Open Plan Kitchen Extension London: Costs, Planning & What to Expect

Creating an open-plan ground floor in a London terrace or semi typically combines: (1) a rear extension of 3-6m beyond the original back wall, (2) removal of the ground-floor rear reception room wall to open the original back room into the extension, (3) a new kitchen fitted across the rear of the extended space, (4) bi-fold or sliding doors to the garden. Each element has its own planning, structural, and regulatory implications. The extension itself may be under Permitted Development (PD) or require planning permission depending on size, location, and whether the property is in a Conservation Area. The internal wall removal always requires structural engineer sign-off and Building Regulations approval. The new opening into the extension requires a steel beam. If the rear wall is a party wall with an adjoining terraced neighbour, Party Wall Act obligations apply.

Under Permitted Development, a single-storey rear extension to a terrace or semi can be up to 3m deep without prior approval (up to 6m under the Larger Home Extension Scheme with neighbour notification). Detached houses have limits of 4m (or 8m under the scheme). The extension must not exceed 4m in height, must not project beyond the side elevation, and must use materials that match the existing house. These PD rights do not apply if: the property is in a Conservation Area, the property is Listed, or Article 4 Directions have removed PD rights. In Conservation Areas a rear extension may still be possible but requires a planning application. Extensions exceeding PD limits need a full planning application. For extensions under the Neighbour Consultation Scheme (3-6m terraces, 4-8m detached), a Prior Approval application is submitted to the council and neighbours have 21 days to object.

The most critical structural element is the beam spanning the opening between the existing house and the extension. In a terrace, the rear wall typically carries load from upper floors (floor joists and the rear wall of bedrooms above). Removing or creating a large opening in this wall requires a structural engineer to calculate and specify an appropriate RSJ or steel beam, temporary propping during works, and padstones to distribute load at each end of the beam. The extension roof (typically a flat roof with glazing, a lantern, or full-width glazed roof panels) must also be structurally specified. Party wall implications: if the extension adjoins the garden boundary of a terrace, the existing party wall with neighbours may be affected and a Party Wall Award may be required.

All extensions of more than 30m2, or any extension regardless of size that involves structural alterations, require Building Regulations approval. This covers: structural calculations, insulation (new walls, roof, and floor must meet current U-value requirements), drainage (if altering drainage runs), glazing (safety glass requirements), electrical works, and ventilation. Approval is obtained either via Full Plans (submitted before work begins, plans approved, and inspections during build) or Building Notice (notified before work begins, no plans approved, inspections during build). Full Plans gives certainty before spending money on construction; Building Notice is faster but leaves more risk that work may need alteration to satisfy the inspector.

Relocating or extending the kitchen involves significant M&E (mechanical and electrical) works: moving the gas supply (if gas cooking), extending or rerouting soil pipes for sink drainage, relocating boiler pipework if affected, adding circuits for kitchen appliances, installing extraction (typically ceiling extract or island hob extraction where a rear run is not possible), and specifying underfloor heating in the slab if a new concrete floor is poured. Extension floor construction matters: a concrete raft floor allows underfloor heating but takes longer to dry; a beam-and-block floor is faster and suitable for timber finish. Bi-fold or sliding glass doors to the garden require a structural head and threshold drainage.

A typical open-plan kitchen extension project in London (3-4m deep rear extension, ground floor wall removal, new kitchen) costs: £45,000-£85,000 for the full build including kitchen fitting. Structural engineer fees: £800-£1,800. Architect/drawing fees for planning and Building Regs: £2,500-£6,000. Planning application fee (if needed): £258 (2025 rate). Building Regulations fee: £700-£1,500 depending on council/approved inspector. Kitchen supply only: £8,000-£30,000 depending on specification. The main build (groundworks, structure, roofing, glazed doors, M&E rough-in, plastering): £30,000-£55,000 for a 3-4m extension. Significant variables: specification of glazed roof (a simple flat roof with a skylight costs £2,500-£4,500 less than a full-width glazed roof or lantern), quality of bi-fold doors (£3,000-£8,000 for a 3-panel set), and kitchen quality.