Quick summary: House extension cost UK
The typical house extension cost UK homeowners should budget for in 2026 is around £1,800–£3,000 per m² for standard single-storey construction, before VAT, professional fees and certain finishing costs. Therefore, a straightforward 20m² extension could have a basic construction budget of approximately £36,000–£60,000, with the total project cost potentially higher.
- Single-storey extensions: approximately £1,800–£3,000 per m² for standard construction.
- 20m² extension: approximately £36,000–£60,000 before additional project costs.
- 30m² extension: approximately £54,000–£90,000 on the same basis.
- Two-storey extensions: often cheaper per m² of total added floor area, although the overall project costs more.
- Additional allowances: professional drawings, structural calculations, Building Control, VAT, kitchens, drainage and contingency.
The important point: an extension price per square metre is a starting estimate, not a complete quotation. Ground conditions, access, roof design, glazing, structural alterations and your chosen specification can change the final figure considerably.
In this guide, we explain realistic 2026 prices, what affects your building budget and how to estimate the full cost before committing to construction.
Planning an extension but unsure what to budget?
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How Much Does a House Extension Cost in the UK?
If you're thinking about extending your home, one of your first questions will probably be how much it will cost. Unfortunately, there's no single answer because even two extensions of identical size can have very different construction requirements.
For 2026, a useful starting point is approximately £1,800–£3,000 per square metre for a standard single-storey extension. This is a broad construction estimate rather than an all-inclusive project price.
At Plans Made Easy, we recommend separating the building estimate from your overall project budget. This helps you avoid comparing a builder's basic construction quotation against another contractor's more complete price.
Quick costs snapshot: 2026 extension prices
| Extension size | Indicative construction cost |
|---|---|
| 10m² | £18,000–£30,000 |
| 15m² | £27,000–£45,000 |
| 20m² | £36,000–£60,000 |
| 25m² | £45,000–£75,000 |
| 30m² | £54,000–£90,000 |
| 40m² | £72,000–£120,000 |
Illustrative calculations using £1,800–£3,000/m². Figures exclude VAT, professional fees, specialist fit-out and contingency. Small or complex projects may exceed these rates.
House Extension Cost Per m² UK: 2026 Prices Explained
The cost per square metre is useful because it gives you a quick way to compare differently sized proposals. However, the rate depends on the specification and exactly what the builder includes.
Basic, standard and premium extension costs
- Basic construction: approximately £1,800–£2,200/m², where the design is straightforward and finishes are relatively simple.
- Standard specification: approximately £2,200–£2,700/m² for a conventional finished extension.
- Higher specification: approximately £2,700–£3,500+/m² where extensive glazing, complex structures or premium materials increase costs.
These are indicative construction bands, not guaranteed market rates. Moreover, the exact boundary between basic and standard finishes differs between contractors.
Single-Storey vs Two-Storey House Extension Costs
The extension's shape and number of storeys can make a substantial difference to your budget. Although two-storey construction costs more overall, it can offer better value for each square metre of additional living space.
Single-storey house extension cost
A single-storey extension is commonly used for a larger kitchen, dining area or family room. Straightforward rear extensions are often simpler than side-return or wrap-around designs.
For example, a 20m² extension priced at £2,400/m² would have an indicative construction cost of £48,000, before additional project expenses.
For a more detailed breakdown, see our guide to budgeting for a single-storey project.
Two-storey house extension cost
A two-storey extension typically shares foundations and a roof across two levels. Consequently, the average price per square metre of new floor area can be lower than for a single-storey scheme.
However, additional floors introduce structural, access, fire-safety and design considerations. A larger project also requires a higher total budget.
Our two-storey project cost breakdown explains these considerations in greater detail.
Side, rear and wooden extensions
- Rear extensions: commonly offer straightforward layouts, although large openings into the existing house may require substantial steelwork.
- Side-return extensions: can become expensive per m² because of narrow access, drainage and neighbouring structures.
- Timber-frame extensions: may offer a quicker structural installation, but the total cost still depends on foundations, external finishes and insulation.
- Kitchen extensions: require an additional allowance for cabinets, appliances, plumbing and electrical work.
How to Calculate Your House Extension Cost
You don't need a complicated calculator to produce an initial estimate. Start with the additional floor area and an appropriate construction rate, then allow separately for other expenses.
Example: 20m² rear extension
Step 1: Calculate the floor area.
5 metres × 4 metres = 20m².
Step 2: Apply an indicative construction rate.
20m² × £2,400 = £48,000.
Step 3: Allow for VAT where applicable.
At the standard 20% rate: £9,600.
Step 4: Add illustrative additional allowances.
- Professional and approval costs: £5,000.
- Contingency: £7,200.
- Kitchen or specialist fit-out: priced separately.
Illustrative working budget: £69,800 before kitchen fit-out.
This is a worked budgeting example, not a quotation. The £5,000 fee allowance is illustrative, and contingency is calculated here at 15% of the construction figure. Actual VAT treatment and professional costs must be confirmed.
A basic online extension calculator may overlook large structural openings, drainage diversions and specialist foundations. Therefore, use the result as an initial budget rather than a price you can confidently build for.
What Additional Costs Should You Budget For?
One of the most common budgeting mistakes is assuming that the builder's initial figure covers the entire project. In practice, several important costs may sit outside the construction quotation.
Architect, drawings and structural engineer costs
You may need measured surveys, design drawings, planning drawings, technical construction details and structural calculations. The cost depends on the complexity of your property and who is responsible for each service.
Our guide to the drawings required for an extension explains what different drawing packages contain.
Other expenses to check
- Planning fees: where a formal application or other council procedure is required.
- Building Control: application and inspection charges.
- Structural alterations: steel beams, supports and additional foundation work.
- Drainage: sewer surveys, diversions or build-over agreements where applicable.
- Party wall matters: notices and potentially surveyor fees.
- Finishes: flooring, decorating, lighting, kitchens and bathrooms.
- VAT: normally charged at 20% on standard-rated building services supplied by VAT-registered businesses.
Why Are House Extensions So Expensive?
An extension is more than a new room. It requires the builder to connect a new structure safely to an existing property while protecting the rest of your home.
Foundations, drainage, roofing, insulation, structural openings and weatherproofing all require labour and materials. In addition, builders may have to work around occupied rooms, neighbouring buildings and restricted access.
What increases the final building price?
- Large bi-fold doors, roof lanterns and bespoke glazing.
- Complicated roof junctions or unusual building shapes.
- Limited access for machinery and material deliveries.
- Poor soil conditions or foundations near trees.
- Substantial changes to the existing house layout.
- Premium kitchens, bathrooms and heating installations.
London and parts of the South East may also carry higher labour and site-management costs. Conversely, straightforward construction with good access can help contain expenditure.
Want to avoid expensive changes after work starts?
We can help establish the right planning and technical drawing requirements so your builder has clearer information to price.
Planning Permission and Building Regulations Costs
Before committing to construction, establish whether your proposal needs planning permission. Some extensions in England qualify as permitted development, provided all relevant limitations and conditions are satisfied.
Your property's history, previous extensions, location and any restrictions on permitted development rights can affect the answer. Our complete homeowner extension guide provides a broader starting point.
Do you need planning permission?
A straightforward proposal might fall within permitted development rights. Nevertheless, larger, two-storey or more unusual schemes may need formal permission, and certain properties have additional restrictions.
For further guidance, see how to check the planning position for your proposal.
Building Regulations still apply
Even where planning permission is unnecessary, an extension will normally require Building Regulations compliance. Requirements may cover structure under Part A, fire safety under Part B, ventilation under Part F and energy efficiency under Part L.
Our Building Regulations homeowner guide explains these requirements and how Building Control fits into the process.
How to Budget for a House Extension: Step by Step
A sensible budget starts before you request detailed building quotations. The following checklist helps you organise the project and reduce the risk of unexpected costs.
House extension budgeting checklist
| Step | What to do |
|---|---|
| 1 | Decide what additional space you need and measure the proposed area. |
| 2 | Choose an initial price-per-m² range suitable for your design. |
| 3 | Check planning restrictions and Building Regulations requirements. |
| 4 | Arrange appropriate drawings and structural design information. |
| 5 | Ask several builders to price the same specification. |
| 6 | Add VAT, professional fees, fittings and other exclusions. |
| 7 | Allow a contingency, commonly around 10–15% for a straightforward project. |
| 8 | Review the total budget before entering into a building contract. |
Older properties or uncertain ground conditions may justify a larger contingency. Equally, detailed drawings and clear specifications can make competing quotations much easier to assess.
Is a House Extension Worth the Cost?
An extension can make your home more practical without requiring you to move. However, whether it represents good financial value depends on the property, local housing market and quality of the finished space.
Before proceeding, compare the total project cost against the price of buying a larger property locally. Consider moving costs, disruption and whether the proposed layout genuinely improves how you live.
It can also be useful to speak with local estate agents about likely resale appeal. They may help you understand whether additional bedrooms, improved kitchen space or better access to the garden are particularly desirable in your area.
Conclusion: Getting Your Extension Budget Right
The typical house extension cost UK homeowners encounter depends on far more than floor area alone. While £1,800–£3,000 per m² provides a useful starting benchmark, the final bill must account for design, structure, approvals, finishes and unexpected site conditions.
At Plans Made Easy, we believe that understanding the requirements early helps you make informed decisions. With appropriate drawings, realistic allowances and comparable builder quotations, you can approach your project with a clearer picture of the likely investment.
FAQs: House Extension Cost UK
How much does a house extension cost in the UK?
A standard single-storey extension commonly has an indicative construction rate of £1,800–£3,000 per m². The total project budget may be higher once VAT, design fees, specialist finishes and contingency are included.
How much does the average house extension cost?
A 20m² extension at £2,400/m² would have an illustrative construction cost of £48,000 before additional expenses. There is no reliable universal average for every home because specification, location and site conditions vary considerably.
How much does a small house extension cost?
A basic 10m² project might initially be estimated at £18,000–£30,000 for construction. However, smaller extensions sometimes cost more per m² because foundations, design work and site setup remain necessary regardless of size.
How much does a single-storey extension cost per m²?
A useful 2026 planning range is approximately £1,800–£3,000/m² for standard single-storey construction. Premium specifications, restricted access and substantial structural alterations can increase that figure.
How much does a two-storey house extension cost?
Two-storey extensions generally cost more overall than single-storey extensions, although their average cost per square metre of total added floor area can be lower. The amount depends on structural design, roof complexity and the total floor area.
How much does a 3-metre house extension cost?
A 3-metre projection does not provide enough information on its own. For example, a 3m-deep by 5m-wide extension adds 15m². At £1,800–£3,000/m², the indicative construction estimate is £27,000–£45,000 before extras.
Why are house extensions so expensive?
Extensions involve foundations, structure, roofing, insulation, drainage, labour and integration with the existing house. Complex glazing, limited site access and unexpected ground conditions can increase the price further.
How much does an architect cost for a house extension?
Fees vary according to whether you need concept design, planning drawings, Building Regulations details, tender information or construction-stage services. Ask for an itemised quotation rather than assuming all services are included in one figure.
Are wooden house extensions cheaper?
Timber-frame construction can offer advantages in build speed and structural weight. However, it is not automatically cheaper overall because foundations, cladding, windows, insulation and internal finishing still contribute significantly to the cost.
Is a house extension worth it?
It can be worthwhile where the extra space substantially improves daily life or helps you avoid moving. Financial value depends on the property and local market, so consider the full project cost alongside independent estate-agent advice.
How much should I allow for unexpected extension costs?
A contingency of approximately 10–15% of construction costs is a common starting allowance for straightforward projects. Older houses, difficult access or unknown ground conditions may justify more.
Next Steps & Useful Guides
If you're comparing options or preparing to speak with a designer or builder, these additional Plans Made Easy resources may help:
- Complete house extension planning and approvals guide
- Understanding single-storey project budgets
- Financial considerations for building across two floors
- What technical and design drawings include
- Understanding Building Regulations requirements
- Typical construction rate £1,800–£3,000/m² for a standard single-storey extension.
- 20m² example £36,000–£60,000 indicative construction cost before extras.
- 30m² example £54,000–£90,000 on the same basis.
- Contingency allowance Commonly 10–15%, potentially higher for complex projects.
- Planning permission Depends on the proposal, property and applicable permitted development rights.
- Building Regulations Normally apply even where full planning permission is unnecessary.
- Biggest cost variables Size, specification, structure, glazing, location and site conditions.
Official References & Further Reading
For additional guidance on costs, regulations and approval requirements, consult these independent sources:
- Planning Portal – extension planning and Building Regulations guidance.
- GOV.UK – when Building Regulations approval is required.
- Office for National Statistics – construction output price indices.
- HomeOwners Alliance – UK house extension price guidance.
Ready to start planning your extension?
Plans Made Easy can help with planning drawings, Building Regulations plans and understanding the approval requirements for your proposed extension.
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