Double Storey Extension London: Costs, Planning, and What to Expect
Title: Double Storey Extension London: Costs, Planning Permission, and Design Guide (2026) Target keyword: double storey extension London
A double storey extension — extending at both ground and first floor level simultaneously — is one of the most efficient ways to add significant living space to a London home. You're effectively building two rooms for the price of a structure that's only marginally more expensive than a single-storey extension of the same footprint.
Why Double Storey?
The cost efficiency is the main argument. A single-storey 20m² extension might cost £65,000–£95,000 all-in. A double-storey extension of the same footprint adds a second room directly above for an additional £30,000–£50,000 — the foundations, roofline, and most of the labour are shared.
For families who need both a ground-floor kitchen/diner and an additional bedroom or bathroom above, a double-storey extension is usually a more cost-effective solution than building the ground-floor extension and then separately converting the loft.
Costs
| Footprint | Ground floor only | Double storey |
|---|---|---|
| 16m² | £60,000–£85,000 | £90,000–£125,000 |
| 24m² | £85,000–£120,000 | £125,000–£175,000 |
| 32m² | £110,000–£155,000 | £165,000–£225,000 |
All figures are rough guides for London, including structure, build, and basic finishes but excluding kitchen and bathroom fitout.
Planning Permission
Double-storey extensions almost always require full planning permission — they generally exceed PD thresholds, particularly when they extend rearward and upward simultaneously. Planning considerations include:
- Overlooking: The first-floor level of the extension may overlook neighbouring gardens or rear windows. Planning authorities will assess this carefully.
- Overbearing impact: A two-storey structure at the rear of a terrace can feel overbearing to neighbours whose gardens adjoin yours. The relationship between the extension and the rear boundary is assessed.
- Permitted development: Rear extensions over one storey are generally not PD (with very limited exceptions). Full planning is normally required.
- Design quality: First-floor extensions need to "read well" — their design needs to be coherent with the main property, not simply bolted on.
Planning applications for double-storey extensions in London have varied success rates depending on borough and design. A well-designed application with appropriate set-back from boundaries and considered overlooking solutions typically fares better.
Structural Considerations
A double-storey extension is structurally more complex than a single-storey: - Foundations: Must carry the load of two storeys — deeper and more reinforced than a single-storey spec - First-floor structure: An intermediate floor must be designed to building regulations standards - Structural steelwork: The ground-floor openings carry first-floor loads above — a properly sized structural engineer-specified RSJ is essential - Roof: Typically pitched to match or complement the existing property - Party wall: The Act almost certainly applies — notify both neighbours if the extension runs close to both boundaries
Programme
A double-storey extension typically takes 18–26 weeks from start on site, depending on size and complexity. The main phases are: - Groundworks and foundations: 3–5 weeks - Structure (ground and first floor): 5–8 weeks - First-fix across both floors: 3–4 weeks - Plasterwork, second-fix, and finishes: 5–7 weeks - Snagging and BCO final: 2 weeks
At rcbGroup, we manage every phase from pre-construction through to practical completion. One contract. One team. One clear line of communication.
Call +44 7359 872594 or visit rcbGroup.co.uk to discuss your double-storey extension.
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