Introduction
Planning a house extension is one of the most significant investments you can make in your home. Whether you want more living space for a growing family, a larger kitchen for entertaining or a home office that actually works, an extension can transform the way you use your property. But before you start choosing tiles and paint colours, you need to understand the costs involved.
Extension costs in the UK vary enormously depending on the type of extension, the specification of finishes, your location and the complexity of the build. In this comprehensive guide, we break down the average costs for every type of house extension in 2026, explain the key factors that influence pricing and help you understand what to budget for before you speak to a builder.
For a year-specific breakdown with the latest pricing data, see our companion article on house extension costs in 2026, which is updated regularly with current market rates.
Average Extension Costs by Type
The table below provides a summary of typical costs for the most common types of house extension in 2026. These figures are based on a mid-range specification and assume reasonably straightforward site conditions. Costs include foundations, structure, roofing, electrics, plumbing, plastering and decoration but exclude kitchen and bathroom fit-outs unless stated.
| Extension Type | Typical Size | Cost Range (2026) |
|---|---|---|
| Single-storey rear | 15–25m² | £30,000 – £65,000 |
| Side return / infill | 8–15m² | £25,000 – £50,000 |
| Wrap-around | 20–35m² | £45,000 – £90,000 |
| Two-storey rear | 30–50m² | £50,000 – £110,000 |
| Over-garage / side | 15–25m² | £35,000 – £70,000 |
| Orangery / garden room | 12–20m² | £30,000 – £60,000 |
| Basement extension | 30–60m² | £150,000 – £350,000+ |
Important: These are guide prices only. Every project is different and actual costs will depend on your specific drawings, specification and site conditions. A professional budget estimate is the most reliable way to understand what your extension will cost.
Cost Per Square Metre Breakdown
Understanding cost per square metre is one of the most useful ways to benchmark extension pricing. It allows you to compare quotes on a like-for-like basis and gives you a rough way to scale costs up or down as your plans evolve.
What Does Cost Per Square Metre Include?
When estimators and quantity surveyors quote a cost per square metre rate, it typically includes the structural shell of the extension: foundations, external walls, roof structure and coverings, windows and external doors, internal walls and partitions, first and second fix electrics and plumbing, plastering, decoration and floor finishes. It does not usually include kitchen or bathroom installations, which are priced separately based on the specific products and fittings you choose.
Typical Rates in 2026
For a standard specification single-storey extension, you can expect to pay in the region of £1,500 to £2,200 per m² in most parts of England and Wales. A mid-range specification with features such as bi-fold doors, underfloor heating and quality finishes typically falls between £2,200 and £2,800 per m². High-end builds with bespoke joinery, structural glazing, premium materials and complex design features can exceed £3,000 per m².
Two-storey extensions benefit from economies of scale, sharing foundations and roof costs across two floors. The effective cost per square metre for a two-storey build is typically 25 to 35 per cent lower than a single-storey extension of the same total floor area.
What Affects Extension Costs
No two extension projects are identical, and a wide range of factors can push your costs up or down from the averages quoted above. Understanding these variables will help you make informed decisions at the design stage and avoid budget overruns during construction.
Size and Complexity
Larger extensions cost more in total but often less per square metre, because many fixed costs such as scaffolding, skip hire, building control fees and preliminaries are spread across a bigger floor area. Complex shapes, split-level floors, vaulted ceilings and structural glazing all increase costs compared to a straightforward rectangular build.
Specification and Finishes
The level of finish you choose has perhaps the single biggest impact on overall cost. A basic plastered shell with standard windows, vinyl flooring and painted walls will cost a fraction of a build featuring engineered timber flooring, bespoke cabinetry, large-format porcelain tiles and designer sanitaryware. It is common for the internal fit-out to account for 30 to 40 per cent of the total extension cost.
Ground Conditions
Poor ground conditions, high water tables, proximity to mature trees or the presence of filled ground can all require deeper or more complex foundation solutions. Standard strip foundations might cost £150 to £250 per linear metre, while piled foundations can cost £5,000 to £15,000 depending on the number of piles required. A desktop site investigation before you start can help identify potential issues early.
Access and Site Constraints
Restricted access for deliveries, narrow side passages, sloping sites and the need to work over existing structures all increase labour costs. If materials have to be barrowed in by hand rather than lifted by crane or telehandler, you can expect to pay a premium of 10 to 20 per cent on construction costs.
Existing Services
If your extension is built over existing drainage runs, gas mains, water pipes or electrical cables, these services may need to be diverted. Service diversions are a common source of unexpected cost, typically adding £2,000 to £8,000 depending on the complexity of the diversion.
Planning and Building Regulation Fees
Beyond the physical construction costs, there are a number of professional and statutory fees that need to be included in your overall budget. These are often overlooked in early cost estimates but can add up to a significant sum.
Planning Permission
Many single-storey rear extensions fall within permitted development rights and do not need planning permission. However, if your project exceeds the permitted development limits, you will need to submit a householder planning application. The fee in England is currently £258, though your architect or planning consultant will charge a separate fee for preparing and submitting the application, typically £500 to £1,500.
Building Regulations
All extensions must comply with the Building Regulations regardless of whether planning permission is required. Building control fees typically range from £500 to £1,200 depending on the size of the extension and whether you use local authority building control or a private approved inspector. For more detail on what building regulation compliance involves, see our building regulation estimates page.
Structural Engineering
Most extensions require input from a structural engineer, particularly where new openings are formed in existing walls or where the extension connects to the original building. Structural engineering fees for a domestic extension typically range from £500 to £1,500. More complex projects involving steel frames, underpinning or basement works will incur higher fees.
Party Wall Costs
If your extension involves work within three or six metres of a neighbouring property, you may need to serve notice under the Party Wall etc. Act 1996. If your neighbour dissents, both parties will need to appoint a surveyor. Party wall costs typically range from £700 to £1,500 per neighbour, though they can be higher for complex situations.
Regional Cost Variations
Where you live has a significant bearing on what your extension will cost. Labour rates, material transport costs and local market conditions all contribute to regional price differences. The table below shows approximate cost multipliers relative to a national average baseline.
| Region | Cost Multiplier | 20m² Single-Storey (Mid Spec) |
|---|---|---|
| London (Inner) | 1.30 – 1.45 | £55,000 – £70,000 |
| London (Outer) & South East | 1.15 – 1.30 | £48,000 – £60,000 |
| South West | 1.05 – 1.15 | £42,000 – £52,000 |
| Midlands | 0.95 – 1.05 | £38,000 – £48,000 |
| North of England | 0.85 – 0.95 | £32,000 – £42,000 |
| Wales | 0.85 – 0.95 | £32,000 – £42,000 |
| Scotland | 0.90 – 1.00 | £35,000 – £45,000 |
| Northern Ireland | 0.80 – 0.90 | £30,000 – £40,000 |
These figures are indicative and should be used for budgeting purposes only. For location-specific pricing, we offer estimating services across the UK, including dedicated pages for London, Manchester, Birmingham and many other cities.
Tip: Regional cost data is useful for early budgeting, but the only way to get an accurate figure for your specific project is to have a detailed estimate prepared from your drawings. Contact us for a fixed-price quotation.
How to Budget Accurately
Accurate budgeting is essential if you want to avoid the stress and financial pressure of cost overruns during construction. Here are the steps we recommend to every homeowner planning an extension:
1. Start With a Realistic Budget Range
Use the cost tables and per-square-metre rates in this guide to establish a rough budget range before you commission detailed drawings. This helps you and your architect design something that is achievable within your means, rather than having to strip back an over-ambitious design later.
2. Get a Professional Estimate Early
Once you have planning or sketch drawings, commission a professional building estimate from a qualified Quantity Surveyor. This gives you an independent, itemised cost breakdown that you can use to compare builder quotations and identify any gaps or underpricing. At First4Estimating, we can prepare a budget estimate from planning drawings in just two to three working days.
3. Include All Costs, Not Just the Build
Your budget should include professional fees (architect, structural engineer, party wall surveyor), statutory fees (planning, building control), furniture and fittings, landscaping and a contingency allowance. It is common for these additional items to account for 15 to 25 per cent of the total project cost on top of the construction figure.
4. Allow a Contingency
We always recommend setting aside a contingency of 10 to 15 per cent of the total build cost to cover unexpected issues during construction. This is standard practice in the industry and provides a financial cushion for unforeseen ground conditions, structural surprises or specification changes.
5. Compare at Least Three Quotations
Once you have your estimate, use it to benchmark builder quotations. Aim to obtain at least three competitive quotes on a like-for-like basis. If any quote is significantly cheaper than your estimate, investigate why. Missing items, underpriced labour or unrealistic timescales are common reasons for low quotes, and they almost always lead to disputes and additional costs later in the project.
Getting a Professional Estimate
At First4Estimating, we prepare detailed cost estimates for house extensions of all sizes across the UK. Our team of qualified Quantity Surveyors work from your drawings to produce a comprehensive, itemised breakdown that covers every element of your project.
Whether you need a quick budget estimate from planning drawings, a full building regulation estimate from detailed drawings, or a complete bill of quantities for competitive tendering, we can help. Our estimates are prepared to industry standards and are trusted by builders, architects and homeowners across the UK.
Ready to find out what your extension will cost? Send us your plans for a fixed-price quotation with no obligation, or browse our full range of estimating services. We typically turn around domestic extension estimates within five working days of receiving your drawings.