundations and roofing serve two floors instead of one. You can gain roughly 200% more space for only 50–75% extra budget compared with a ground-floor-only project.
Shell costs for a double-storey extension start at around £1,200/m², whereas single-storey work averages closer to £1,800/m². London and the South East carry a premium due to higher labour rates and logistics.
If your existing single-storey extension has foundations designed to bear additional load, building upward can keep costs nearer the £1,200/m² mark. Internal fit-out also matters: bedrooms cost less per square metre than kitchens.
Budget separately for structural alterations to the original house, plus architect and engineer fees. A well-executed design can raise both property value and day-to-day practicality.
Side Return Extension Costs
A side return extension is a way of filling in the narrow alley that runs alongside many Victorian and Edwardian terraced houses. This slim strip—often around two to three metres wide and six metres deep—sits between the main house and the boundary wall, and reclaiming it can turn an awkward L-shaped ground floor into a single open-plan space.
Costs for side return projects in London typically range from £150,000 to £300,000 including VAT and professional fees, with mid-range specifications averaging around £2,600 per square metre or higher. The per-square-metre rate tends to be steeper than for larger rear extensions because builders face tighter access and must often strip back existing outrigger brickwork.
A common budgeting surprise is the refurbishment ratio: roughly 80 per cent of spending may go on upgrading the existing ground floor rather than the new build itself. To create a realistic budget for your family home expansion, including garden room uses, start with a preliminary estimate of the build cost, add a contingency fund for unforeseen expenses and subtract any potential grants or subsidies.
Basement Extension Costs
Basement extension costs are the highest of any domestic build, typically ranging from £6,000 to £8,000 per square metre in London and £3,000 to £5,000 per square metre elsewhere in the UK. These figures reflect the complex engineering required to excavate beneath an existing property.
The expense stems from fixed structural requirements that cannot be reduced. Underpinning strengthens foundations to support the dig, while tanking creates a watertight membrane around the new space. Both are non-negotiable for a sound build, which means there is no realistic budget route to a basement.
Unlike loft or rear extensions, basements lack economies of scale. Doubling the floor area does not halve the cost per square metre because groundworks, drainage and waterproofing remain labour-intensive throughout.
In high-density areas such as South West London, tight planning restrictions limit outward growth, so adding space downward often makes financial sense despite the price. Homeowners should weigh the cost-to-value ratio carefully, factoring in local property prices and long-term plans.
Conservatory Extension Costs
A conservatory extension is a glazed addition to your home that typically costs between £10,000 and £50,000 or more, depending on size, materials and design complexity.
uPVC frames sit at the affordable end, whilst timber and aluminium options command premium prices but often add more to your property’s resale value. Shape matters too: a simple lean-to design costs less per square metre than an ornate Victorian or P-shaped layout.
Structural Insulated Panel (SIP) builds can reduce onsite labour time from months to weeks, trimming overall costs despite higher material prices. Traditional brick-and-mortar extensions remain labour-intensive, pushing hourly trade costs higher.
Watch for hidden fees. Quotes often exclude site clearance, drainage modifications and internal finishing. Utility connections can add hundreds of pounds.
Larger projects benefit from economies of scale, as the cost per square metre tends to fall when you build bigger. Better thermal performance also pays back through lower energy bills over time.
What Drives House Extension Costs Per m²?
House extension costs per m² depend on several interlinked factors rather than a single flat rate. In 2026, UK prices typically range from £1,800 to £3,000 per m², but your actual figure will shift based on location, build complexity and room type.
Location matters. London and South East projects often carry premiums of 30–50% above the national average, driven by higher labour rates and material transport costs.
Build type affects efficiency. Adding a second storey costs roughly 50–60% more than a single-storey extension—not double—because both share the same foundations and groundworks. Spreading these fixed costs over more floor space lowers your effective rate per m².
Room function changes the maths. Kitchens and bathrooms (‘wet’ rooms) need plumbing, drainage and specialist finishes, pushing their m² cost well above bedrooms or living spaces.
Hidden costs add up. Soil surveys, drainage rerouting, structural steelwork for knocked-through walls and professional fees often inflate the headline rate by 10–15%. Always budget for these before comparing quotes.
Location and Regional Differences
Location is one of the largest factors affecting timber-frame extension costs across the UK. Projects in London and the South East typically carry a premium of 15–25% compared with the national average, driven by higher trade rates and site logistics.
Urban builds face extra expenses that suburban plots often avoid. Restricted access in dense areas can increase labour time by 10–15% because materials must be moved by hand rather than machinery. Councils may require parking permits, skip licences and storage fees, all of which add to the final bill.
Trade pricing also varies by region. Contractors in cities often charge fixed daily rates rather than hourly, which inflates costs when deliveries are delayed. In contrast, rural or remote locations may attract mobilisation fees if specialist timber-frame teams need to travel long distances.
Kitchen-led extensions highlight these differences sharply, with budgets ranging from £25,000 to £100,000 depending on fittings and local labour markets.
Structural Complexity and Ground Conditions
Structural complexity and ground conditions are the two factors most likely to push a kitchen extension over budget. A straightforward single-storey rear addition on stable ground uses economical strip foundations, typically costing £150–£200 per linear metre. Poor soil, nearby tree roots, or a high water table may demand piled foundations instead, adding £15,000–£40,000 to the total.
Above ground, the design itself affects cost. Removing a load-bearing wall for an open-plan layout requires structural steel beams, usually adding £5,000–£15,000. Wraparound (L-shaped) extensions cost more per square metre because they involve complex dual-wall connections and additional steelwork at the corner junction.
Access matters too. Mid-terrace homes without rear entry force labourers to carry materials through the house, raising labour bills noticeably.
Structural engineer and architect fees typically run 12–18 per cent of the build cost. Party Wall Agreements with neighbours can add £1,500–£3,000 each. Building in a contingency of at least 10 per cent helps absorb these surprises.
Specification and Finish Quality
Specification and finish quality is the single biggest variable in your final cost per square metre. A standard rear extension using off-the-shelf materials typically costs between £1,000 and £2,500 per m², while premium or bespoke finishes can push that figure considerably higher.
Standard specifications include painted plasterboard, vinyl flooring and basic double glazing. Upgrading to engineered timber flooring, structural glazing or handmade tiles adds a substantial premium. Large glass panels require larger steel beams and specialist installation, increasing both material and labour costs.
‘Invisible’ upgrades also add up. Smart home wiring, underfloor heating controls and audio-visual systems can quietly absorb thousands of pounds. Higher-grade insulation improves long-term thermal performance but raises initial outlay.
Location matters too. Projects in London and the South East typically cost 30–50% more per m² than elsewhere in the UK. Listed buildings or conservation areas often require heritage-sensitive materials and additional approvals, inflating budgets further. Always allow a 10–15% contingency for unforeseen issues.
Typical Extension Cost Breakdown
A typical extension cost breakdown divides spending into four main categories: shell and structure, internal fit-out, professional fees and utility integration.
For a 30m² kitchen extension at around £82,500, expect roughly 40–45% to cover the shell—foundations, walls, roof and glazing. Internal fit-out, including plastering, flooring and decoration, accounts for about 25–30%. Professional fees for architects, structural engineers and building control typically run to 10–15% of the total.
The remaining portion covers the kitchen utility premium. Adding plumbing, gas and electrical services pushes costs £15,000–£35,000 higher than a simple dry room, representing a 20–40% increase overall.
Material choices shift the budget significantly. Standard uPVC patio doors might cost £1,500–£3,000, whereas premium aluminium bifolds often reach £8,000–£15,000 for a comparable opening.
Prefabricated timber-frame kits offer an alternative route. Factory-built panels arrive ready for assembly, cutting onsite labour time and providing better thermal performance than traditional masonry. This approach suits projects where predictable costs matter.
Finish-level tiers also affect the final figure. A basic specification keeps costs lower, mid-range adds quality fixtures and premium specifications include high-end appliances and bespoke joinery.
Hidden and Additional Costs
Hidden and additional costs are the expenses that sit outside your main construction contract, often adding 20–40% to the headline price. Knowing about them early helps you set a realistic budget.
Statutory fees cover planning applications, Building Control sign-off and structural-engineer calculations. Surveys—party-wall agreements, drainage checks and asbestos testing—add further charges. In London and the South East, expect a 30–50% uplift on labour and materials compared with other UK regions.
Glazing is another area where prices rise sharply. Large bifold or sliding doors cost more per square metre than smaller units because they must meet stricter thermal-performance standards.
Timber-frame or prefab extensions sometimes need crane hire and clear lorry access. If your street is narrow or your garden is hard to reach, logistics costs climb quickly.
Softer expenses also drain contingency funds: temporary kitchens, storage for furniture, landscaping to repair a churned-up lawn and making good any fencing or paving disturbed during the build.
Professional Fees and Compliance Costs
Professional fees and compliance costs are the charges you pay to qualified experts and local authorities before and during construction. These ‘soft costs’ typically add 15–20% to your base build price but protect you from legal problems and structural failures.
Architect fees cover design work and planning drawings, while a structural engineer confirms your extension is safe. Building control inspections, charged by your local council or an approved inspector, verify the work meets regulations. If your extension sits near a neighbour’s boundary, you may need a Party Wall surveyor—costs can climb to £1,000–£3,000 per surveyor per neighbour if disputes arise.
Properties in conservation areas or listed buildings face a ‘heritage premium’ of 15–40%, as councils often require period-accurate materials and detailed heritage impact assessments. Measured surveys cost extra before design begins.
VAT applies at 20% on most professional services. Always set aside a 10–15% contingency fund for unexpected ground or structural issues.
VAT on House Extensions
VAT on house extensions is charged at 20% on both labour and materials in the UK. This standard rate applies to most residential building work, adding a significant sum to your final bill—often £400 to £750 per square metre on high-end London projects where base costs already reach £3,050 to £3,750 per square metre.
However, certain works qualify for reduced rates. Energy-saving measures such as insulation, solar panels and air source heat pumps can attract 0% VAT under current HMRC incentives. If your property has stood empty for at least two years, the entire project may qualify for 5% VAT. Combining two flats into one dwelling can also trigger this lower rate.
Two-storey extensions spread VAT across greater floor area, improving cost efficiency per square metre.
Build Routes and Their Impact on Cost Per m²
A build route is the way you organise and pay for the people who construct your project. The route you choose affects your final cost per m² as much as the materials themselves.
Main contractor packages are the simplest option. A single firm handles everything, but you pay a management premium of around 15–20% for that convenience. Project management, where you hire a professional to coordinate separate trades, sits in the middle—lower overheads but still structured oversight. Self-managed builds can cut costs further, though they demand time, knowledge and confidence in handling disputes.
Taking over at the watertight stage—when the shell, roof and windows are complete—lets you manage finishing trades yourself. This handover point can reduce overall spend by roughly 10%.
Smaller projects suffer from a scale paradox. Fixed fees for structural engineers, surveys and Building Control approvals do not shrink with floor area, so a 3×3 m extension carries disproportionately high costs per m². In London and the South East, labour rates push management-route premiums 30–50% above national averages, tightening budgets further regardless of which route you select.
Construction Methods and Their Costs
Construction methods determine both the price per square metre and the overall build timeline of an extension. Traditional masonry remains the most common approach in the UK, with costs typically ranging from £1,800 to £2,500 per square metre. Brick and block construction uses widely available materials and familiar techniques, though it relies heavily on on-site labour and is vulnerable to weather delays.
Timber frame and structural insulated panels (SIPs) offer faster build times and better thermal performance. Factory-controlled manufacturing means tighter tolerances for airtightness—a key requirement under the 2025/26 Future Homes Standard. However, design fees tend to be higher upfront because precise engineering drawings are needed before production begins.
Modern methods of construction (MMC), including pre-manufactured kits, can reduce on-site labour costs by shifting work to controlled factory environments. Be cautious with ‘shell-only’ quotes; these often exclude plumbing, electrics and internal finishing.
Beyond materials, budget for structural surveys, planning fees (around £548 in England), Building Regulations applications and any Party Wall notices required for boundary work.
Getting Quotes and Finding a Builder
Getting quotes and finding a builder is the step where your budget meets reality. Labour typically accounts for 40–45% of your total extension cost, so choosing the right contractor matters as much as the design itself.
Start by requesting at least three quotes to compare. Understand the difference between an estimate, which is a rough guide that can change, and a fixed-price quote, which locks in the cost for agreed work. Always ask whether the price includes a ‘material flux clause’ allowing increases if supply costs rise.
Before committing, verify each builder’s credentials. Check they hold valid public liability insurance and, where relevant, membership of trade bodies such as the Federation of Master Builders. Ask for references and visit completed projects where possible.
Regional labour rates vary considerably. Builders in London and the South East often charge 15–25% more than those in other parts of the UK, so factor this into your comparisons. When reviewing quotes, ensure each one reflects the same finish standard—a basic specification at around £1,500 per square metre looks cheaper than a high-end quote at £2,500 or more, but they are not like-for-like.
Protect your finances by agreeing a staged payment schedule tied to milestones. Formalise everything in a written contract before any work begins.
Will an Extension Add Value to Your Home?
A home extension is an investment, and whether it adds value depends on several factors. In most cases, a well-designed extension will return between 50 and 75 per cent of what you spend, but the type of space you create matters enormously.
Kitchen extensions and open-plan kitchen-diners tend to perform best, potentially adding 15 to 20 per cent to your property’s value. Loft conversions follow closely, often yielding around 15 per cent. An extra bedroom with an ensuite typically adds 10 to 12 per cent, while a standard single-storey extension might contribute five to 10 per cent.
However, every street has a ceiling price—the maximum buyers will pay regardless of improvements. Once your home reaches this limit, further spending rarely translates into profit. Sacrificing too much garden space can also reduce appeal, particularly in family-oriented areas.
Consider the broader picture before deciding. If you plan to stay long-term, an extension helps you avoid stamp duty, legal fees and removal costs. These savings can justify the project even if the return on investment is modest.
Before committing, speak with local estate agents about realistic valuations and check whether your extension might push you into a higher council tax band.
Planning Permission and Permitted Development
Planning permission and permitted development are two separate routes for approving a house extension in the United Kingdom. Permitted development (PD) rights allow certain works without a full planning application, but strict rules apply.
Under PD, single-storey rear extensions can reach up to eight metres for detached homes or six metres for other types, provided you follow the Prior Approval process. The ‘50% land rule’ matters here: any sheds, garages or outbuildings already on your plot count towards the total area you can cover, reducing what remains for a new extension.
Timber and oak-frame builds must also pass the ‘similar appearance’ test. Materials should match the principal look of the existing dwelling, which can require creative interpretation when exposed oak is involved.
A Lawful Development Certificate (LDC) costs roughly half a standard planning fee—around £206 in 2026—but proves your project was lawful on the date issued. Full planning permission, by contrast, grants a three-year window to begin work. For larger schemes, the Community Infrastructure Levy may also apply, adding to overall costs.
Frequently Asked Questions
How much does a house extension cost per m2 in the UK in 2024?
House extension costs in the UK typically range from £1,200 to £2,500 per m² depending on the build type, with standard masonry construction sitting at the lower end and premium timber-frame or steel-integrated builds commanding higher rates. London and the South East generally see prices 20–30% above the national average.
Is it cheaper per m2 to build a single or double-storey extension?
Double-storey extensions offer better value per m² because they share foundation and roof costs across a larger floor area, with shell costs dropping to around £1,200 per m² compared to approximately £1,800 per m² for single-storey builds.
How long does a typical house extension take to build?
Most single-storey extensions take between eight and twelve weeks to complete, whilst double-storey projects typically require twelve to sixteen weeks. Complex builds involving structural integration or basement work can extend timelines considerably.
Do I need an architect for a house extension?
Whilst not legally required for all projects, engaging an architect or architectural technologist is advisable for extensions over 20 m² or those involving structural alterations. Professional design input helps maximise space, ensures Building Regulations compliance and often improves the final value added to your property.
What is the most cost-effective type of house extension?
Rear single-storey extensions and side returns on Victorian properties typically offer the best balance of cost and additional living space, particularly when kept within permitted development limits to avoid planning application fees. Double-storey builds deliver even greater value per m² for those with the budget and suitable property.
