If you need more space and a loft conversion isn't going to give you enough, the next question is: single-storey or double-storey extension? Both add real footprint. They cost differently per square metre, they need different planning, and they suit different homes.
The headline difference
A single-storey extension adds ground-floor space — typically extending a kitchen or living area outwards. A double-storey extension adds the same ground-floor space, plus a matching first-floor extension above it. The first floor usually becomes a bedroom or bathroom extension.
Cost comparison
| Extension type | Footprint | Typical cost (London 2026) | £ per m² |
|---|---|---|---|
| Single-storey (3m × 5m) | 15 m² | £45,000-70,000 | £3,000-4,700 |
| Double-storey (3m × 5m × 2 floors) | 30 m² | £90,000-150,000 | £3,000-5,000 |
| Single-storey (4m × 5m) | 20 m² | £55,000-85,000 | £2,750-4,250 |
| Double-storey (4m × 5m × 2 floors) | 40 m² | £110,000-180,000 | £2,750-4,500 |
Per square metre, double-storey is roughly the same or slightly cheaper. Foundations, roof, and groundworks are shared across two floors — efficient. The absolute cost is significantly higher because you're building twice the area and adding more glazing, more services, more steel for the openings between rooms.
Planning permission — the big decision
Single-storey rear extensions on London terraces and semi-detached homes often fall under permitted development rights — up to 6m deep on terraces (8m on detached) under the larger PD prior-approval route. No full planning permission needed.
Double-storey extensions almost always need full planning permission. Permitted development for two-storey rear extensions is capped at 3m depth, must not exceed the eaves height of the original roof, and is rarely available in conservation areas (where most central London sits). Plan for 8-12 weeks of planning lead time.
Light and overlooking — the planning hot spots
Double-storey extensions raise two planning concerns single-storey doesn't:
- Light loss to neighbours — a two-storey rear extension can cast significant shadow on the adjacent garden/windows. Planning officers apply the 25°/45° rule (the Building Research Establishment 'Site Layout Planning for Daylight' guidance) to test this.
- Overlooking — first-floor windows looking directly into neighbours' gardens or windows. Solutions include obscure glazing, high cill heights (1.7m+), or angled windows.
Address these in the design stage. A good architect or builder familiar with your borough's planning team will know what flies and what doesn't.
Value added on resale
Bedrooms count. In London especially, going from a 2-bed to a 3-bed house, or 3-bed to 4-bed, jumps the property into a different buyer pool with materially higher search filters. A double-storey extension that adds an extra bedroom + bathroom typically returns 80-120% of build cost in property value uplift in family-oriented boroughs (Wandsworth, Battersea, Clapham, Greenwich). Single-storey extensions usually return 60-90% — still positive, but less.
When single-storey is the right call
- You already have enough bedrooms — the issue is downstairs living space
- Permitted development covers it (faster, cheaper start)
- Budget is capped at £45-85k
- You don't want the disruption of removing/extending the upper-floor roof
- You're in a conservation area where double-storey rarely gets permission
When double-storey makes more sense
- You need an extra bedroom AND extra living space
- You're staying long-term (5+ years) — better resale ROI
- Your house has room to extend without dominating the neighbour's plot
- Budget allows £90-180k
- You'd otherwise consider moving — a double-storey extension is often cheaper than buying a bigger house in the same area (especially with stamp duty factored in)
Disruption and timeline
Single-storey extensions: typically 10-16 weeks from site start. Most clients stay in the house throughout — the existing rear wall stays in place until the 'break-through' moment late in the build (often week 8-10), when the new extension is weatherproof and ready to connect to the old kitchen.
Double-storey extensions: 16-22 weeks site time. The upper floor build phase is more disruptive — scaffolding goes higher, the existing upper-floor rear bedroom is unusable for several weeks during the connection. Many clients with double-storey extensions move out for 4-6 weeks during the most intense phase.
Frequently asked questions
Can I build a single-storey first and add a double-storey later?
Yes, but it's almost always more expensive than building double-storey in one go. You pay for foundations, scaffolding, building control, etc. twice. If you know you'll eventually want the upper floor, build it now.
Will a double-storey extension need an architect?
Yes — you'll need detailed drawings for planning permission, structural calculations, and Building Regulations approval. A good architect for a London double-storey extension typically charges £4,000-8,000 for the full design + planning service. Some builders include this in their quote via a partner architect.
Can I do a double-storey on a terraced house?
Yes, but the planning team in your borough will look hard at it. Terraced houses are usually constrained by the neighbours' rear extensions setting a precedent — match the depth and height of what's already been approved either side, and approval is much more likely.
How does the Party Wall Act apply to double-storey extensions?
More notices, potentially. You serve notice for any excavation within 6m of the boundary (foundations are deeper for double-storey), for any new wall on or near the boundary, and for cutting into the existing party wall. Most double-storey extensions trigger 2-3 Party Wall notices.
Are double-storey extensions allowed on flats?
Almost never on the upper floor of a converted flat (you don't own the airspace). On a freehold semi or terraced house, yes — that's the standard use case.
Related services
House Extension
Single-storey and double-storey extensions across London — we build both regularly.
Learn more →Loft Conversion
Sometimes the answer to 'more bedrooms' is up rather than out — see our loft conversion service.
Learn more →Extension Cost Factors
The 11 things that drive your extension price.
Learn more →Glass Extension Cost Guide
What a glazed-box or glass extension really costs in London, and what drives the price.
Learn more →Single-storey or double — which is right for your home?
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