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How Much Does a House Extension Cost in the UK?
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How Much Does a House Extension Cost in the UK?
The cost of a house extension can vary significantly.
Size matters, but it is only one part of the calculation.
The final cost can also be affected by:
- Extension type
- Structural complexity
- Location
- Ground conditions
- Access
- Specification
- Glazing
- Kitchens and bathrooms
- Heating and electrical work
- External works
- Professional fees
- Building Control
- Unexpected work to the existing property
As a broad 2026 UK guide, a fully finished house extension is commonly estimated at around £1,800–£3,000 per m², while a basic shell-only build may be closer to £1,200–£1,700 per m².
These figures are useful for early budgeting, but they are not a substitute for a project-specific cost estimate.
At Tholos Architects, we help homeowners develop extensions that respond to both the property and the available budget before detailed construction decisions are made.
House Extension Cost per Square Metre
Cost per square metre is one of the most common ways to estimate an extension at an early stage.
Current UK guidance puts typical finished extension costs broadly around:
- £1,800–£2,200 per m² for a relatively straightforward specification
- £2,200–£3,000 per m² for higher specifications or more complicated projects
- £3,000+ per m² where the design, location, structure or finishes significantly increase costs
Published 2026 industry guidance places the broad UK range for a finished extension at approximately £1,800–£3,000 per m². FMB
These figures should be treated as budgeting ranges rather than fixed prices.
Why Cost per Square Metre Can Be Misleading
A simple calculation such as:
30 m² × £2,400 = £72,000
can be useful for an initial budget.
But two 30 m² extensions can cost very different amounts.
One might contain:
- Simple brick walls
- Standard windows
- Basic finishes
- No major internal alterations
Another might include:
- Large structural openings
- Bespoke glazing
- New kitchen
- Underfloor heating
- Rooflights
- High-end finishes
- Significant remodelling to the existing house
The floor area is identical.
The project is not.
House Extension Costs
Typical House Extension Costs by Size
Using a broad finished-build range of approximately £1,800–£3,000 per m², indicative construction costs might look like this:
| Extension Size | Indicative Build Cost |
|---|---|
| 15 m² | £27,000–£45,000 |
| 20 m² | £36,000–£60,000 |
| 25 m² | £45,000–£75,000 |
| 30 m² | £54,000–£90,000 |
| 40 m² | £72,000–£120,000 |
| 50 m² | £90,000–£150,000 |
These are broad budgeting figures only.
Published 2026 cost data gives approximately £40,000–£56,000 for a 20 m² single-storey extension, illustrating how actual project pricing can differ from a simple square-metre calculation.
What Is Included in an Extension Cost?
This depends entirely on the estimate or quotation you are looking at.
A fully finished extension may include:
- Groundworks
- Foundations
- Walls
- Structural work
- Roof
- Insulation
- Windows and doors
- Plumbing
- Electrical work
- Heating
- Plastering
- Joinery
- Flooring
- Decoration
But there is no universal definition.
Some published square-metre figures exclude:
- Kitchen
- Sanitaryware
- Professional fees
- Planning fees
- Building Control
- Party-wall costs
- Landscaping
- Specialist finishes
Always check what a figure actually includes before using it to set your budget.
Shell-only Extension Cost
A shell-only extension generally means the main external structure is constructed, but much of the internal work is left unfinished.
This may include:
- Foundations
- External walls
- Roof
while excluding items such as:
- Internal finishes
- Services
- Flooring
- Kitchen
- Decoration
Current UK estimates place shell-only extension construction broadly around £1,200–£1,700 per m².
A shell build can appear significantly cheaper, but make sure you also budget for everything required to turn it into usable space.
Single-storey Extension Cost
Single-storey extensions are one of the most common ways to increase living space.
They may be used for:
- Kitchen extensions
- Dining areas
- Family rooms
- Utility rooms
- Home offices
- Open-plan living spaces
The Federation of Master Builders currently gives a broad UK guide of approximately £1,800–£3,000 per m² for a single-storey extension, with regional and specification differences affecting the final cost.
Double-storey Extension Cost
A double-storey extension adds accommodation over two levels.
It might create:
- Larger kitchen or living space downstairs
- Additional bedroom upstairs
- New bathroom
- Office
- Expanded circulation
Although a double-storey extension costs more overall than a single-storey extension, the additional upper-floor space does not necessarily double the price because some elements such as foundations and roof are shared.
Current 2026 guidance puts double-storey extension construction broadly around £1,650–£3,000 per m², depending on specification and complexity
Rear Extension Cost
Rear extensions can range from relatively simple additions to substantial transformations of the ground floor.
Costs may increase where the project includes:
- Large structural opening
- Steelwork
- Large sliding or bifold doors
- Rooflights
- New kitchen
- Internal remodelling
- Drainage changes
A small rear extension and a large open-plan kitchen extension should therefore not be expected to cost the same amount per square metre.
Side-return Extension Cost
Side-return extensions are common on Victorian and Edwardian properties.
Although the additional floor area may be relatively small, the cost per square metre can be comparatively high because the project may still require:
- Structural work
- Drainage alterations
- Roof glazing
- Boundary detailing
- Full kitchen remodelling
Current UK cost guidance suggests side-return projects can range roughly from £30,000–£65,000, depending on size and type, with costs per square metre varying substantially.
Wraparound Extension Cost
A wraparound extension combines rear and side extension space.
It can create a substantial new ground-floor footprint but may require:
- Extensive structural steelwork
- Multiple roof forms
- Large glazed openings
- Drainage alterations
- Significant internal remodelling
Because of this, the cost should be assessed as an individual project rather than simply multiplying floor area by a national average.
Kitchen Extension Cost
Kitchen extensions often cost more than a basic living-space extension because the project includes both construction and a major fitted room.
Additional costs can include:
- Kitchen units
- Worktops
- Appliances
- Plumbing
- Electrical circuits
- Extraction
- Lighting
- Flooring
A £50,000 construction budget and a £50,000 completed kitchen-extension budget are therefore not necessarily describing the same thing.
House Extension Costs
What Makes a House Extension More Expensive?
There is rarely one single factor.
Several design and site decisions can push the cost upward.
Structural Work
Large open-plan spaces often require significant structural intervention.
This may involve:
- Steel beams
- Columns
- Padstones
- Temporary support
- Removal of load-bearing walls
Large spans and complex structural arrangements usually increase both material and labour costs.
Large Areas of Glazing
Glazing can become a significant part of the budget.
Costs may increase with:
- Large sliding doors
- Bifold doors
- Structural glazing
- Rooflights
- Roof lanterns
- Bespoke windows
The specification can make an enormous difference.
Kitchens
A kitchen can add anything from a relatively modest amount to tens of thousands of pounds.
Cost depends on:
- Cabinetry
- Appliances
- Worktops
- Joinery
- Electrical requirements
- Plumbing
- Finishes
Keep the kitchen budget separate during early cost planning rather than hiding it inside a generic £/m² figure.
Bathrooms
Adding a new bathroom or WC increases cost through:
- Drainage
- Water supply
- Sanitaryware
- Tiling
- Ventilation
- Waterproofing
- Electrical work
The location of existing services can also affect cost.
Internal Remodelling
Many extensions involve substantial work inside the existing house.
For example:
- Removing walls
- Relocating doors
- Changing circulation
- Moving the kitchen
- Altering utilities
- Redecorating adjoining rooms
These works increase the project cost without increasing the extension floor area.
That is another reason why cost per square metre has limitations.
Foundations
Foundation costs depend partly on what is found below ground.
Factors can include:
- Soil conditions
- Trees
- Existing drains
- Previous structures
- Foundation depth
- Access
Unusual ground conditions can increase cost considerably.
Drainage
Extensions frequently interact with existing drainage.
The project may require:
- Moving manholes
- Rerouting drains
- New connections
- Additional inspection chambers
These costs can be difficult to predict until the existing drainage arrangement is understood.
Site Access
Restricted access can increase labour and logistics costs.
Examples include:
- Terraced houses
- No side access
- Narrow passages
- Difficult parking
- Materials carried through the house
A project that is easy to access may be considerably easier and cheaper to build than an identical extension on a constrained site.
Location
Building costs vary across the UK.
London and parts of the South East will commonly cost more than many other regions because of:
- Labour rates
- Contractor demand
- Logistics
- Overheads
This is one reason national averages should never be treated as quotations.
Specification
Specification has a major impact on cost.
Compare:
Standard specification
- Standard windows
- Standard rooflights
- Basic flooring
- Standard sanitaryware
- Simple lighting
with:
Higher specification
- Bespoke glazing
- Premium flooring
- Custom joinery
- High-end kitchen
- Architectural lighting
- Premium finishes
The building may have exactly the same footprint but a very different final cost.
house extension cost
Professional Fees
The builder’s construction price is not necessarily the complete project budget.
Professional costs may include:
- Architect
- Structural engineer
- Party-wall surveyor
- Other consultants
The exact appointments depend on the project.
These should be budgeted separately from the construction cost unless expressly included.
Planning Costs
Some extensions require planning permission.
Others may fall within permitted development rights, depending on the property and proposal.
Potential planning-related expenditure may include:
- Application fee
- Drawings
- Supporting information
- Ventilation
- Specialist reports
Building Regulations Costs
Extensions generally need to meet applicable Building Regulations requirements.
Costs can include:
- Technical drawings
- Structural calculations
- Building Control charges
Building Control fees vary by local authority or provider and by the scale of the project.
Current 2026 guidance gives broad single-storey extension Full Plans charges of around £250–£550, although actual charges vary considerably.
Structural Engineer Costs
Structural engineering may be required where the project involves:
- Steel beams
- Load-bearing wall removal
- Complex foundations
- Significant structural alterations
Fees depend on the amount and complexity of engineering required.
Party Wall Costs
Some extensions can involve work covered by the Party Wall etc. Act 1996.
Whether this applies depends on factors such as:
- Proximity to neighbouring structures
- Boundary work
- Excavation depth
- Party structures
If surveyors are required, this should be included in the overall project budget.
Do Not Forget VAT
When comparing extension prices, check whether VAT is included.
A figure that excludes VAT can appear considerably cheaper than the amount you will actually pay.
Always establish whether published costs, estimates and builder quotations are:
- Including VAT
- Excluding VAT
before comparing them.
Contingency Budget
Even carefully planned extension projects can uncover unexpected issues.
A contingency gives you financial room to deal with them.
A commonly used starting point is around 10–15% of the construction budget, with more potentially appropriate for older buildings or projects containing significant uncertainty. Current consumer cost guidance similarly recommends allowing at least 10–15%.
Contingency is not money that must be spent.
It is money you do not want to discover you needed after the budget has already run out.
Older Properties May Need a Larger Contingency
Renovating and extending an older property can reveal:
- Poor previous work
- Damp
- Rotten timber
- Old services
- Structural movement
- Unusual construction
Some of these issues may only become visible after work begins.
House Extension Costs
How Much Does a 20 m² Extension Cost?
As a broad guide, a 20 m² extension might fall somewhere around £36,000–£60,000 using the £1,800–£3,000 per m² range.
Published 2026 UK cost data gives approximately £40,000–£56,000 as an indicative range for a 20 m² single-storey extension.
A kitchen, premium glazing or significant structural alteration could push the total higher.
How Much Does a 30 m² Extension Cost?
Using the same broad range, a 30 m² extension might suggest approximately:
£54,000–£90,000
for construction.
But this could rise depending on:
- Kitchen
- Glazing
- Structural work
- Specification
- Existing-house alterations
How Much Does a 40 m² Extension Cost?
A broad early-stage range might be:
£72,000–£120,000
before considering whether all professional fees, specialist items and VAT are included.
How Much Does a 50 m² Extension Cost?
At approximately £1,800–£3,000 per m², a simple starting range would be:
£90,000–£150,000
Again, this is only an early budgeting tool.
Why Small Extensions Can Cost More per Square Metre
A small extension does not necessarily have a proportionally small cost.
Every extension still needs core elements such as:
- Site setup
- Foundations
- Walls
- Roof
- Drainage
- Electrical work
- Building Control
A 12 m² extension therefore does not necessarily cost 40% of a 30 m² extension.
Many fixed costs are spread across a smaller floor area.
Can You Build an Extension for £30,000?
Possibly, but expectations need to be realistic.
A £30,000 budget is much more likely to suit:
- Small extension
- Straightforward construction
- Modest specification
- Limited structural alterations
than a large completed kitchen extension with extensive glazing and internal remodelling.
Published cost guidance indicates that £30,000 may be at or below the lower end of many typical complete extension projects.
Can You Build an Extension for £50,000?
A £50,000 construction budget can potentially deliver a modest extension, depending on:
- Location
- Size
- Specification
- Structure
- Kitchen requirements
But the design needs to respond to the available budget.
Designing a £100,000 project and then attempting to remove half of it later is rarely an efficient process.
Can You Build an Extension for £100,000?
A £100,000 budget gives considerably more flexibility.
Depending on the project and location, it may support:
- Larger extension
- Better specification
- Structural remodelling
- Kitchen
- More substantial glazing
But even £100,000 can be exceeded quickly on a complex or high-specification project.
house extension cost
Design to the Budget
One of the most useful things you can do is establish a realistic budget before the design becomes too developed.
An extension can then be designed around:
- Priorities
- Floor area
- Structural strategy
- Glazing
- Specification
rather than designing first and discovering later that the project is unaffordable.
Bigger Is Not Always Better
An extension does not need to maximise every available square metre.
Sometimes a smaller, better-planned extension combined with internal remodelling can achieve more than a much larger addition.
Good design considers:
- Existing rooms
- Circulation
- Natural light
- Furniture
- Storage
- Relationship with the garden
not simply total floor area.
Where to Spend More
Some elements are difficult or expensive to replace later.
It can make sense to prioritise budget toward:
- Building fabric
- Insulation
- Waterproofing
- Windows
- Structure
- Core services
Decorative finishes are generally easier to change later.
Where Costs Can Be Reduced
Cost savings may sometimes be found through:
- Simpler structure
- Simpler roof form
- Standard window sizes
- Reduced bespoke glazing
- Standard materials
- Keeping kitchens or bathrooms near existing services
- Reducing unnecessary floor area
The objective should be cost-efficient design, not simply cheap construction.
Get More Than One Builder Quote
Once the project has developed sufficiently, obtaining comparable contractor quotations can give a much better indication of actual market cost.
Builders should ideally receive consistent information.
Otherwise, one may price a very different scope from another.
Do Not Compare Quotes by Total Price Alone
Check:
- Scope
- Exclusions
- Allowances
- VAT
- Materials
- Programme
- Payment terms
A £70,000 quote can be more expensive than an £80,000 quote once missing work is added back in.
House Extension Budget Checklist
Your total extension budget may need to include:
- Construction
- Kitchen
- Bathrooms
- Flooring
- Decoration
- Glazing
- Professional fees
- Structural engineer
- Planning
- Building Control
- Party-wall matters
- Specialist reports
- External works
- Temporary accommodation if required
- Contingency
- VAT where applicable
Do not assume the builder’s headline figure represents the complete cost of the project.
How Tholos Can Help Control Extension Costs
Cost control begins during design, not when the builder submits the final quotation.
At Tholos Architects, we can help clients consider the relationship between:
- Budget
- Floor area
- Layout
- Structure
- Specification
- Glazing
- Existing property
before the design becomes unnecessarily expensive.
We can also help prepare clearer project information so builders have a better basis for pricing the work.
Client experience
"
Lorem ipsum dolor sit amet, consectetur adipiscing elit. We had a vision for our home but no idea how to realise it. Tholos Interiors took complete ownership of the project — every detail was considered, every decision explained. The result exceeded everything we had imagined.
– Lawrence Ipsum, Homeowner · London –
Frequently Asked Questions
How much does a house extension cost per m² in the UK?
A broad 2026 guide is approximately £1,800–£3,000 per m² for a finished extension, although location, design and specification can move costs significantly outside that range.
What is the most expensive part of an extension?
There is no single answer. Structure, glazing, kitchens, groundworks and high-end finishes can all significantly affect cost.
Is a double-storey extension cheaper per square metre?
It can be more cost-efficient per additional square metre because the two floors share elements such as foundations and roof, although the total construction cost will still be higher.
Should I add a contingency?
Yes. Around 10–15% is a common starting point for budgeting, although older or more uncertain buildings may justify a larger allowance.
How can I get an accurate extension cost?
The most reliable price normally comes once there is enough design and technical information for suitable builders to quote the same defined project.
Plan the Extension Around a Realistic Budget
There is no single answer to “How much does a house extension cost?”
A useful starting point is the current UK range of approximately £1,800–£3,000 per m², but the real cost depends on the project.
The strongest approach is to establish the budget early and allow it to inform:
- Size
- Layout
- Structure
- Specification
- Glazing
- Finishes
That produces a much healthier project than designing without a budget and trying to cut costs afterwards.
At Tholos Architects, we can help develop an extension around your property, priorities and available budget.
Disclaimer
This article is part of a series created by our dynamic marketing team for informational purposes only. While we strive to review all content thoroughly, occasional oversights may occur, even after publication. Therefore, we cannot accept liability for any interpretations or use of the information provided. For the most up-to-date information or to report any discrepancies, please contact us at hello@tholos-architects.com.
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