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How Much Does a Single-storey Extension Cost in the UK?
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Single-storey Extension Cost intro
How Much Does a Single-storey Extension Cost in the UK?
A single-storey extension is one of the most common ways to create more space without moving home.
It can be used to create:
- A larger kitchen
- Open-plan kitchen and dining space
- Family room
- Home office
- Utility room
- Additional bedroom
- Accessible ground-floor accommodation
- Who is responsible for different parts of the project
- What happens at completion
- How disputes are dealt with
As a broad 2026 UK guide, a single-storey extension typically costs around £1,800–£3,000 per m², although location, specification and complexity can push the final figure above or below that range. FMB
Current published examples suggest approximately:
- 20 m²: £40,000–£56,000
- 30 m²: £60,000–£84,000
- 50 m²: £100,000–£140,000 Checkatrade
These figures are useful for early budgeting, but they are not quotations.
The actual cost depends on what you are building, what needs to happen to the existing house and what level of finish you expect.
At Tholos Architects, we help homeowners develop single-storey extensions around both their property and their available budget.
Single-storey Extension Cost explained
Single-storey Extension Cost per Square Metre
A cost-per-square-metre calculation is useful during the early stages of a project.
For 2026, a reasonable UK starting range is approximately:
- £1,800–£2,200 per m² for relatively straightforward construction and specification
- £2,200–£3,000 per m² for more complex or higher-specification projects
- £3,000+ per m² where there is substantial structural work, premium glazing, difficult access or expensive finishes
The Federation of Master Builders currently gives a broad average of approximately £1,800–£3,000 per m² for single-storey extensions in the UK.
These figures should be used as an initial budgeting tool rather than a fixed construction price.
Typical Single-storey Extension Costs by Size
The following gives a useful starting point:
| Extension Size | Indicative 2026 Cost |
|---|---|
| 15 m² | £27,000–£45,000 |
| 20 m² | £40,000–£56,000 |
| 25 m² | £45,000–£75,000 |
| 30 m² | £60,000–£84,000 |
| 40 m² | £72,000–£120,000 |
| 50 m² | £100,000–£140,000 |
Published 2026 figures from Checkatrade place a 20 m² single-storey extension at around £40,000–£56,000, a 30 m² extension at approximately £60,000–£84,000, and a 50 m² project at around £100,000–£140,000. Checkatrade
The wider ranges above reflect the fact that specification and project complexity vary considerably.
Why Two Single-storey Extensions Can Cost Completely Different Amounts
Imagine two 30 m² extensions.
The first includes:
- Simple brick construction
- Standard windows
- Straightforward roof
- Limited internal alteration
- Basic finishes
The second includes:
- Removal of most of the existing rear wall
- Large structural beams
- Sliding glazing
- Rooflights
- New kitchen
- Underfloor heating
- Bespoke joinery
- Premium flooring
They may have exactly the same floor area.
They are not remotely the same construction project.
That is why £/m² figures should always be treated cautiously.
What Does a 20 m² Single-storey Extension Cost?
This depends on the source of the price.
A typical construction figure may include:
- Kitchen extension
- Kitchen/dining room
- Family space
- Small bedroom and bathroom arrangement
The cost may rise where the project includes major structural alterations, premium glazing or a new kitchen.
What Does a 30 m² Single-storey Extension Cost?
A 30 m² single-storey extension is currently estimated at approximately:
£60,000–£84,000
in current UK cost guidance. Checkatrade
That size can provide enough space for a substantial kitchen/dining/living area, depending on the layout.
But internal works to the original house can add considerably to the project cost.
What Does a 40 m² Single-storey Extension Cost?
Using a broad £1,800–£3,000 per m² range, an early allowance would be approximately:
£72,000–£120,000
The final cost will depend heavily on the specification and structural arrangement.
What Does a 50 m² Single-storey Extension Cost?
Current published UK data suggests approximately:
£100,000–£140,000
for a 50 m² single-storey extension. Checkatrade
A project of this scale is likely to involve substantial design decisions around:
- Structure
- Glazing
- Heating
- Internal layout
- Kitchen
- Finishes
These costs are spread across fewer square metres.
That means a small 10–15 m² extension may have a higher cost per square metre than a larger straightforward project.
Single-storey Extension Cost
Small Single-storey Extensions Can Cost More per m²
A smaller extension is not necessarily proportionally cheaper.
Every project still needs many fixed elements, including:
- Site setup
- Excavation
- Foundations
- Drainage
- Structural work
- Roof
- Electrical work
- Building Control
These costs are spread across fewer square metres.
That means a small 10–15 m² extension may have a higher cost per square metre than a larger straightforward project.
Rear Single-storey Extension Cost
Rear extensions are among the most common single-storey projects.
They are often used to create:
- Larger kitchens
- Kitchen/dining spaces
- Open-plan family rooms
- Better access to the garden
Cost can increase where the design requires:
- Large structural openings
- Sliding or bifold doors
- Rooflights
- New kitchen
- Significant internal remodelling
Side-return Extension Cost
Side-return extensions often add relatively little floor area but can transform narrow Victorian or Edwardian layouts.
The cost per square metre can be comparatively high because the project may still involve:
- Structural steel
- Drainage alterations
- Boundary construction
- Roof glazing
- Kitchen remodelling
Wraparound Single-storey Extension Cost
A wraparound extension combines rear and side extension space.
This can create a substantial ground-floor transformation.
However, it may also require:
- More complex structure
- More extensive foundations
- Multiple roof junctions
- Drainage alterations
- Significant internal remodelling
For that reason, a wraparound extension should normally be costed as an individual project rather than simply applying a generic £/m² rate.
Side Extension Cost
A single-storey side extension can make use of space beside a detached or semi-detached property.
The construction cost depends on:
- Width
- Length
- Boundary position
- Roof form
- Drainage
- Access
- Existing services
Kitchen Extension Cost
Many single-storey extensions contain a new kitchen.
This can significantly increase the overall budget.
Kitchen costs may include:
- Units
- Worktops
- Appliances
- Plumbing
- Electrical work
- Extraction
- Lighting
- Flooring
- Installation
A basic construction cost per square metre may not include any of these.
Open-plan Extension Cost
Creating a large open-plan space often requires structural alterations to the existing house.
This may involve:
- Removing load-bearing walls
- Steel beams
- Columns
- Temporary support
- Making good
The cost of these works is not necessarily reflected in the size of the extension itself.
A relatively small extension combined with substantial internal remodelling can therefore cost more than a larger but structurally simpler addition.
Structural Steelwork
Structural work is one of the main variables in extension pricing.
Costs can increase where you want:
- Very large openings
- Corner openings
- Minimal columns
- Large spans
- Complex roof structures
The architectural design and structural strategy should therefore be developed together.
Foundations
Foundations can represent a significant part of a single-storey extension budget.
Cost depends on factors such as:
- Ground conditions
- Nearby trees
- Drainage
- Existing structures
- Foundation depth
Unexpected ground conditions can create additional costs once excavation begins.
Drainage Costs
Extensions frequently affect existing drains.
Work may involve:
- Moving manholes
- Rerouting drains
- New connections
- Rainwater drainage
- Soil pipes
Understanding the existing drainage arrangement early can reduce surprises during construction.
Roof Cost
The roof design can significantly affect price.
Common options include:
- Flat roof
- Pitched roof
- Mono-pitch roof
- Complex junctions with existing roofs
Additional elements may include:
- Rooflights
- Roof lanterns
- Bespoke glazing
A simple roof form will usually be more economical than a complicated one.
single-storey extension cost
Rooflights
Rooflights are popular in single-storey extensions because they can bring daylight deeper into the original house.
Cost depends on:
- Size
- Number
- Brand
- Opening mechanism
- Installation complexity
Large bespoke roof glazing can increase the budget considerably.
Sliding and Bifold Doors
Large glazed doors can become one of the more expensive individual elements of an extension.
Cost depends on:
- Width
- Number of panels
- Frame material
- Thermal performance
- Glass specification
- Installation
Standard-sized openings are generally easier to budget than bespoke structural glazing.
Internal Remodelling
The extension itself may represent only part of the project.
Many homeowners also want to alter the existing ground floor.
This can involve:
- Removing walls
- Relocating doors
- Moving kitchens
- Creating utility rooms
- Changing circulation
- Redecorating adjoining spaces
These works increase cost without increasing the new floor area.
Electrical Costs
Electrical work may include:
- Additional sockets
- Lighting
- Kitchen circuits
- External lighting
- Smoke alarms
- Consumer-unit alterations
A simple specification will cost considerably less than a detailed architectural-lighting scheme.
Plumbing and Heating
Depending on the project, work may involve:
- New radiators
- Underfloor heating
- Plumbing for kitchen
- Utility connections
- Bathroom plumbing
- Boiler alterations
Changing existing systems can increase cost beyond the extension shell itself.
Underfloor Heating
Underfloor heating is popular in open-plan extensions.
Its cost depends on:
- Floor area
- Existing heating system
- Floor construction
- Controls
It should be considered early because it affects both services and floor build-up.
Flooring
Flooring costs vary considerably.
Options may include:
- Vinyl
- Laminate
- Tiles
- Engineered timber
- Natural stone
Check whether flooring is included in the builder’s quotation or expected to be supplied separately.
Decoration
Some contractors include final decoration.
Others may leave the extension:
- Plastered
- Mist coated
- Completely undecorated
Always check the quotation before assuming decoration is included.
External Works
The construction process can affect:
- Patios
- Paths
- Fencing
- Garden
- Drainage
- Driveways
Making good around the extension may not automatically be included in the construction price.
single-storey extension cost
Difficult Site Access
Access can have a major impact on costs.
Projects may become more expensive where:
- There is no side access
- Materials must pass through the house
- Parking is restricted
- Excavation machinery cannot reach the site
- Waste must be carried manually
The same extension can therefore cost different amounts on two different properties.
Regional Differences
Single-storey extension costs vary across the UK.
London and parts of the South East commonly experience higher labour and contractor costs than many other regions.
National cost guides are therefore useful only as starting points.
Local contractor quotations provide a more accurate indication once the design has developed.
Planning Permission Costs
Some single-storey extensions require planning permission.
Others may fall within permitted development rights depending on:
- Property type
- Size
- Height
- Position
- Previous alterations
- Location
Potential planning-related costs can include:
- Application fee
- Drawings
- Supporting documents
- Specialist reports
Building Regulations Costs
Single-storey extensions generally need to comply with Building Regulations even where planning permission is not required.
Requirements may cover:
- Structure
- Insulation
- Ventilation
- Drainage
- Electrical work
- Fire safety
- Glazing
Technical drawings, structural calculations and Building Control charges should therefore be included in the wider project budget.
Structural Engineer Costs
A structural engineer may be required where the project involves:
- Large openings
- Steel beams
- Columns
- Complex roof structures
- Unusual foundations
The fee will depend on the complexity and extent of the structural design.
Professional Fees
The construction price is not necessarily the complete project cost.
The overall budget may also need to include:
- Architectural services
- Structural engineer
- Building Control
- Party-wall surveyor
- Other specialist consultants
These should be considered before deciding how much can realistically be spent on construction.
Party Wall Costs
Depending on the location of the proposed extension, the Party Wall etc. Act 1996 may apply.
Relevant work can include:
- Excavation near neighbouring structures
- Building at or near a boundary
- Work affecting an existing party structure
If surveyors are needed, the associated fees should be included in the budget.
VAT
Always check whether VAT has been included in:
- Cost guides
- Estimates
- Builder quotations
A price excluding VAT can appear substantially lower than the actual amount payable.
Allow for a Contingency
Even a well-designed extension can encounter unforeseen costs.
A sensible budget normally includes a contingency rather than spending every available pound on the original construction estimate.
Around 10–15% is commonly used as an early budgeting allowance, depending on the age and uncertainty of the property.
Older buildings may justify a larger contingency because hidden defects and previous alterations are more likely.
Single-storey Extension cost
Can I Build a Single-storey Extension for £30,000?
Possibly, but the project would need to be relatively modest.
A £30,000 budget is more realistic for:
- Small extension
- Straightforward structure
- Basic specification
- Limited glazing
- Minimal internal alterations
Current published cost guidance puts many complete single-storey extensions above this level.
Can I Build a Single-storey Extension for £50,000?
Potentially.
A £50,000 budget might support a relatively modest extension depending on:
- Size
- Location
- Specification
- Structure
- Internal alterations
For context, current estimates place a typical 20 m² single-storey extension at around £40,000–£56,000.
Can I Build a Single-storey Extension for £75,000?
A £75,000 construction budget gives significantly more flexibility.
Depending on the project, it may accommodate:
- Larger floor area
- Better glazing
- Structural remodelling
- Kitchen
- Higher specification
But a complex kitchen extension can still exceed this figure.
Can I Build a Single-storey Extension for £100,000?
A £100,000 budget can support a substantial single-storey project in many parts of the UK.
However, large extensions with:
- Bespoke glazing
- Premium kitchens
- Complex structural work
- Significant internal remodelling
can exceed £100,000 relatively easily.
Single-storey Extension cost
How to Reduce the Cost of a Single-storey Extension
The biggest savings often come from design decisions made early.
Possible ways to control costs include:
- Keep the footprint efficient
- Use a simple roof form
- Avoid unnecessary structural spans
- Use standard-sized glazing
- Minimise complicated junctions
- Keep kitchens and bathrooms close to existing services
- Limit bespoke items
- Prioritise the most important spaces
Cost-efficient design is usually more effective than trying to remove quality from a complicated design later.
Keep the Structure Simple
Large column-free openings can look impressive, but they may require:
- Bigger steel beams
- More temporary support
- More complex structural connections
If budget is important, the structural strategy should be considered during design rather than after planning has been completed.
Avoid Unnecessary Floor Area
Every extra square metre adds:
- Foundations
- Walls
- Roof
- Insulation
- Finishes
Sometimes a smaller extension combined with a better internal layout can achieve the same practical result for less money.
Use Standard Glazing Sizes Where Appropriate
Bespoke glazing can significantly increase cost.
Standard window and door systems can often provide excellent results at a more predictable price.
Design Around Existing Services
Moving:
- Boiler
- Kitchen drainage
- Soil pipes
- Utility connections
can increase project cost.
Where practical, retaining services close to existing locations can make the design more economical.
Spend Where It Matters
Elements such as:
- Structure
- Insulation
- Waterproofing
- Roof
- Windows
are difficult to change once construction is complete.
Decorative finishes can often be upgraded later.
Budget priorities should reflect that.
single-storey extension cost
Get Comparable Builder Quotes
Once enough technical information exists, several suitable contractors can be asked to price the same project.
They should ideally receive:
- Same drawings
- Same structural information
- Same specification
- Same scope
This makes quotations much easier to compare.
Single-storey Extension Budget Checklist
Your overall budget may need to cover:
- Main construction
- Structural work
- Windows and doors
- Kitchen
- Bathrooms
- Heating
- Electrical work
- Flooring
- Decoration
- Architectural fees
- Structural engineer
- Planning
- Building Control
- Party-wall matters
- External works
- VAT
- Contingency
Do not assume the builder’s construction figure represents the entire cost of the project.
How Tholos Can Help Control Single-storey Extension Costs
Budget should influence the design from the beginning.
At Tholos Architects, we can help clients consider:
- Floor area
- Layout
- Structure
- Glazing
- Specification
- Internal alterations
- Existing services
in relation to the available budget.
The aim is to create an extension that achieves what the household actually needs without adding unnecessary complexity or cost.
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 single-storey extension cost per m²?
A broad 2026 UK guide is approximately £1,800–£3,000 per m², depending on location, specification and complexity.
Is a rear extension cheaper than a side extension?
Not necessarily.
Cost depends on the size, access, structure, drainage and specification rather than simply whether the extension is at the rear or side.
How much does a 20 m² single-storey extension cost?
Current UK guidance suggests approximately £40,000–£56,000.
How much does a 30 m² single-storey extension cost?
Current published estimates suggest approximately £60,000–£84,000.
How much does a 50 m² single-storey extension cost?
Current guidance suggests approximately £100,000–£140,000, depending on the project.
Do I need planning permission for a single-storey extension?
Not always.
Some projects can fall within permitted development rights, depending on the property and proposal.
Planning requirements should be checked for the specific property.
Plan the Cost Before You Finalise the Design
A single-storey extension can cost anywhere from a relatively modest amount to well over £100,000.
The floor area matters, but so do:
- Structure
- Glazing
- Kitchen
- Internal remodelling
- Specification
- Access
- Location
A useful early benchmark is approximately £1,800–£3,000 per m², but the design should ultimately be developed around the actual budget available. FMB
At Tholos Architects, we can help develop a single-storey extension that balances space, design and cost from the beginning.
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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