Kitchen Extensions in London: Design, Costs and Planning Guide

The standard London kitchen extension combines several elements in a single project: extending the footprint of the house at the rear (single-storey, typically 3-5m depth, spanning the full width of the house); removing the rear external wall of the original kitchen and the wall between the kitchen and the reception room behind to create a continuous open-plan space; installing a large glazed opening at the rear of the extension (bi-fold doors, sliding doors, or a combination of fixed glazing and opening panels); moving or relocating the kitchen to the new extended footprint; and often — simultaneously — opening up the ground floor reception rooms by removing the wall between front and rear reception to create an even more open ground floor plan. This package of works — extension plus knockthrough plus kitchen renovation — is the most commonly requested and most transformative project in London residential construction. The combination of better light (from the large rear glazing), better connection to the garden, and a modern open-plan layout adds substantial value and significantly improves the quality of daily life.

A kitchen extension project in London has three cost components: the extension shell (structural works, foundation, walls, roof, glazing); the knockthrough and structural works within the existing house; and the kitchen fit-out. All-in budget for a single-storey kitchen extension in a London terrace: Small extension (3m x 3m = 9m2), basic kitchen, standard specification, simple bi-fold doors: £45,000-£70,000. Medium extension (4m x 4.5m = 18m2), mid-range kitchen, large bi-fold doors, knockthrough to front reception: £75,000-£120,000. Large extension (5m x 4.5m = 22.5m2) plus internal alterations, high-spec kitchen and glazing, underfloor heating: £130,000-£200,000. These are total project costs including the extension structure, bi-fold doors, internal structural works, kitchen supply and installation, electrics, plumbing, and decoration. They assume a contractor managing all trades. The largest cost variables within this range are: the depth and width of the extension (which drives the structural cost, foundation cost, and roof cost directly); the glazing specification (basic bi-fold doors vs. premium slim-profile system with solar control glass); the kitchen specification (IKEA-based vs. semi-custom vs. bespoke).

The most successful London kitchen extensions share several design characteristics: Connection to the garden: the extension should open as fully as possible to the rear garden — bi-fold doors, sliding doors, or fixed glass across the full width of the extension rear wall. Where the garden is small (common in London terraces), sliding-fold or pivot doors that open the rear wall completely give the greatest sense of inside-outside connection. Natural light from above: a flat-roof extension with roof lights (or a glass roof or glass lantern) dramatically increases the light in what would otherwise be a dark space hemmed in by rear additions. South-facing extensions with roof lights can be extraordinary in their light quality year-round. Flush floor level: matching the floor level of the extension precisely to the interior floor level (and designing the external patio to sit just below the threshold) gives the seamless indoor-outdoor connection that is the aesthetic aspiration of most London kitchen extension projects. Open-plan flow: the kitchen island or peninsula, the dining area, and the seating area should flow naturally through the space without barriers. Kitchen design and spatial design should be developed together — the island position relative to the bi-fold doors, the cooking zone relative to the natural light, and the seating area relative to the garden view all interact. Material palette: consistent floor material running from inside to outside (large-format porcelain tiles, or engineered wood inside with matching porcelain on the patio) reinforces the indoor-outdoor connection. Consistent material colour and tone through the kitchen, dining area, and living area creates coherence in the open-plan space.

Single-storey kitchen extensions at the rear of a London terrace are covered by the same planning rules as any rear extension. Under Permitted Development: a single-storey rear extension of up to 3m beyond the original rear wall (or 6m under the Neighbour Consultation Scheme) does not require planning permission for a terraced house, provided it meets all PD conditions. In a conservation area, planning permission is almost always required for rear extensions visible from the highway. The knockthrough between the kitchen and the front reception room is an internal alteration and does not require planning permission (but may require Building Regulations if structural). The bi-fold doors are part of the extension and are covered by the extension's PD or planning permission — no separate consent is required for the doors themselves. If the extension requires planning permission (conservation area, or exceeding PD limits), the full design of the extension including the kitchen layout, glazing, and materials is submitted as part of the planning application.

A kitchen extension project involves multiple specialist trades working in sequence — groundworks, structural (steelwork), building (brickwork, blockwork, roof), glazing (bi-fold door installation), plumbing (kitchen and underfloor heating), electrics, tiling, flooring, plastering, decoration, and kitchen fitting. The two approaches are: use a principal contractor who manages all of these trades; or manage the trades directly as a homeowner project manager. For most homeowners, a principal contractor is the better choice — the coordination complexity and time commitment of directly managing 8-10 specialist trades simultaneously is significant. When evaluating kitchen extension contractors in London: ask to see completed similar projects (ideally visit one); check references specifically from kitchen extension projects, not just general building references; ensure they have experience of the specific challenge of open-plan conversions involving both structural alterations and kitchen fit-out; confirm their approach to the glazing specification and installation (bi-fold door installation quality is a common source of problems — leaks, sticking panels, thermal performance issues); and ensure the contract includes a clear scope, specification, programme, and payment schedule tied to stages.