Double Storey Extensions in London: Design, Planning and Costs

A double storey rear extension at the rear of a London Victorian terrace or semi-detached house typically provides: Ground floor: enlarging the existing kitchen, adding a utility room, extending the dining/living space, or creating a new family room at the rear of the house. The ground floor addition is typically the same as a single-storey rear extension in its spatial quality — but the structural system above (the first floor extension wall) means the ground floor may need to be designed to carry the load of the floor and wall above, rather than as a lightweight single-storey structure. First floor: adding a new bedroom (often an en-suite master bedroom or a guest bedroom with en-suite), or enlarging an existing small first-floor bedroom. In many London Victorian terraces, the first floor above the existing back addition is a small bathroom or a single undersized bedroom — a double-storey extension replacing or extending the back addition creates a significantly larger first floor bedroom and bathroom footprint. Combined value uplift: for a London three-bedroom terrace, adding a ground-floor extension plus a first-floor bedroom and bathroom moves the property from 3 bedrooms to 4 bedrooms, and from 1 bathroom to 2 bathrooms — a step-change in both functionality and market value, typically adding 10-20% to the property's value depending on the area and the quality of the extension.

Unlike single-storey rear extensions, which in many cases can be built under Permitted Development rights without a planning application, double storey rear extensions in London almost always require full planning permission. Planning rules for double storey extensions in London (as of April 2025): Permitted Development for double storey rear extensions: a two-storey rear extension is permitted under Class A of the Permitted Development Order only where: the extension does not extend more than 3m beyond the rear wall of the original house; the extension does not exceed the height of the highest part of the original roof; the extension does not include a balcony, verandah, or raised platform; the materials used are similar in appearance to the existing house; and the extension is not in front of the building's principal elevation or side elevation fronting a highway. In practice, 3m is a very modest extension depth at two storeys — most clients wanting a meaningful double storey extension are extending further than 3m, and therefore require full planning permission. The Larger Home Extension scheme (Prior Approval): this scheme only applies to single-storey rear extensions — it does not extend to two-storey rear extensions. Conservation area restrictions: in conservation areas, the planning authority assesses the design of a double storey extension against the conservation area's character appraisal. Rear double storey extensions that are not visible from the public realm are generally assessed more leniently in conservation areas than those visible from the street. Overlooking and privacy: a key material planning consideration for double storey rear extensions in London is the impact of first-floor windows on the privacy of neighbouring properties. A first-floor rear window at the end of a 4m extension looks directly into the rear gardens and rear windows of the neighbouring properties — the planning officer will assess this carefully and may require obscure glazing, raised sill levels, or rooflights in lieu of vertical windows.

A double storey rear extension is structurally more complex than a single-storey extension — the wall and floor structure must carry both the first floor loads and the roof load above. Key structural elements of a London double storey rear extension: Foundations: the foundations of a two-storey extension must be designed to carry a significantly greater load than a single-storey extension — typically requiring wider and/or deeper strip foundations, or a mini-piled scheme where London Clay shrinkage or tree proximity is a factor. The structural engineer designs the foundation to the specific loading. Ground floor walls: the external walls of a double storey extension (masonry or timber frame) must carry the combined load of the first floor slab/joists plus the first floor walls plus the roof structure. Wall thickness and structural specification is designed to the specific load and span. First floor structure: the first floor of the extension is typically a timber joist floor (joist size and spacing to BS 5268 or Eurocode 5 to the structural engineer's design) spanning between the external walls of the extension. Acoustic insulation between floors (where the first floor is a habitable room and the ground floor below is occupied): required under Part E of Building Regulations — typically mineral wool acoustic batts between the joists and resilient floor build-up. Opening from the existing first floor into the new first-floor extension: the connection from the existing first floor landing or bedroom to the new first-floor extension requires a structural opening in the existing rear first-floor wall — designed by the structural engineer. First floor walls: the first floor external walls of the extension carry the roof structure above. The structural engineer specifies the wall construction (masonry, timber frame, blockwork inner leaf). Roof structure: the roof of a double storey extension may be a flat roof (more common for rear extensions in London terraces, where a flat roof allows the extension to be built up to the first floor window level of the existing house without blocking light), a pitched roof (where there is sufficient space for a pitched roof above the first floor extension without running into the existing roof), or a parapet wall with flat roof (a common design treatment in contemporary London extension design).

The design of a double storey rear extension must balance: Natural light to the existing first floor rear bedrooms: a full-width double storey extension built to the full width of the house at first-floor level can significantly reduce light to the existing first-floor rear bedroom windows — which are now looking into the side wall of the extension rather than over the garden. Design solutions: set the first-floor extension back from the full rear width (a stepped or chamfered plan); incorporate a Juliet balcony or full-height glazing on the rear elevation to maximise natural light; use a rooflight in the first-floor ceiling rather than a rear vertical window; or accept the reduced natural light as a trade-off against the additional bedroom floor area. Neighbour light impact: in many London Victorian terraces, properties are built back-to-back or close to neighbouring rear elevations. A full-height double storey extension may have a material impact on the daylight to neighbouring rear windows — the planning officer will assess this using the BRE Daylight and Sunlight Guide (the 45° rule and vertical sky component analysis). Privacy: first-floor rear windows of the new extension overlook neighbouring rear gardens and rear windows. Design solutions for the planning application: use rooflights in the first-floor ceiling rather than rear vertical windows; use obscure glazing in any first-floor rear windows; position any vertical rear first-floor windows to minimise direct overlooking by setting them back from the end wall of the extension or to face obliquely. External materials: at two storeys, the extension is more visually prominent than at single storey — external materials choice is a significant factor in planning approval and in the quality of the finished result. Matching or complementary facing brick (or render) and roof material to the existing house are typically required in conservation areas.

Indicative all-in costs for a double storey rear extension in London 2025: Double storey rear extension, 20m² per floor (total 40m² addition), mid-terrace Victorian house, full planning required: standard specification (masonry walls, timber joist first floor, flat roof at first-floor level, standard double-glazed windows, internal finish to plaster level): £95,000-£150,000 supply and construct. Mid-range specification (brick facing, bi-fold doors at ground floor, rooflights, en-suite to new first-floor bedroom, underfloor heating at ground floor level): £130,000-£185,000 fully fitted and decorated. Premium specification (contemporary design, full-width glazed rear facade at ground floor, Juliet balcony, bespoke kitchen, premium bathroom): £170,000-£240,000+. Comparison with two separate extensions (single storey first, double storey second): building a single-storey extension first and then later extending upward at first-floor level is more expensive in total (two planning applications, two Building Control applications, two scaffold erections and strikes, two mobilisations, rebuilding or modifying a recently built single-storey roof) and more disruptive than building the double storey in one programme. Cost premium for double storey vs single storey: building both floors simultaneously typically adds 60-80% to the cost of the single-storey element alone — versus 100-130% extra if built as two separate operations at different times. Professional fee allowance: architect and structural engineer fees for a double storey extension with a planning application typically add £8,000-£15,000 to the project cost — reflecting the additional drawing, planning, structural design, and contract administration work compared with a single-storey PD extension.