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How Much Does a Double-storey Extension Cost in the UK?
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Double-storey Extension Cost intro
How Much Does a Double-storey Extension Cost in the UK?
A double-storey extension can add significantly more space than a single-storey addition because it creates accommodation across two levels.
It may be used to create:
- Larger kitchen or living space downstairs
- Additional bedrooms upstairs
- New bathroom
- Home office
- Improved circulation
- Additional storage
As a broad 2026 UK guide, double-storey extension construction typically falls around £1,650–£3,000 per m², depending on location, specification and complexity.
Because the upper floor shares some elements with the ground floor, such as foundations and part of the roof structure, a double-storey extension can sometimes provide better value per additional square metre than building the same extra area as a single-storey extension.
At Tholos Architects, we help homeowners assess whether a double-storey extension makes sense for the property, the planning context and the available budget.
Double-storey Extension Cost explained
Double-storey Extension Cost per Square Metre
A cost-per-square-metre calculation can be useful at the early budgeting stage.
A broad 2026 UK guide is approximately:
- £1,650–£2,200 per m² for relatively straightforward construction
- £2,200–£3,000 per m² for higher-specification or more complex projects
- £3,000+ per m² where the extension involves significant structural work, difficult access, premium finishes or complex detailing
These are broad budgeting figures rather than fixed prices.
The actual cost depends on what is being built and what needs to happen to the existing house.
Typical Double-storey Extension Costs by Size
The following gives a useful early-stage guide:
| Extension Size | Indicative Cost |
|---|---|
| 30 m² | £49,500–£90,000 |
| 40 m² | £66,000–£120,000 |
| 50 m² | £82,500–£150,000 |
| 60 m² | £99,000–£180,000 |
| 70 m² | £115,500–£210,000 |
These figures assume the total floor area across both levels.
For example, a 50 m² double-storey extension might mean approximately:
- 25 m² ground floor
- 25 m² first floor
The final project cost may be higher once kitchens, bathrooms, finishes, professional fees and external works are included.
Why Double-storey Extensions Can Be Cost-efficient
A double-storey extension generally costs more overall than a single-storey extension.
However, the upper floor can sometimes be comparatively efficient because some major construction elements are shared.
For example:
- Foundations serve both floors
- Groundworks are already required
- Roof covers both levels
- External walls extend upward rather than creating a separate footprint
This can make a double-storey extension attractive where more space is needed and planning constraints allow it.
Why Double-storey Extensions Still Cost More Overall
Although the cost per square metre can be efficient, the total project is larger.
Additional costs may include:
- Upper-floor structure
- Additional external walls
- Windows
- Internal partitions
- Bedrooms
- Bathrooms
- Heating
- Electrical work
- - Additional finishes
The project may also involve more complicated work to the existing first floor.
What Is Included in a Double-storey Extension Cost?
A typical construction figure may include:
- Groundworks
- Foundations
- Structural work
- External walls
- Floor structure
- Roof
- Insulation
- Windows and doors
- Plumbing
- Electrical work
- Heating
- Plastering
- Joinery
- Basic finishes
But published figures and builder quotations do not all include the same items.
Some may exclude:
- Kitchen
- Bathroom fittings
- Flooring
- Decoration
- Professional fees
- Structural engineer
- Planning fees
- Building Control fees
- Party-wall costs
- Landscaping
Always check what the price actually includes.
What Does a 30 m² Double-storey Extension Cost?
Using a broad £1,650–£3,000 per m² range, a 30 m² double-storey extension might cost approximately:
£49,500–£90,000
This could represent around 15 m² per floor.
That might be enough to create:
- Additional ground-floor living space
- One additional bedroom upstairs
depending on the existing layout.
What Does a 40 m² Double-storey Extension Cost?
A 40 m² project might cost approximately:
£66,000–£120,000
This could create around 20 m² on each level.
Depending on the design, that may allow:
- Larger kitchen or family space downstairs
- Additional bedroom and bathroom upstairs
What Does a 50 m² Double-storey Extension Cost?
A broad early-stage range would be:
£82,500–£150,000
This could represent approximately 25 m² per floor.
The final figure will depend heavily on:
- Structure
- Glazing
- Kitchen
- Bathroom
- Specification
- Internal remodelling
Double-storey Extension Cost
What Does a 60 m² Double-storey Extension Cost?
A 60 m² double-storey extension might broadly fall around:
£99,000–£180,000
At this scale, the project may substantially change both the ground and first-floor layouts.
Rear Double-storey Extension Cost
A rear double-storey extension can create significant additional space.
It may provide:
- Kitchen or family space downstairs
- Bedroom upstairs
- Bathroom upstairs
- Improved circulation
Costs may increase where the project requires:
- Large structural openings
- Significant roof alteration
- Complex first-floor connections
- Large glazing
Side Double-storey Extension Cost
A side double-storey extension can make effective use of space beside a detached or semi-detached house.
Cost depends on:
- Width
- Boundary conditions
- Existing roof
- Foundations
- First-floor layout
- Access
Double-storey Extension With Kitchen
Many double-storey extensions include a larger kitchen at ground-floor level.
Kitchen costs may include:
- Units
- Worktops
- Appliances
- Plumbing
- Electrical work
- Lighting
- Flooring
- Installation
These costs can add significantly to the project budget.
Double-storey Extension With Bathroom
The upper floor may include:
- New bathroom
- Ensuite
- Shower room
This adds cost through:
- Plumbing
- Drainage
- Sanitaryware
- Tiling
- Ventilation
- Waterproofing
The position of existing drainage can affect how expensive this work becomes.
Structural Costs
Double-storey extensions can require significant structural work.
This may include:
- Steel beams
- Columns
- Floor structure
- Roof structure
- Temporary support
- Openings into the existing house
The complexity of these elements can have a major effect on price.
Foundation Costs
Foundations support both levels of the extension.
Cost depends on:
- Ground conditions
- Nearby trees
- Drainage
- Existing structures
- Foundation depth
Poor or uncertain ground conditions can increase costs considerably.
Upper-floor Structure
The first-floor structure must support:
- Bedrooms
- Bathrooms
- Internal partitions
- Furniture
- Services
The design may use:
- Timber joists
- Engineered joists
- Steelwork
- Other structural systems
The chosen system affects both cost and buildability.
Roof Costs
The roof may be:
- Pitched
- Hipped
- Gabled
- Flat
- Combined with the existing roof
A simple roof form is generally easier and cheaper to build than a complex one.
Roof complexity can increase where the extension must integrate into:
- Existing valleys
- Chimneys
- Dormers
- Multiple roof slopes
Existing Roof Alterations
A double-storey extension often affects the existing roof more than a single-storey project.
Work may involve:
- Extending the roof
- Rebuilding part of the roof
- New gutters
- New valleys
- New drainage
- Structural alterations
These works should be considered early because they can materially affect the cost.
Windows
The upper floor will usually require additional windows.
Cost depends on:
- Number
- Size
- Material
- Glazing specification
- Bespoke design
Larger or more complex windows increase both material and installation costs.
Large Ground-floor Glazing
Many homeowners combine a double-storey extension with large sliding or bifold doors at ground-floor level.
These can increase costs significantly.
The structure above the opening may also require larger beams or more complex support.
Internal Remodelling
A double-storey extension rarely affects only the new area.
The existing house may also need changes such as:
- Removing walls
- Relocating doors
- Changing circulation
- Reconfiguring bedrooms
- Creating a landing
- Moving bathrooms
These costs are not captured simply by measuring the new floor area.
double-storey extension cost
Stair and Circulation Changes
Depending on the design, the first-floor extension may require changes to:
- Landing
- Corridors
- Door positions
- Stairs
A poor circulation arrangement can waste floor area.
Good planning can help maximise usable space without unnecessarily increasing the footprint.
Heating Costs
Additional rooms need heating.
This may involve:
- New radiators
- Underfloor heating
- Boiler upgrades
- Heating controls
The existing heating system should be assessed to determine whether it can support the additional floor area.
Electrical Costs
New rooms require:
- Sockets
- Lighting
- Smoke alarms
- Switches
- Data connections
The cost rises where the project includes more sophisticated lighting or smart-home systems.
Plumbing Costs
New bathrooms or kitchens may require:
- New hot and cold supplies
- Waste pipes
- Soil pipes
- Drainage alterations
Locating wet rooms close to existing services can sometimes reduce cost.
Drainage
Drainage is especially important where the extension footprint covers existing drains.
Work may involve:
- Moving manholes
- Rerouting drains
- New connections
- Additional inspection chambers
Drainage information should be understood before construction where possible.
Site Access
Restricted access can increase costs.
Common issues include:
- Narrow side passages
- Terraced properties
- Difficult scaffolding
- Limited parking
- Restricted delivery space
Double-storey construction can also require more scaffolding than a single-storey extension.
Scaffolding
Scaffolding is normally a significant consideration.
It may be required for:
- External walls
- Roof
- Windows
- Gutters
- Finishes
The duration of scaffold hire can also affect cost if the programme is extended.
double-storey extension cost
Planning Permission
Double-storey extensions are generally more sensitive in planning terms than single-storey additions.
Considerations may include:
- Impact on neighbours
- Overlooking
- Daylight
- Scale
- Character
- Roof form
- Separation distances
Some proposals may fall within permitted development rights, but many will require planning permission.
Building Regulations
Double-storey extensions must comply with applicable Building Regulations requirements.
These may include:
- Structure
- Fire safety
- Insulation
- Ventilation
- Drainage
- Stairs
- Glazing
- Electrical work
Fire Safety
Adding another storey of accommodation can affect fire-safety requirements.
Design may need to consider:
- Escape routes
- Doors
- Smoke alarms
- Protected circulation
- Windows
The requirements depend on the existing property and proposal.
Structural Engineer Costs
A structural engineer may be required to design:
- Steel beams
- Columns
- Foundations
- Floor structure
- Roof structure
Engineering fees vary with the amount and complexity of structural design.
Party Wall Costs
Some double-storey extensions may fall within the scope of the Party Wall etc. Act 1996.
This can depend on:
- Boundary position
- Excavation
- Adjacent structures
- Existing party walls
Any potential surveyor fees should be considered within the overall budget.
Professional Fees
In addition to construction, the budget may need to include:
- Architectural services
- Structural engineer
- Planning
- Building Control
- Party-wall surveyor
- Specialist consultants
These costs should be considered from the outset.
VAT
Check whether VAT is included in all:
- Cost estimates
- Builder quotes
- Published figures
Comparing an inclusive figure with an exclusive one can give a misleading impression of cost.
Contingency
A double-storey extension should include a realistic contingency.
Around 10–15% is a sensible early starting point for many projects.
Older properties or projects with greater uncertainty may justify more.
Double-storey Extension cost
Can I Build a Double-storey Extension for £50,000?
Possibly, but the project would need to be small and straightforward.
At current broad cost ranges, £50,000 is close to the lower end of what might be achievable for a modest double-storey extension.
Expectations around:
- Size
- Specification
- Glazing
- Kitchens
- Bathrooms
would need to remain realistic.
Can I Build a Double-storey Extension for £75,000?
Potentially.
A £75,000 budget may support a relatively modest project depending on:
- Location
- Floor area
- Structure
- Specification
However, kitchens, bathrooms and major internal remodelling could quickly push the cost higher.
Can I Build a Double-storey Extension for £100,000?
Yes, in many cases £100,000 can support a meaningful double-storey extension.
But the final scope will depend heavily on:
- Location
- Size
- Internal works
- Structure
- Specification
Can I Build a Double-storey Extension for £150,000?
A £150,000 budget provides considerably more flexibility.
Depending on the project, it may accommodate:
- Larger floor area
- Better specification
- Additional bathroom
- Structural remodelling
- Higher-quality glazing
But complex or premium projects can still exceed this figure.
Double-storey Extension cost
Is a Double-storey Extension Better Value Than a Single-storey Extension?
Sometimes.
The key advantage is that additional floor area is created without proportionally increasing:
- Foundations
- Roof area
- Ground footprint
This can make the cost per additional square metre attractive.
However, planning constraints and first-floor alterations may make a double-storey solution unsuitable for some properties.
Is a Double-storey Extension Worth It?
That depends on what the house needs.
A double-storey extension can be particularly effective where you want both:
- More living space downstairs
- More bedrooms or bathrooms upstairs
It can avoid creating a large ground-floor footprint while leaving the first floor undersized.
How to Reduce Double-storey Extension Costs
Possible ways to control the budget include:
- Keep the footprint compact
- Use a simple structural grid
- Keep the roof form simple
- Avoid unnecessary bespoke glazing
- Align upper and lower walls
- Keep bathrooms near existing drainage
- Minimise unnecessary internal alterations
- Minimise unnecessary internal alterations
Simple, coordinated design generally costs less to build.
Align the Structure Between Floors
Where practical, placing upper-floor walls over supporting walls below can simplify the structural design.
Complex offsets may require:
- Additional beams
- Transfer structures
- Columns
Good coordination can therefore reduce cost.
Keep the Roof Simple
Complex roof junctions can increase:
- Labour
- Materials
- Weatherproofing risk
- Construction time
A simple roof often provides better cost certainty.
Avoid Oversizing the Extension
More floor area does not automatically mean a better house.
A well-designed 40 m² extension can sometimes work better than a poorly planned 60 m² extension.
The design should respond to:
- Furniture
- Circulation
- Storage
- Natural light
- Existing rooms
not just maximum area.
double-storey extension cost
Get Comparable Builder Quotes
Once the drawings and technical information are developed, suitable builders can be asked to price the same scope.
This provides a better indication of the actual market cost.
Double-storey Extension Budget Checklist
Allow for:
- Main construction
- Groundworks
- Structure
- Upper floor
- Roof
- Windows
- Kitchen
- Bathrooms
- Heating
- Electrical work
- Flooring
- Decoration
- Scaffolding
- Architectural fees
- Structural engineer
- Planning
- Building Control
- Party-wall matters
- External works
- VAT
- Contingency
The builder’s headline construction figure may not represent the total amount required to complete the project.
How Tholos Can Help Control Double-storey Extension Costs
The best time to control cost is during design.
At Tholos Architects, we can help clients consider:
- Floor area
- Planning constraints
- Structural arrangement
- Roof form
- Layout
- Bathrooms
- Glazing
- Specification
in relation to the available budget.
A well-planned double-storey extension can deliver substantial additional space without unnecessary structural or design complexity.
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 double-storey extension cost per m²?
A broad 2026 UK guide is approximately £1,650–£3,000 per m², depending on location, specification and structural complexity.
How much does a 40 m² double-storey extension cost?
A broad early-stage estimate is approximately £66,000–£120,000.
How much does a 50 m² double-storey extension cost?
A broad budgeting range is approximately £82,500–£150,000.
Is a double-storey extension cheaper than two separate extensions?
It can be more cost-efficient because foundations, groundworks and roof serve both floors.
Does a double-storey extension cost twice as much as a single-storey extension?
Usually not.
Adding the second storey increases the total cost substantially, but it does not necessarily double it because some major elements are shared.
Do I need planning permission for a double-storey extension?
Many double-storey extensions require planning permission, although some may fall within permitted development rights depending on the property and proposal.
Is a double-storey extension good value?
It can be, particularly where the household needs both additional ground-floor and first-floor accommodation.
Plan the Extra Floor Around the Budget
A double-storey extension can be one of the most efficient ways to add substantial space to a house.
A useful early benchmark is around:
£1,650–£3,000 per m²
but the real cost depends on much more than floor area.
Structure, roof design, bathrooms, glazing, internal remodelling and specification can all materially affect the final figure.
At Tholos Architects, we can help develop a double-storey extension that balances additional space, planning constraints and construction cost from the outset.
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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