Quick summary: average cost of house extension
The average cost of house extension work in the UK depends heavily on size, specification, location and structural complexity. As a broad 2026 budgeting guide, a straightforward single-storey extension will commonly fall around £2,200–£3,300 per m² excluding VAT. That means a typical 30m² extension could cost roughly £66,000–£99,000 before VAT, although individual projects can sit well outside this range.
- 20m² extension: roughly £44,000–£66,000 before VAT using a broad £2,200–£3,300/m² allowance.
- 30m² extension: roughly £66,000–£99,000 before VAT.
- 40m² extension: roughly £88,000–£132,000 before VAT.
- Double-storey extensions: usually cost more overall, but the cost per square metre can be lower because foundations and roof costs are shared across two floors.
However, these figures should be treated as an early-stage budget rather than a quotation. Kitchens, glazing, structural steelwork, drainage alterations, professional fees, Building Control, planning requirements and the standard of finishes can all change the final figure substantially.
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What is the average cost of a house extension in the UK?
The average cost of house extension work is best thought of as a range rather than one national figure. For a reasonably straightforward single-storey extension in 2026, a useful early budgeting allowance is around £2,200–£3,300 per m² excluding VAT.
Therefore, a 30m² rear or kitchen extension might have a basic construction budget of around £66,000–£99,000 before VAT. However, the finished project can cost considerably more once you add a new kitchen, large areas of glazing, structural work, professional fees and other project-specific costs.
If you want a broader explanation of where these figures sit within an extension budget, read our complete guide to budgeting for an extension.
Quick costs snapshot: average house extension costs
- 20m²: approximately £44,000–£66,000 before VAT.
- 30m²: approximately £66,000–£99,000 before VAT.
- 40m²: approximately £88,000–£132,000 before VAT.
- Higher specification: premium glazing, kitchens, finishes and complex structures can push costs above these figures.
- Location: London and parts of the South East can cost noticeably more than many other areas.
These examples use a broad £2,200–£3,300/m² construction allowance and are intended for initial budgeting only. VAT, professional fees and project-specific extras may apply.
Average house extension cost per square metre
Cost per square metre is one of the quickest ways to estimate a project before detailed plans exist. However, it works best when comparing extensions of a reasonably similar type and specification.
What does the average cost per m² actually include?
A contractor's construction figure will generally relate to building the extension itself. Depending on the quotation, that may include foundations, walls, roof, insulation, plastering, electrics and basic finishes.
It may not include everything needed to take your idea from the first sketch to a completed room. For example, you may still need to budget separately for:
- architectural or technical drawings,
- structural engineer calculations,
- planning or lawful development applications,
- Building Control charges,
- party wall advice where applicable,
- drainage alterations,
- kitchens and appliances,
- flooring and decorating, and
- external landscaping or making good.
Our guide to what goes into a complete house extension budget looks at these additional costs in more detail.
What is the typical cost of a small, medium or large house extension?
Extension size clearly affects the overall budget. However, smaller extensions do not always have the lowest cost per square metre because foundations, drainage connections, structural design and site setup create fixed costs regardless of floor area.
Average cost of a small house extension
A compact 15–20m² extension might appear simple, but it can still involve nearly all the same trades as a much larger project. Using a broad 2026 construction allowance, a 20m² extension could therefore sit around £44,000–£66,000 before VAT.
Average cost of a 30m² house extension
Around 25–30m² is common for a rear kitchen-diner extension. At approximately £2,200–£3,300/m², a 30m² project gives an early construction allowance of £66,000–£99,000 before VAT.
Average cost of a 40m² house extension
A larger 40m² project could broadly fall around £88,000–£132,000 before VAT. Nevertheless, structural spans, bifold or sliding doors, rooflights and a high-specification kitchen can quickly increase this figure.
What affects the average cost of a house extension?
Two extensions with exactly the same floor area can have very different final costs. The main reasons are usually design complexity, specification and site conditions.
Design and structural complexity
Straightforward rectangular extensions are generally easier to construct. By contrast, large openings in existing walls, unusual roof forms, extensive glazing and cantilevered sections can require additional engineering and labour.
Kitchen extension costs
The average cost of a kitchen extension is often higher than the cost of creating a simple living room because the budget may also include cabinetry, worktops, appliances, plumbing, electrics and ventilation.
If your project is mainly about creating a larger kitchen-diner, our kitchen extension homeowner guide explains the planning, design and cost considerations in more detail.
Where you live
Labour rates, contractor demand and access vary around the country. Consequently, an extension in London or another high-cost area may be noticeably more expensive than an equivalent project elsewhere.
Access and existing ground conditions
Restricted access can make excavation and material handling slower. Similarly, unusual foundations, trees, drains or poor ground can add costs that may not have been obvious during the earliest design stage.
Single-storey vs double-storey extension costs
A double-storey extension costs more overall than a similar single-storey project because you are creating substantially more floor area. However, it can offer better value per square metre.
This is because several major costs — including foundations, site setup and much of the roof — are already required for the ground-floor extension. Adding another floor does not necessarily double every part of the construction cost.
For a more detailed comparison, see our guide to budgeting for a two-storey extension and our separate breakdown of single-storey project costs.
Not sure how much extension your budget can realistically deliver?
We can help turn your ideas into a clear set of plans so you can understand the likely approvals, design constraints and construction scope before approaching builders.
What should you include in your house extension budget?
The building contract is only one part of the overall cost. A sensible homeowner budget should also consider the work needed before construction begins and the finishes required afterwards.
- Survey and design: measured surveys and extension drawings.
- Structural design: calculations for beams, foundations and other structural alterations.
- Planning: application costs or permitted development confirmation where appropriate.
- Building Regulations: technical drawings, calculations and Building Control charges.
- Party wall matters: potentially relevant where work affects shared walls or excavations near neighbouring structures.
- Kitchen and bathroom fittings: often a significant separate budget.
- Contingency: money reserved for unexpected work or design changes.
Most house extensions require Building Regulations approval even where planning permission is not required. Our guide to Building Regulations for extensions explains what Building Control will normally check.
How to budget for a house extension step by step
Rather than starting with a builder's quotation, it is usually better to work through your budget in stages.
House extension budgeting checklist
- Set your maximum overall budget. Include any borrowing costs and keep a contingency rather than committing every pound to construction.
- Decide what space you actually need. A carefully designed smaller extension can sometimes solve the problem better than simply adding more floor area.
- Check the planning position. Establish whether the proposal could fall under permitted development or requires an application.
- Prepare proper drawings. Accurate plans make it much easier for builders to price the same scope of work.
- Identify structural requirements. Large openings and unusual roof designs can materially affect cost.
- Specify the finish. Decide whether the quotation should include kitchens, flooring, decorating, glazing and external works.
- Obtain comparable quotations. Ask contractors to price from the same drawings and specification.
- Keep a contingency. Older properties in particular can reveal unforeseen conditions after construction begins.
Before requesting quotations, it is also worth understanding what a proper extension plan package should contain.
Do planning rules change the cost of an extension?
They can. Some house extensions fall within permitted development rights, while others require planning permission. Restrictions can also apply because of conservation areas, Article 4 Directions, listed buildings, previous extensions or planning conditions affecting the property.
Therefore, it is worth checking the planning position before assuming that the construction budget is the only figure that matters. Our step-by-step extension planning guide explains the process.
FAQs: average cost of house extension
What's the average cost of a house extension in the UK?
As an early 2026 budgeting guide, a straightforward single-storey extension can commonly be estimated at around £2,200–£3,300 per m² excluding VAT. A 30m² extension could therefore be around £66,000–£99,000 before VAT, although design, location and specification can move the final figure considerably.
How much is a 20m² house extension?
Using the same broad £2,200–£3,300/m² allowance, a 20m² extension might have a construction budget of approximately £44,000–£66,000 before VAT. Professional fees, kitchen fittings and other extras may sit outside this amount.
What is the average cost of a kitchen extension?
Kitchen extensions often sit towards the higher end of extension budgets because the room itself can include cabinetry, worktops, appliances, plumbing, ventilation, lighting and additional electrical work. Separate the basic construction cost from the kitchen fit-out when preparing your budget.
What is the average cost of a small house extension?
A small extension does not necessarily have a low cost per square metre. Foundations, drainage, design work and site setup are still required, so fixed costs are spread across less new floor area. This is why very small extensions can sometimes appear expensive on a per-m² basis.
Are house extensions more expensive in London?
They often are. Labour, contractor demand, access restrictions and general operating costs can all be higher in London and parts of the South East. For that reason, national average figures should only be used as a starting point before obtaining local quotations.
Can I use a house extension cost calculator to set my budget?
A calculator is useful during the feasibility stage because it can estimate the effect of changing the extension size. However, it cannot fully account for structural requirements, ground conditions, access, specification or the condition of your existing property. Detailed drawings and contractor quotations are therefore much more reliable before committing to construction.
Does a double-storey extension cost twice as much as a single-storey extension?
Not usually. Although the overall project will cost more, foundations, site setup and roof costs can be shared across the two floors. Consequently, a double-storey extension can sometimes provide more floor area for a lower cost per square metre.
Have a budget but not sure what is realistic?
We can help you turn your ideas into a practical extension proposal and explain which drawings and approvals you are likely to need.
Next steps & useful guides
If you are now working out what your extension might involve, these related Plans Made Easy guides are useful next steps:
- Extension budgeting explained from start to finish
- What goes into a complete extension budget
- Budgeting for a single-storey project
- Budgeting for a two-storey project
- What drawings you need before building
- What Building Control will check
- How the extension planning process works
- 2026 broad single-storey allowance Around £2,200–£3,300 per m² excluding VAT for a straightforward project.
- Typical 20m² example Approximately £44,000–£66,000 before VAT.
- Typical 30m² example Approximately £66,000–£99,000 before VAT.
- Typical 40m² example Approximately £88,000–£132,000 before VAT.
- Biggest cost variables Location, structural complexity, glazing, kitchen specification, access, groundwork and finish level.
- Do not forget Professional fees, Building Control, structural calculations, planning requirements, fittings and contingency may sit outside the basic construction figure.
- Planning position Some extensions can use permitted development rights, but restrictions and local circumstances can change what is allowed.
Planning an extension? Ask Plans Made Easy to review your project before you commit to drawings or construction.
Official guidance and useful cost references
National rules and market prices change, so it is sensible to check current guidance as your project develops.
- Planning Portal guidance on extension planning rules and permitted development
- GOV.UK technical guidance on permitted development rights for householders
- RICS consumer guidance on home extensions and improvements
- Homebuilding & Renovating 2026 extension budgeting guidance
So, how much should you budget for your house extension?
The average cost of a house extension gives you a useful starting point, but your own design matters far more than a national average. Size is important, yet structural work, glazing, kitchens, access and specification can change the figure just as much.
For that reason, the best way to move from a rough budget to a dependable figure is to establish what you can build, prepare accurate plans and then ask builders to quote against the same information. This gives you a far better comparison than collecting informal estimates based on floor area alone.
Plans Made Easy can help you develop the proposal, prepare the required drawings and guide you through planning and Building Regulations so that you know what you are asking contractors to price before construction begins.
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