Double-Storey Extensions in London: What to Expect
A double-storey rear extension adds habitable space on two floors in a single project — a ground-floor kitchen, dining, or living addition below and an additional bedroom, study, or bathroom above. It almost always requires full planning permission, but it typically represents significantly better value per square metre than two separate single-storey projects phased over time.
What a Double-Storey Extension Adds
The most common configuration in London is: ground floor — extension of the kitchen into a full open-plan kitchen-dining-living room; first floor — addition of a new bedroom, or expansion of the main bedroom, or addition of an en-suite bathroom. In some cases, the first floor element is used for a home office, a utility room with roof terrace access, or additional bathroom capacity.
On properties with a side return, a double-storey project can also extend across the side at first-floor level — adding width to a first-floor bedroom and significantly transforming what can feel like the cramped upper floors of a Victorian terrace.
Planning Considerations
Double-storey rear extensions almost always require full planning permission. The key planning concerns are:
Overlooking. A first-floor rear extension with rear-facing windows can directly overlook neighbouring gardens. Many boroughs apply a 45-degree test or similar guideline to assess overshadowing impact. Roof lights rather than rear-facing windows on the upper floor can sometimes resolve this.
Daylight and sunlight to neighbours. A two-storey mass at the rear of a property has more impact on neighbouring properties than a single-storey addition. BRE daylight and sunlight assessments may be required for more complex or sensitive schemes.
Relationship to the host building. Planning policy generally requires that extensions respect the scale, proportion, and character of the original building. A double-storey extension that is too wide, too tall, or uses incompatible materials is more likely to attract refusal than a well-designed, properly proportioned one.
Structural Considerations
A double-storey extension requires more robust foundations than a single-storey addition — the foundation design must carry the additional load of the upper floor. The structural connection between the new extension walls and the existing building requires careful engineering, particularly at first-floor level where the new floor structure ties into the existing house.
Cost Range for Double-Storey Extensions in London
Small to medium (3m x 4m footprint, two floors): £90,000–£140,000 for build-only cost.
Medium to large (4m x 5m+, two floors): £140,000–£220,000+ depending on specification, glazing system, and structural complexity.
The cost per square metre for a double-storey extension is typically lower than two separate single-storey projects of the same floor area — reflecting the efficiencies of building in one mobilisation and under one set of scaffolding and temporary works.
RCB designs and builds double-storey extensions across London — managing the planning, structural coordination, and principal contractor delivery from first meeting to handover.
07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk