Kitchen Extension Design in London 2025: Layout, Specification and What to Expect

Kitchen extension layout options for London Victorian and Edwardian terraces: the starting point for a London kitchen extension is the existing house layout and the extent of the proposed extension. Common existing ground floor arrangements in a London Victorian terrace: A two-room arrangement: front reception room (living room); rear reception room (dining room); galley kitchen (in the original rear return or as a small extension at the back of the house); external W.C. and yard. The extension strategy: knock through from the rear reception room into the new extension to create a large open-plan kitchen-diner-family room; the original front reception room remains as a living room or is left as a study or playroom. Extension footprint: the width of the original house minus any side return (typically 3.5m-5m); depth of new extension typically 3.0m-5.5m (anything deeper than 5.5m starts to affect the daylight quality at the rear of the existing house). Common extension layouts: L-shaped layout (extension running across the full width of the existing house, with the kitchen along one wall, dining in the centre, and living/family area towards the garden): most common layout for a London rear extension; provides clear zoning between cooking, dining, and relaxing without requiring walls or partitions. Kitchen in the extension (linear galley along one wall): if the existing kitchen is small and dark, relocating the kitchen into the brighter rear extension space is transformative; the kitchen runs along the rear wall or a side wall of the extension, with the run facing the garden; the island (if specified) runs parallel to the kitchen run, providing additional worktop and seating. Kitchen along the rear wall facing out to the garden: provides the cook with a view of the garden and visual connection to outdoor dining or a terrace; suits an island run parallel to the main run with bar seating on the garden side of the island. Structural opening between original house and new extension: creating a large structural opening between the existing rear wall of the house and the new extension is a key element of the kitchen extension design — it is what makes the space feel connected and open. Structural opening size: for a standard Victorian terrace (4m-5m wide at the rear elevation), a single steel beam spanning the full width of the rear wall (minus the structural jambs at each end) creates the maximum possible opening. Typical opening widths: 3.0m-4.5m clear opening (after allowing for structural jambs at each end). Structural beam depth: the depth of the beam (the RSJ or LVL) depends on the span and the loads imposed. For a typical 3.5-4.0m span carrying a first floor load: a 254x146x37 UB (approximately 254mm deep) is commonly specified; for a 4.0-5.0m span: a 305x165x40 UB or similar. The structural engineer specifies the exact beam size — the designer cannot assume a standard beam without structural engineer input. Note: a deep beam (250mm+) affects the finished ceiling height at the junction between old and new — if the finished ceiling height of the new extension is 2.7m, and the beam is 250mm deep, the finished soffit of the beam is at 2.45m above the finished floor level — which is acceptable in most kitchen extensions but worth considering in the design. The beam can be boxed in (enclosed in a timber or plasterboard box with a flat soffit) or expressed (left visible as a feature steel with a painted finish). Glazing choices for London kitchen extensions: the choice of glazing strategy — the type, size, and proportion of glazing in the kitchen extension — is one of the most significant design decisions and one of the largest cost variables. Rear elevation glazing: full-width bi-fold or sliding door set across the rear of the extension: opening the entire rear elevation to the garden; the single most effective way to connect indoor and outdoor space; costs vary significantly by glazing type and quality. Flat roof glazing (roof lantern vs skylights vs full-width structural glass rooflights): if the extension has a flat roof (the most common choice for a single-storey rear extension in London), natural light from above is essential to avoid the back of the space feeling dark. Options: Roof lantern (a pyramid or ridge-form glazed roof light sitting on an upstand on the flat roof): traditional and warm-looking, particularly in a period terrace; sizes typically 1.0m x 2.0m to 2.0m x 4.0m; costs £1,500-£8,000 supply and install depending on size. Fixed skylight (a flat, flush-mounted glazed skylight — VELUX or equivalent): more contemporary than a roof lantern; larger sizes possible; 1.0m x 2.0m to 2.0m x 4.0m; costs £800-£4,000 supply and install. Full-width structural glass rooflight (a frameless or minimal-frame structural glass panel flush with the flat roof, running full width or full length of the extension): most expensive option; most dramatic light quality; costs £3,000-£15,000 supply and install. Part L 2021 compliance for glazing: all new glazing (windows, doors, rooflights) in an extension must comply with Building Regulations Part L 2021. The total area of glazing in an extension is limited to a formula: total area of windows + doors + rooflights shall not exceed the sum of: 25% of the floor area of the extension + 25% of the floor area of any rooms lost from the existing house to form the extension. In practice, for a 4m x 5m kitchen extension (20m² floor area), the maximum glazing area is typically 5.0m² (roof lantern) + 4.5m² (bi-fold door) = 9.5m² of glazing — which is around 47% of the floor area. This sometimes conflicts with the homeowner's desire for maximum glazing — a SAP (Standard Assessment Procedure) calculation or Part L compliance check by the designer or energy assessor is needed to confirm compliance or identify whether compensatory measures (additional insulation) are needed.

Kitchen specification levels in a London extension: the kitchen unit specification is one of the largest single cost variables in a kitchen extension project. The spectrum of kitchen specification options for London projects: Budget/mid-range kitchen (flat-pack or rigid box carcasses, standard door finishes, laminate worktop): brands: IKEA (Metod range with AXSTAD/BODBYN etc. doors), HOWDENS (Clerkenwell range, etc.), Wickes or similar; approximate supply cost (units only, ex-VAT): £4,000-£10,000 for a standard L-shaped kitchen in a 4m x 5m extension. Mid-range kitchen (rigid-box carcasses, painted shaker-style or handleless doors, stone worktops): brands: Wren, Magnet, or equivalent supplier; approximate supply cost (units only, ex-VAT): £8,000-£20,000. High-end kitchen (custom or semi-custom rigid-box or solid timber carcasses, hand-painted lacquer or solid timber slab doors, stone worktops — marble, quartz, or engineered stone): brands: Roundhouse, deVOL, Neptune, Smallbone, or equivalent bespoke kitchen designer; approximate supply cost (units only, ex-VAT): £20,000-£80,000+. Second-fix materials and appliances (all supplied by client): a key RCB operating principle — all second-fix materials (kitchen units, worktops, appliances, taps, sinks) are client-supplied unless the client specifically requests procurement support. This protects the client's ability to select and source their own kitchen independently of the construction contract. Kitchen island considerations: a kitchen island is now a near-standard element of any London kitchen extension. Key island design decisions: fixed vs unfixed: a fixed island (plumbed in, with a hob or sink, requiring structural connection to the floor) is part of the first-fix programme and must be specified early. An unfixed island (a freestanding unit with no plumbing or electrical connection) can be installed at second-fix stage. Island size and overhang for bar seating: standard island depth 600-900mm (base unit depth); worktop overhang for bar seating 300-400mm; bar stool seat height approximately 650-680mm (matching a standard 900mm-high island worktop with a 300mm overhang). Minimum clearances around the island: 900mm minimum circulation space on all sides of the island between the island edge and the base unit or wall (1,000-1,200mm preferred for comfortable movement when cooking and passing). Island with waterfall end: a worktop that wraps around one or both ends of the island (creating a 'waterfall' of stone from the top down the end panel) — adds visual drama but adds approximately £800-£2,500 to the worktop cost depending on material and overhang. Utility connections to a kitchen extension: the kitchen extension requires the following utility connections to be resolved during the first-fix programme (before plastering and second fix): Drainage: a new kitchen extension drainage run (110mm UPVC waste pipe) from the kitchen sink waste and dishwasher waste to the existing below-ground drainage system (see london-plumbing-guide for detail); Cold water supply: a new 15mm cold water supply branch from the existing cold water main to the new kitchen sink position; Hot water supply: a new hot water supply pipe from the existing cylinder or combi boiler to the new kitchen sink; Gas: if the kitchen has a gas hob, a new gas supply branch must be connected by a Gas Safe registered engineer — typically from the existing gas meter or the existing kitchen gas point; Electrical: minimum electrical outlets required for a kitchen extension: double sockets at 600mm intervals around the work surfaces (minimum 8-12 double sockets for a full kitchen run); dedicated circuits: dishwasher (13A socket), washing machine (13A socket if in kitchen extension), fridge-freezer (13A socket), extractor (1A fused spur or 13A socket), electric oven (30A or 45A dedicated circuit, not a 13A socket), induction hob (32A or 45A dedicated circuit — check with the appliance manufacturer); underfloor heating wiring (if specified); under-cabinet LED lighting (low-voltage transformer and wiring); CCTV/CAT6 data point/doorbell point where relevant. Extractor fan and ventilation: a kitchen extractor is required for both ventilation effectiveness and Building Regulations Part F compliance. For a new kitchen extension: a centrifugal in-line extractor or an axial kitchen extractor (typically 150mm ducted extractor — from the hob, run through the flat roof or external wall to a roof or wall termination); minimum extraction rate 60 litres/second for a general kitchen space (BS EN 13141 / Part F guidance). Alternatively: a recirculating extractor with activated carbon filters (no external duct required) — Part F compliance using a higher-rated recirculation unit. Underfloor heating in a kitchen extension: electric underfloor heating (UFH) under floor tiles is the most common choice for a kitchen extension floor — it is simple to install, relatively low cost, and compatible with the flat, solid-floor construction of a typical extension. Electric UFH heating mat (15-17W/m²): typically £300-£600 supply and install for a 15-20m² extension floor area, not including tile floor finish. Wet underfloor heating (plumbed UFH): more efficient over the long term (particularly with a heat pump or a high-efficiency condensing boiler) but more expensive to install; requires a manifold and connection to the central heating circuit; costs typically £800-£2,000 for a kitchen extension, depending on area and complexity.

Kitchen extension costs London 2025: a London single-storey rear kitchen extension is typically priced on a total project basis (all-in design-and-build contract price) or split between the structural/shell/wet trades (construction contract) and the kitchen and finishes (client-direct purchase). Structural and building works — typical costs (supply and all wet trades, including structural opening, flat roof, bifold doors, drainage, plumbing first fix, electrical first fix, plastering — NOT including kitchen units, worktops, appliances, or second fix fittings — all ex-VAT): Small kitchen extension (3m x 3m, single-storey flat roof, single structural opening, basic bifold door set, standard roof lantern, drainage, first fix — structural and weathertight shell only): £30,000-£50,000 + VAT. Standard kitchen extension (4m x 5m, single-storey flat roof, full-width structural opening, 3-4 panel bi-fold or sliding door set, roof lantern or structural glass, drainage, plumbing first fix, electrical first fix, insulation, plastering — structural and weathertight shell with plastered walls and ceiling, level screeded floor ready for tiles): £45,000-£80,000 + VAT. Larger or more complex extension (4m x 6m+, or with a complex roof form, or combining extension with rear return infill, or with a side return, or with a glass structure): £70,000-£130,000+ + VAT. Kitchen units, worktops, and appliances (client-supplied, separate budget): budget kitchen: £4,000-£15,000 for units and worktop; mid-range kitchen: £10,000-£30,000; high-end: £25,000-£80,000+. Kitchen installation (second fix — installing client-supplied kitchen units, worktops, appliances, taps): typically £2,000-£6,000 for a standard kitchen installation by a specialist kitchen fitter, included in (or separate from) the construction contract. Indicative total project budgets (structural works + kitchen including installation, ex-VAT): small practical extension with IKEA-level kitchen: £40,000-£70,000; standard 4m x 5m extension with mid-range kitchen: £60,000-£120,000; large or complex extension with high-specification kitchen: £100,000-£250,000+. Note: these are 2025 London market prices and represent the range from competitively-tendered builder-only contracts to full design-and-build principal contractor contracts. Prices can vary significantly by London borough, site access conditions, specification choices, and market conditions. The programme for a London kitchen extension: design and pre-construction (4-10 weeks): architect produces drawings; planning permitted development or planning application submitted; structural engineer calculations produced; Building Regulations application submitted. Planning (if required — most single-storey rear extensions in London are PD but check): 8 weeks statutory period for a householder planning application. Building Control plan check (4-8 weeks from submission). Start on site — construction programme: Week 1-2: groundworks (excavations for extension foundations, drainage run); Week 2-3: foundations (concrete strip or pad) poured and cured; Week 3-4: external walls built up to eaves level (or structural frame erected if a steel-frame extension); Week 4-6: flat roof structure and waterproof membrane installed; Week 5-6: internal structural opening (RSJ beam installed, temporary works removed); Week 6-7: external brickwork and rendering completed; Week 7-8: glazing installed (bifold doors, roof lantern, windows); Week 8-10: first-fix plumbing and electrical; Week 10-11: floor screed (or concrete slab if no UFH — if UFH then mats installed and screed on top); Week 11-13: plastering (walls and ceilings); Week 13-16: second fix — kitchen units installed, worktops fitted, appliances connected, electrical second fix, plumbing second fix, floor tiles, decoration. Total programme (structural works to completion): typically 14-18 weeks on site for a standard 4m x 5m single-storey kitchen extension. What the principal contractor manages in a London kitchen extension: as principal contractor, RCB coordinates and manages: structural contractor and groundworks; brickwork and blockwork; roofing and waterproofing; glazing and door installation; plumber (first fix and second fix); electrician (first fix and second fix); plasterer; kitchen fitter (second fix); floor tiler; decorator. The principal contractor takes responsibility for: CDM 2015 compliance (construction phase health and safety plan, site safety); programme management and sequencing; quality control of each trade; coordination between trades (particularly at the critical interfaces — plumbing and electrical before screed; glazing before plastering; kitchen unit installation before worktop measure); liaising with Building Control for inspections; client communication and progress updates.