A double storey extension in 2026 costs £1,800–£3,000 per m² per floor outside London, or £2,500–£4,000/m² in the capital. A typical 4m×5m footprint (40m² total across two floors) costs £65,000–£100,000. Total project costs range from £50,000 to £120,000+ depending on size, specification, and location. Crucially, this is not double the cost of a single storey — expect 50–70% more, because you share the foundations, roof, and groundwork.
Cost by Size
3m × 4m footprint (24m² total)
One bedroom and en-suite above, utility or study below. The smallest practical double storey. 14–16 weeks build time.
4m × 5m footprint (40m² total)
Kitchen-diner downstairs, master bedroom with en-suite above. The sweet spot for cost vs space. 16–20 weeks.
4m × 6m footprint (48m² total)
Large open-plan ground floor, two bedrooms or one bedroom plus bathroom above. Serious family upgrade. 18–22 weeks.
5m × 6m footprint (60m² total)
Substantial addition — equivalent to a small house. Open-plan living below, two bedrooms and family bathroom above. 20–24 weeks.
3m × 8m footprint (48m² total)
Full-width rear extension on a terrace. Long and narrow. More steelwork due to the span. Kitchen-diner below, two bedrooms above.
5m × 8m footprint (80m² total)
Major project on a detached house. Three bedrooms and a bathroom above, full living space below. Equivalent to a small semi-detached. 22–28 weeks.
Why a Double Storey Isn't Double the Price
This is the single most important thing to understand about double storey extensions. You're not building two separate structures — you're building one structure with two floors. The expensive bits (foundations, groundwork, roof, scaffolding, drainage connections) are shared between both floors.
What you share (no extra cost)
- Foundations — same footprint, just slightly deeper (typically 150–200mm more depth for the extra load)
- Roof — one roof covers both floors, same as a single storey
- Groundwork and drainage — identical excavation, identical connection to the existing drainage
- Scaffolding — you'd need scaffolding for a single storey roof anyway; going higher adds marginally
- Planning and design fees — one application, one set of drawings
What the second floor adds
- First-floor structure: Steel beams, timber joists, or engineered joists for the floor — £2,000–£5,000
- Staircase: Standard timber staircase £2,000–£3,500, oak or bespoke £4,000–£8,000
- Additional blockwork: Another 2.4m of external walls — £3,000–£6,000
- Extra windows and doors: First-floor windows, fire doors — £1,500–£4,000
- Second fix upstairs: Electrics, plumbing, plastering, decorating — £4,000–£10,000
- Fire regulation upgrades: FD30 doors, mains-wired alarms, protected stairwell — £1,500–£3,000
- Deeper foundations: The additional load means foundations go 150–200mm deeper — adds £800–£2,000
💰 The maths in plain English
A 4m×5m single storey extension might cost £40,000. The same footprint as a double storey costs £75,000–£90,000 — roughly 90–125% more space for 50–70% more money. That's why double storey extensions are the best value per square metre of any extension type. You're getting a bedroom and bathroom upstairs essentially at half the per-m² rate of the ground floor.
Planning Permission — Always Required
Unlike single storey rear extensions, double storey extensions do not fall under permitted development in England. You will need full planning permission. No shortcuts, no prior approval route.
- Planning application fee: £462 (England, 2026)
- Typical decision time: 8 weeks (minor application), but allow 10–12 weeks in practice
- Maximum rear projection: 3m from the original rear wall of the house
- Minimum distance to boundary: Must be at least 7m from the rear boundary
- Ridge height: Must not exceed the ridge of the existing house
- Eaves must match: The eaves of the extension should match or be lower than the existing eaves
- Materials: Must match the existing house (same brick, same tiles, or similar)
- Windows: Side-facing first-floor windows must be obscure-glazed and non-opening below 1.7m (to protect neighbour privacy)
Scotland and Wales: Different planning rules apply. Scotland uses a different PD system and has its own householder application process. Wales has separate PD legislation (the Town and Country Planning (General Permitted Development) (Amendment) (Wales) Order). Always check with your local planning authority.
Party Wall Agreements
If you live in a semi-detached, terraced, or end-of-terrace house, you will almost certainly need a Party Wall Award under the Party Wall etc. Act 1996. This applies when:
- Building on or at the boundary line with your neighbour
- Excavating foundations within 3m of a neighbouring structure
- Excavating within 6m of a neighbouring structure if your foundations will go deeper than theirs
- Cutting into, raising, or underpinning a shared party wall
Cost: If your neighbour agrees (a "consent" under the Act), it's effectively free — just a written agreement. If they dissent and appoint their own surveyor, expect to pay £700–£1,500 per neighbour. With two neighbours (e.g., a mid-terrace), that's potentially £1,500–£3,000. This is your cost — the Act says the building owner pays for the neighbour's surveyor.
Timing: Serve your Party Wall Notice at least 2 months before you plan to start. If neighbours are slow to respond, this can delay your start date. Serve notices as soon as you have planning permission — don't wait for building regs.
Fire Regulations (Building Regs Part B)
Adding a second floor triggers additional fire safety requirements under Building Regulations Approved Document B. These aren't optional and your building control officer will inspect them:
- 30-minute fire doors (FD30): Required on all habitable rooms opening onto the stairwell — bedrooms, living rooms, kitchen. Budget £150–£300 per door including frame and intumescent strips. You'll need these on existing doors too, not just the new ones.
- Mains-wired smoke alarms: Interlinked alarms on every floor, including existing floors. Battery-only is not acceptable for new building work. A heat detector is required in the kitchen. Budget £400–£800 for the full system.
- Protected stairwell: The staircase must be enclosed with fire-rated construction so occupants can escape. This means fire-rated plasterboard (or existing masonry walls) around the stair from ground to first floor.
- Escape windows: First-floor habitable rooms must have at least one window that opens wide enough to escape through (minimum 450mm × 450mm clear opening, with the bottom no more than 1100mm above floor level).
- Self-closing devices: Fire doors must be fitted with self-closing devices (overhead closers or rising butt hinges).
Important: The fire door requirement applies to the entire house, not just the extension. If your existing bedroom doors aren't FD30, they'll need replacing or upgrading. This catches many homeowners off guard — budget £1,500–£3,000 for door replacements across the whole property.
Staircase Options and Costs
You need to get upstairs somehow, and the staircase is one of the trickiest design decisions. Where it goes affects the layout of both floors.
Internal staircase (from existing hallway)
The neatest solution — extending or rerouting the existing staircase. This only works if you have space in the existing hallway or landing. You'll lose some ground-floor space. Cost: £2,500–£5,000 for a standard softwood staircase with modification to the existing structure.
Staircase within the extension
The most common approach. The staircase sits inside the new ground floor, eating into your downstairs space (typically 3–4m²). Keep it against an external wall to minimise the impact on the open-plan layout. Cost: £2,000–£3,500 for standard softwood; £4,000–£8,000 for oak or glass balustrades.
External access (rare)
Occasionally used when the upper floor is a self-contained space (home office, annex). An external staircase requires planning permission consideration and weather protection. Cost: £3,000–£6,000 for a steel external stair with platform.
📐 Building Regs for stairs
Minimum width 800mm for domestic stairs. Maximum rise per step: 220mm. Minimum going (tread depth): 220mm. Headroom: minimum 2m measured vertically above the pitch line. The staircase must comply with Approved Document K — your architect will design this, but be aware that tight spaces can make compliance tricky.
Structural Steelwork
Double storey extensions need more steel than single storey. The structural engineer's design is critical — this is the skeleton of your extension.
- Ground floor opening (RSJ): To open up the rear wall into the existing house. Typically a 203×203 UC or similar. £1,200–£3,000 supplied and installed.
- First-floor beam: Supporting the new first floor. Size depends on span and load. £1,000–£2,500.
- Goalpost frame: For wide openings at ground floor (4m+ bi-fold or sliding doors). Two columns and a beam. £2,500–£5,500.
- Padstones and bearing plates: Where steels sit on blockwork. £50–£150 each.
- Connection steels: Tying new structure to existing. £500–£2,000 depending on complexity.
Total steelwork budget: £4,000–£12,000 for a typical double storey extension, versus £1,500–£5,000 for a single storey. The structural engineer's fees (£500–£1,500) are on top.
Regional Pricing (Per m² Per Floor)
These rates are for a standard-specification double storey extension — cavity blockwork walls, pitched tile roof, standard windows, plastered and ready for decoration. Kitchen, bathroom, and premium finishes are extra.
What Affects the Price
Foundation depth and ground conditions
Double storey extensions need deeper foundations — typically 1m minimum, deeper on clay or near trees. Near mature oaks on shrinkable clay, you might need piled foundations at £1,500–£3,000 per pile. A ground investigation (£500–£800) can save tens of thousands by identifying problems early.
Roof design
Most double storey extensions have a pitched tile roof to match the existing house. A simple lean-to or gable end is cheapest (£5,000–£10,000). A hipped roof matching existing adds complexity (£8,000–£15,000). Integrating into the existing roof line (cutting in) costs more due to lead work and weather-tightness.
Structural complexity
Wide openings on the ground floor, large spans, and connections to the existing structure all add cost. A simple extension bolted onto the rear with one opening is much cheaper than removing the entire rear wall and integrating open-plan across the full width. Budget £4,000–£12,000 for steelwork.
First-floor specification
What goes upstairs matters. A basic bedroom with a radiator and some sockets: relatively cheap. A master suite with en-suite shower, fitted wardrobes, and underfloor heating: add £8,000–£15,000. An en-suite bathroom alone costs £3,000–£8,000 to fit out.
Staircase position and design
A staircase within the new extension is simpler. Modifying the existing staircase or creating a new opening through an existing floor costs more in structural work and making good. Budget £2,000–£8,000 depending on material and complexity.
Site access and scaffolding
Double storey means full scaffolding — not just a tower but a proper scaffold wrap. Budget £2,500–£6,000 for scaffolding hire over the build period. Restricted access (no side passage, everything through the house) adds £3,000–£8,000 in labour and logistics.
Matching existing materials
Planning will likely require matching bricks, tiles, and window styles. Sourcing matching bricks for a 1930s house can cost £1–£2 per brick versus £0.50 for standard. Matching clay roof tiles add a premium over concrete. Budget an extra £2,000–£5,000 for material matching.
Fire safety upgrades
FD30 fire doors throughout the house (not just the extension), mains-wired interlinked alarms, protected stairwell construction. This affects existing rooms too. Budget £1,500–£3,500 for the fire safety package across the whole property.
Heating system capacity
Adding 40–80m² of heated space may exceed your existing boiler's capacity. A boiler upgrade costs £2,500–£4,500. Extending the central heating system with radiators: £1,500–£3,000. Underfloor heating on both floors: £3,000–£6,000.
Electrics — new consumer unit
The additional circuits for a double storey extension often require a new consumer unit or sub-board. If your existing board is an old rewireable fuse box, building regs will require a full upgrade. Budget £400–£800 for the unit itself, plus £2,000–£4,000 for first and second fix across both floors.
How to Save Money on a Double Storey Extension
💡 Practical cost-cutting strategies
- Keep the footprint rectangular. L-shaped or stepped plans add complexity to the roof, foundations, and structure. A simple rectangle minimises steelwork, speeds up the build, and keeps costs predictable.
- Match the existing roof pitch and line. If the extension roof ties into the existing roof seamlessly, it's cheaper than creating a new roof form. It also helps with planning approval.
- Avoid moving the soil stack. Position the upstairs bathroom or en-suite directly above or very close to the existing soil stack. Running a new soil pipe up is far cheaper than relocating an existing one (saves £2,000–£4,000).
- Use the existing staircase route. If you can extend the existing landing rather than building a completely new staircase, you save the cost of a new stair (£2,000–£5,000) and preserve ground-floor space.
- Standardise window sizes. Bespoke windows cost 30–50% more than stock sizes. Plan around standard dimensions (e.g., 600×900mm, 1200×1200mm) wherever possible.
- Keep the first floor simple initially. Budget tight? Have the builder complete the shell upstairs — plastered, basic electrics, heating — and fit it out yourself later. The structure costs the same regardless.
- Build in winter. November–February pricing can be 5–10% lower. Groundwork in wet weather is slower, but the overall savings are real. Most trades welcome the work.
- Choose one en-suite, not two. Every additional bathroom adds £3,000–£8,000. One good en-suite is usually enough — a second bathroom is a luxury that can wait.
- Get the party wall process started immediately. Delays from party wall disputes push back start dates, and builders may increase prices for a later start. Serve notices the day you get planning permission.
What Should Be in a Builder's Quote
A double storey extension quote is more complex than a single storey. Make sure it clearly covers both floors:
- Groundwork and foundations — excavation, concrete, DPM, drainage connections, ground-floor slab
- Ground-floor structure — blockwork, cavity insulation, wall ties, DPC, steels and lintels
- First-floor structure — floor joists or engineered beams, strutting, decking, insulation
- First-floor walls — additional blockwork, cavity, insulation, lintels
- Roof — structure (rafters, purlins), insulation, battens, tiles or slates, fascias, soffits, guttering, lead work
- Staircase — supply and install, including any structural modifications to existing floors
- Windows and external doors — both floors, including bi-folds or sliders if specified
- Fire doors (FD30) — new and existing rooms opening onto the stairwell
- First and second fix electrics — both floors, consumer unit upgrade if required
- First and second fix plumbing — heating, hot/cold supplies, en-suite or bathroom rough-in
- Plastering — walls and ceilings, both floors
- Decoration — mist coat and emulsion (often excluded — clarify)
- Scaffolding — full scaffold for the build duration (often 12–20 weeks)
- Skip hire and waste removal
- Building regs compliance — though the fee is usually separate
Common exclusions: Kitchen fitting, bathroom suites and tiling, flooring, fitted wardrobes, landscaping, building regs fees, planning fees, party wall costs, structural engineer fees, architectural fees. Clarify everything before signing.
Hidden Costs to Budget For
Planning Application
Always required for double storey. No PD route. Allow 8–12 weeks for a decision. Pre-application advice (£250–£600) is worthwhile for complex proposals.
Architectural Drawings
Design, planning drawings, and building regs package. Double storey is more complex than single — more sections, more details. Worth the investment in a good architect.
Structural Engineer
Foundation design, steel calculations, floor structure design. More involved than a single storey due to additional loads and floor structure.
Party Wall Surveyor(s)
£700–£1,500 per neighbour if they dissent. Almost always needed for semi-detached and terrace houses. Start the process early to avoid delays.
Building Regulations
Full plans application for both floors, including fire safety, structural, thermal, and drainage compliance. Inspections throughout the build.
Fire Door Upgrades
FD30 doors required on ALL habitable rooms opening onto the stairwell — including existing rooms. Often 4–8 doors across the whole house. £150–£300 each fitted.
Boiler Upgrade
Your existing boiler may not have the capacity for the extra radiators and hot water demand. Common on 15+ year old systems.
Garden Reinstatement
The garden takes a bigger hit with double storey — more materials, longer build, more scaffolding damage. Budget for patio, fencing, and re-turfing.
Temporary Accommodation
Major disruption for 16–24 weeks. Most families stay put but some need short-term accommodation during the most disruptive phase (roof open, no heating).
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