Best fit
Whole-home expansion where the project needs both ground-floor living space and new first-floor rooms.
Double storey extension costs
Double Storey Extension Cost Calculator UK
Estimate the cost of a two-storey extension using total floor area, layout, finish level and region. This calculator reflects how costs change when foundations, structure and roof work are spread across two levels.
This page is part of the extensions cost calculators section. If you are comparing one-storey and two-storey options, check the single storey extension cost calculator as well. For assumptions and data handling, see our methodology.
Use this estimate before speaking with builders
A double storey extension needs more early planning than a simple ground-floor build. The total budget depends on floor area, foundations, roof design, structural tie-ins, access and the amount of internal fit-out. CostIntel gives you a practical planning range before you ask builders for site-specific quotes.
Is a double storey extension the right project type?
A double storey extension is usually worth considering when you need both downstairs and upstairs space. It can add a larger kitchen or living area on the ground floor while creating bedrooms, a bathroom, office space or storage above.
It may be too much project if you only need a larger kitchen or family room. In that case, compare the numbers with the single storey extension cost calculator. If the main aim is upstairs space only, a loft conversion cost calculator may give a useful comparison.
Main advantage
The build can produce more floor area from the same footprint, which can improve cost efficiency per square metre.
Main caution
The project is more exposed to foundation design, planning constraints, roof integration and structural detailing.
What affects double storey extension cost?
The cost is shaped by more than the headline size. A two-storey structure places extra load on the ground, changes how the roof connects to the existing house and often adds more services and internal work than a single-level project.
Foundations and load
The foundations need to support two levels. Ground conditions, excavation depth and structural design can change the price early in the project.
Roofline integration
The new roof must tie into the existing property. Pitch, drainage, coverings and shape all affect labour and materials.
Internal layout
Two floors can mean more partitions, electrics, heating, plumbing, bathrooms and decorating. The fit-out can move the final quote sharply.
Typical UK double storey extension cost ranges
These figures are planning ranges. They are most useful when compared with your calculator result and the project details a builder will inspect on site.
| Project type | Typical total size | Estimated range | Why the range moves |
|---|---|---|---|
| Small double storey extension | 35–45 m² | £70,000–£120,000 | Fixed costs are spread across a smaller area, so the cost per m² can remain high. |
| Standard rear double storey extension | 45–65 m² | £85,000–£160,000 | Structure, roof integration and internal finish usually drive the spread. |
| Side double storey extension | 50–75 m² | £100,000–£190,000 | Boundary position, side access and roofline work can add cost. |
| Large two-storey addition | 75–110 m² | £150,000–£280,000+ | Multiple rooms, more services, roof changes and higher finishes increase the total. |
Why double storey extensions can be better value per m²
A double storey extension can look expensive because the total budget is higher. The cost per m² can still compare well because the project adds floor area faster than it adds some major fixed costs.
Foundations, scaffolding, site setup and roof work are not simply doubled when a second level is added. That is why a two-storey project can sometimes be more efficient than building the same amount of floor area across one level.
This advantage has limits. Bathrooms, complex roof shapes, higher finishes and major internal reconfiguration can reduce the cost efficiency. For a closer floor-area comparison, use the extension cost per m² calculator.
Real-world pricing scenarios
These examples show why a double storey estimate needs both total cost and cost per m². A project can look efficient by floor area while still needing a large cash budget.
50m² rear double storey extension
A 25m² footprint over two floors can add a larger kitchen or living area below and bedroom or office space above. It may sit near the middle of the range if the roof connection and foundations are straightforward.
60m² side double storey extension
A side extension can add useful rooms, but the price can rise when boundary access is tight or the new roofline is difficult to match with the existing house.
80m² large two-storey addition
A larger project may produce stronger cost per m², but the total budget can rise quickly once bathrooms, heating, glazing and higher specification finishes are included.
Why your quote might be higher than the estimate
The largest increases often come from site-specific details. A builder may adjust the quote after checking ground conditions, access, roof shape, drainage, structural openings and the number of rooms being fitted out.
Deeper foundations
Two levels place more demand on the structure. Poor ground, nearby trees or older foundations can increase excavation and design requirements.
Roof complexity
Matching the existing roofline can be simple on some homes and difficult on others. Pitch, valleys, drainage and coverings all affect cost.
Structural openings
Open-plan ground floors, large rear openings and major wall removal can require steel beams, temporary support and structural calculations.
Upstairs bathrooms and services
Bathrooms, heating, electrics, plumbing and drainage can add more cost than a simple bedroom or office layout.
If the extension includes new bathrooms or plumbing work, compare related allowances with the bathroom plumbing cost calculator and bathroom renovation cost calculator.
Single storey vs double storey extension cost
The better option depends on what space you need. A single storey extension is usually cheaper overall and often enough for a larger kitchen, dining area or family room. A double storey extension costs more, but can be better value if you need upstairs rooms as well.
| Factor | Single storey | Double storey |
|---|---|---|
| Total cost | Usually lower | Usually higher |
| Cost per m² | Often higher | Often lower |
| Best for | Kitchen, dining and living space | Whole-home expansion with upstairs rooms |
| Structural demand | Lower | Higher |
Compare this estimate with the single storey extension cost calculator if the project could work on one level.
Cost per square metre needs context
Cost per m² is useful for comparing one extension option with another, but it can hide the cash difference between projects. A double storey extension may look efficient per m² while still needing a much larger total budget.
Use cost per m² as a comparison tool, not as the only decision point. The number makes most sense when viewed beside the total cost, the rooms created and the amount of structure involved.
How to read the result
Total costBudget reality
Cost per m²Efficiency check
Rooms createdValue context
Planning and design considerations
Double storey extensions usually need closer design attention because they change the external shape of the house more than a ground-floor addition. Roof height, neighbouring windows, side boundaries, overlooking, materials and the relationship with the existing house can all affect what is realistic.
The calculator does not replace planning advice or a design review. It gives an early cost range so you can judge whether the idea is worth developing. For how CostIntel handles assumptions, read pricing data and how costs are calculated.
What to prepare before getting quotes
A quote is more useful when the builder can understand the intended footprint, total floor area, roof connection, likely room layout and finish level. For double storey projects, it also helps to know whether the upstairs space includes bathrooms or mostly bedrooms and storage.
Try to separate the main build from high-cost fit-out items. Bathrooms, large glazing, kitchen work, premium flooring and heating upgrades can make two quotes look very different even when the structure is similar.
Quote-ready summary
Example brief: 50 m² rear double storey extension, standard finish, typical complexity, Rest of England pricing, VAT included and 10% contingency.
Alternative projects worth comparing
A double storey extension is not always the most efficient route. If the main need is downstairs living space, a single-level project may be enough. If the main need is bedrooms or office space, converting existing structure may be worth comparing before committing to a larger build.
Single storey extension
Compare this when the goal is a larger kitchen, dining area or ground-floor living space.
Loft conversion
Useful when the main aim is extra bedroom, office or bathroom space upstairs.
Garage conversion
Compare this if suitable existing structure could be reused for living or work space.
Basement conversion
Relevant where below-ground space is possible, but structure and waterproofing need careful allowance.
Related extension cost calculators
Use these calculators to compare the double storey estimate with nearby project types inside the extensions cost cluster.
Home extension
Broad estimate across common UK extension types.
Single storey extension
Estimate one-level rear, side and kitchen extension costs.
Extension cost per m²
Compare extension costs by floor area and total budget.
Loft conversion
Compare roof-space conversion with building a two-storey addition.
Garage conversion
Estimate costs where existing structure can be reused.
Basement conversion
Estimate below-ground conversion costs and structural allowances.
Double storey extension cost FAQs
How much does a double storey extension cost in the UK?
Many standard double storey extensions fall between £85,000 and £180,000. Large or complex builds with roof changes, bathrooms, difficult access or higher finishes can exceed £250,000.
Is a double storey extension cheaper per m²?
Often, yes. The total cost is higher, but the cost per m² can be lower because foundations, roof work, scaffolding and site setup are spread across two levels.
Is it cheaper to build single storey or double storey?
Single storey is usually cheaper overall. Double storey can be better value when you need both downstairs and upstairs space.
Does a double storey extension need stronger foundations?
Usually, yes. The foundations need to support the extra load, so ground conditions and structural design can affect the quote.
Does adding a bathroom upstairs increase cost?
Yes. Bathrooms add plumbing, drainage, waterproofing, ventilation, tiling, fixtures and extra labour.
Should I include VAT and contingency?
For early budgeting, yes. VAT and contingency give a more realistic project allowance than looking at base build cost alone.