Double-Storey Extensions in London: What You Get, What It Costs, and What to Expect

The decision between a single-storey and a double-storey rear extension comes down to what you need and what planning permission you are likely to achieve. A single-storey extension creates additional ground floor space. A double-storey extension creates additional ground floor space and additional first-floor space. The additional cost of going double-storey over single-storey is relatively modest in the context of the total project cost: you have the same foundations, the same ground floor structure, and similar overall contractor programme. The incremental cost of the first floor (floor structure, walls, staircase modification, and roofing at first floor level rather than ground floor level) is approximately 50-60% of the ground floor build cost per m², rather than 100% — because foundations, ground preparation, and roof are already being built. This means a double-storey extension that adds 25m² on each floor (50m² total) might cost only 40-50% more than a single-storey extension of 25m². The economic argument for going double-storey is therefore strong where you need both living space and bedroom capacity, and where planning permission for the additional storey is achievable. The argument against is planning: a double-storey extension has significantly more planning sensitivity than a single-storey extension, particularly regarding impact on neighbouring properties, and planning permission is not guaranteed.

Double-storey rear extensions in London always require a full householder planning application. Planning authorities assess double-storey extensions against the following criteria: Impact on adjacent properties — the primary concern. A double-storey extension that projects significantly into the rear garden of a terraced house will be seen from the first-floor rear windows of both neighbouring properties, and potentially from the first floor of properties facing the rear of the site. Daylight and sunlight assessment (BRE methodology) is more likely to be required for a double-storey extension than for a single-storey project. Overlooking from new first-floor windows — a new bedroom window at first-floor level that directly faces a neighbour's rear garden or first-floor bedroom window will be scrutinised carefully. Planning authorities typically require either a minimum separation distance (commonly 10-12 metres between facing first-floor windows in urban London conditions) or the use of obscure glazing for windows that directly face neighbours. Overbearing impact — a double-storey extension close to the boundary between properties is likely to present a significantly more overbearing elevation to the neighbouring property than a single-storey extension. The 45-degree rule (assessing whether the proposed extension intrudes into a 45-degree line drawn from the affected window) applies to both plan and section for double-storey extensions. Design and character — the planning authority will assess whether the double-storey extension is designed in a way that complements the existing house and is in keeping with the character of the area. A poorly designed double-storey extension that looks significantly different from the host building or the surrounding street scene is more likely to face objection than a well-integrated design.

The most common double-storey rear extension layout in a London terraced or semi-detached house: Ground floor — kitchen-dining or living-dining space, typically 20-35m², with bi-fold or sliding doors opening to the garden. This mirrors the typical single-storey extension layout in terms of function and specification. First floor — one or two bedrooms plus an ensuite bathroom or shower room. Where space allows, a master bedroom with ensuite is the most common first-floor arrangement, transforming what was a 3-bedroom house into a 4-bedroom house with en suite facilities that it previously lacked. In a semi-detached house with a wider first-floor footprint, two bedrooms on the first floor of the extension (making a 5-bedroom house total) are achievable. The staircase arrangement for a double-storey extension: the existing staircase typically serves the main first-floor landing, and the new first-floor bedroom(s) in the extension are accessed via a corridor or extended landing from the existing staircase — not a separate stair. This requires careful planning of the first-floor layout, particularly in terraced houses where the existing staircase and landing geometry may be constrained. Occasionally, a separate secondary stair is incorporated where the corridor connection to the extension would be very long or impractical.

A double-storey rear extension has structural requirements that are more complex than a single-storey extension: Foundations — the load carried by the double-storey extension structure is approximately twice that of an equivalent single-storey extension. Foundation design must account for these loads, particularly at the corners and beneath structural walls. Strip foundations to typical London depths (900-1,200mm to allow for tree root zones and London Clay seasonal movement) are the standard approach, but deeper piled foundations may be required in areas with significant tree coverage or made-up ground conditions. Ground floor structure — as for a single-storey extension, the ground floor slab or suspended floor is designed to carry domestic floor loads. First floor structure — a structural floor is required at first floor level. The most common approach in a double-storey London extension is a timber joist floor (engineered timber I-joists or solid timber joists spanning between structural walls or steel beams) with acoustic insulation between the joists to reduce sound transmission from the bedroom above to the kitchen or living room below. Where the ground floor of the extension is an open-plan space without intermediate supporting walls, the first floor structure must span the full depth of the extension on steel beams carried by the side walls and the rear wall of the existing house. These structural beams are often visible (boxed in) and affect the ceiling height of the ground floor space beneath. First-floor walls and roof — external walls at first floor level are typically built in the same material as the ground floor walls for consistency; the roof structure at first-floor level (which is typically a flat roof or a pitched roof designed to sit below the existing first-floor windows of the host building) closes the structure. Party wall considerations are more significant for double-storey extensions than single-storey — the additional loading and height of the structure may affect the shared party wall with greater consequence.

Indicative total project costs (build + professional fees + VAT) for a double-storey rear extension on a standard London terrace or semi-detached house: Small double-storey extension (5m x 4m on each floor = 40m² total floor area, mid-range specification): £90,000-£130,000. Medium double-storey extension (5m x 5m on each floor = 50m² total floor area, mid-range specification with mid-range kitchen and bathroom): £120,000-£170,000. Larger double-storey (6m x 5m on each floor = 60m² total floor area, higher specification): £160,000-£220,000. These figures include: all structural work; architect and structural engineer fees; planning and Building Regulations applications; ground floor kitchen or living space fit-out; first-floor bathroom or ensuite to mid-range specification; decoration; and VAT at 20%. They do not include furniture, carpets, or landscaping. The cost per m² of floor area for a double-storey extension (typically £2,200-£3,500 per m² of gross internal area added) is lower than for a single-storey extension of similar plan area (typically £2,500-£4,000 per m²), reflecting the economies of scale from building two floors on one foundation and one structural envelope.