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 RCB, we manage every phase from pre-construction through to practical completion. One contract. One team. One clear line of communication.