Kitchen Extension Ideas and Planning: How to Design the Right Open-Plan Kitchen Space in London

**What a kitchen extension is actually solving**:

• Most London kitchen extensions are solving the same core problem: the original Victorian or Edwardian back addition (typically a narrow 2m–3m wide rear outrigger that housed the original scullery and back kitchen) is too small and dark for modern kitchen living. The kitchen extension project typically involves:
• •Demolishing the existing back addition (the narrow outrigger)
• •Building a new, wider rear extension (typically 3.5m–6m deep × full width of the house, or as wide as permitted development allows)
• •Removing one or more internal walls between the new extension and the ground floor of the existing house
• •Creating an open-plan kitchen / dining / living room that spans from the front of the ground floor to the rear garden
• •Opening the rear elevation of the new extension to the garden with bi-fold doors, sliding doors, or fixed glazing

The key design decisions in this project are: (1) the footprint and depth of the extension; (2) the arrangement of the open-plan space; (3) the structural openings between old and new; (4) the rear and roof glazing; and (5) the relationship between inside and outside.

**Footprint and depth**:

• The most common extension size for a standard London Victorian terrace:
• •*3.0m–4.0m deep (from original rear wall of the main house)*: Achieves the most common 'side return plus rear infill' type that encloses the side passage and extends into the garden — often permitted development
• •*4.0m–6.0m deep (from original rear wall)*: More significant extension — creates a large open-plan space but requires planning permission (beyond the 4m/3m PD depth limit for single-storey rear extensions)
• •*6.0m+ deep*: A larger extension requiring planning permission and typically with planning design conditions; may affect light and outlook for neighbouring properties

Important for single-storey permitted development depth: Under the Prior Approval (neighbour consultation) process, a detached house can extend up to 8m single-storey and a semi-detached/terraced up to 6m — but this requires prior approval from the LPA and neighbour consultation, and may not be available in conservation areas.

**Layout principles for an open-plan kitchen-dining-living space**:

The most successful open-plan kitchen extensions follow clear zoning principles:

• *Kitchen zone (cooking)*:
• •Typically positioned against the wall furthest from the garden doors (the 'back wall' of the extension, or a side wall)
• •Ideally with a window above the sink for natural light during food preparation
• •The cooking zone is a source of smells, heat, and mess — positioning it away from the living and dining zones allows easier management of fumes and noise
• •Extraction: a powerful ceiling-mounted or island extractor fan is essential in an open-plan space — inadequate extraction is one of the most common complaints about open-plan kitchen extensions

• *Dining zone*:
• •Typically in the centre of the extension — between the kitchen zone and the living zone
• •Natural light from above (lantern/rooflight) is particularly valuable over the dining zone — it is the area furthest from both the side windows and the rear doors and without a rooflight can be the darkest part of the extension
• •Proportions: allow minimum 2.5m × 4m for a 6-person dining table with chairs and circulation space

• *Living zone*:
• •Typically at the garden-facing end of the extension, adjacent to the bi-fold or sliding doors
• •The connection to the garden (level threshold between inside and outside, patio directly beyond the doors) is what makes the living zone feel generous and connected
• •In narrow houses, a linear layout (kitchen at back wall, dining in middle, sofa at garden end) works well; in wider extensions, a U-shaped or L-shaped arrangement is possible

• *The connection to the front of the house*:
• •Most kitchen extensions also involve removing the wall or walls between the extension and the existing ground floor of the house — connecting the new kitchen-dining-living space to the existing ground floor hallway and living room
• •These openings typically require steel RSJ beams (sometimes twin beams where loads from above are significant) and structural engineer calculations
• •The scale of the structural opening between old house and extension significantly affects how well the whole ground floor flows — a wide 3.5m–5.0m opening between old and new reads as a single large space; a narrow opening (1.8m–2.4m) reads as two separate rooms with a connection

**The roof and light**:

One of the defining features of a modern London kitchen extension is the treatment of the roof for natural light:

*Flat roof with rooflight(s)*: A flat roof (see `flat-roof-extension-guide`) allows the maximum number of rooflights to be installed at any position over the extension. A large fixed glazed rooflight (e.g., 2000mm × 1000mm or larger) over the dining zone is perhaps the most transformative single element in a kitchen extension — bringing daylight deep into the centre of the plan in a way that side windows and rear doors cannot achieve.

*Lantern roof*: A roof lantern (a traditional pitched-glass rooflight set proud of the main flat roof) provides a more classical appearance and may be more appropriate for period properties. Modern aluminium-framed roof lanterns (e.g., from Korniche, Brett Martin, Atlas) allow large glazed areas with thin frame sightlines.

*Pitched roof with Velux rooflights*: A single-pitch (mono-pitch) or dual-pitch tiled roof is more traditional in appearance but limits rooflight placement to the sloped plane. A lean-to extension with a mono-pitch roof and a Velux at the high point above the dining zone can still provide significant natural light.

**Rear glazing — bi-fold, sliding, or fixed**:

*Bi-fold doors*: Open concertina-style; allow the rear wall to be completely opened to the garden (when fully folded, the opening is 80–90% of its full width); require wall space at one or both sides for the folded door stack; popular but the folded stack takes space from the opening and can obstruct access; aluminium or timber frames

*Sliding doors (slim-framed)*: Slide on tracks; typically two or three large panels; the opening width is limited to 50% of the frame (one panel behind another) but the remaining width is fully open; increasingly popular because they avoid the concertina stack obstruction and have slimmer sightlines

*Fixed glazing + single door*: Where the main visual connection to the garden is desired but the extension is not large enough to justify bi-folds or sliders, a large fixed glazed panel with a single opening door provides the visual connection at lower cost

*Level threshold*: For the 'inside-outside' effect, the floor level inside the extension and the patio level outside must be at the same level — or within 10mm. This requires the patio to be built at a raised level, or the extension floor to be designed at garden level. A level threshold also improves access for wheelchair users and is preferred for DDA compliance.

**Structural openings between the extension and the existing house**:

• The structural opening between the new extension and the existing rear wall of the house is one of the most significant structural elements in the project. The existing rear wall is typically:
• •In a Victorian terrace: solid brick, 225mm (9-inch) or 337mm (13.5-inch) thick, load-bearing (supporting the party walls, the floor joists above, and in some cases the chimney breast)
• •In a 1930s semi-detached: cavity brick, typically load-bearing

• Creating a structural opening in this wall requires:
• •*Structural engineer calculations*: The size and specification of the steel RSJ (rolled steel joist) or universal beam (UB) required to carry the loads above the opening
• •*Temporary support (acro props)*: Before the opening is created, the wall above must be temporarily propped to carry the loads during the formation of the opening
• •*Padstones*: Concrete or engineering brick padstones at each end of the beam — to spread the concentrated load from the beam end into the supporting wall
• •*Building Control inspection*: The structural opening work must be inspected by Building Control (Full Plans or Building Notice route)

Typical RSJ specification for a 3.5m–5m opening in a Victorian terrace: 254×146 UB or 305×165 UB (depending on the span and loads above) — the structural engineer calculates the required section. Cost of structural beam supply and installation in an opening of this width: typically £3,000–£8,000 (including temporary propping, formation of opening, beam supply, padstones, and making good around beam ends).

**Planning permission for kitchen extensions**:

• Single-storey kitchen extensions can be built without planning permission (under Permitted Development) up to:
• •3m deep (from the original rear wall) for a terraced or semi-detached house
• •4m deep for a detached house
• •4m high at the ridge (pitched roof) or 3m high (flat roof within 2m of the boundary)
• •Under Prior Approval (for permitted development): up to 6m (terraced/semi-detached) or 8m (detached) — subject to neighbour consultation and LPA approval

• Planning permission is required if:
• •The extension exceeds PD size limits
• •The property is in a conservation area (where additional restrictions apply)
• •PD rights have been removed by an Article 4 Direction
• •The extension is to a flat

For kitchen extensions in London, many projects are either within PD or use the Prior Approval route (which requires submitting a householder Prior Approval application at £120, and the LPA consults neighbours) — a formal full planning application at £258 is only needed when the project exceeds the Prior Approval limit or PD rights are removed.

**Building Regulations requirements**:

• All kitchen extensions require Building Regulations approval (regardless of whether planning permission is needed):
• •*Part A (Structure)*: Foundation design (strip, pad, or raft depending on soil conditions), structural opening beam calculations, structural connections
• •*Part B (Fire safety)*: Smoke detection (mains-powered smoke alarm in the extension if the extension creates a new habitable room or increases the size of the ground floor kitchen/living space)
• •*Part C (Dampness)*: DPC to the extension walls; DPM to the floor slab
• •*Part F (Ventilation)*: Background ventilation (trickle vents) in new windows and doors; extract ventilation in the kitchen (mechanical extract through the roof or wall)
• •*Part L (Energy efficiency)*: All new elements to Part L standards — wall insulation, floor insulation, roof insulation, double or triple glazed windows and doors, U-value calculations
• •*Part M (Access)*: Level threshold between house and extension and between extension and garden (where practicable)
• •*Part P (Electrics)*: Any new electrical installation must be notified to Building Control

**Gas and drainage**:

• Kitchen extensions often require:
• •*Drainage*: New waste pipes from the kitchen sink, dishwasher, and (if included) utility room sink; these must connect to the existing drainage system and may require a new manhole or inspection chamber at the connection point
• •*Gas*: Moving or extending a gas supply to the new kitchen position; any gas pipework must be installed by a Gas Safe registered engineer and notified to Building Control
• •*Underfloor heating*: Many kitchen extension projects include underfloor heating in the new extension floor — the UFH system (wet or electric) must be connected to the boiler and controlled by a thermostat; confirm the existing boiler has adequate output capacity for the added floor area

**The most common kitchen extension mistakes**:

*1. Underspecifying the extraction*:

• The most common complaint from clients in open-plan kitchen extensions is cooking smells and condensation spreading through the house. In an open-plan space, the kitchen extractor fan must be significantly more powerful than in a closed kitchen. The extraction should:
• •Be recirculating or ducted externally (external ducting is more effective — the duct from the extractor must go through the roof or wall to outside)
• •Have a minimum extraction rate of 60 litres/second (Part F minimum for kitchen extraction in a new or extended kitchen)
• •In a large open-plan space, a ceiling-mounted extractor (island extractor) positioned directly above the hob is most effective
• •Avoid under-specification: a cheap recirculating extractor above an open-plan kitchen will not control cooking smells in the dining and living zones

*2. Underspecifying the glazing for thermal comfort*:

• A kitchen extension with a large flat roof covered in rooflights and fully glazed rear wall can become extremely hot in summer and cold in winter if the glazing specification is not adequate. Key specifications:
• •All glazing should be triple-glazed for best thermal performance (or high-performance double-glazed at minimum)
• •Solar control glass on south-facing rooflights (glass with a solar control coating that reduces heat gain in summer while maintaining light transmission)
• •External blinds on south-facing rooflights (more effective than internal blinds at reducing solar gain)
• •Underfloor heating in the extension floor (to compensate for heat loss through the large glazed areas in winter)

*3. Forgetting the dining zone ceiling height*:

• In a flat-roof extension, the standard flat-ceiling height may be only 2.4m — which can feel low, particularly if the dining zone is surrounded by glazing. Consider:
• •A raised ceiling height in the dining zone (a rooflight box that projects above the main flat roof, creating a higher ceiling over the dining zone)
• •A lantern roof over the dining zone (both increases the ceiling height and provides natural light)
• •A vaulted ceiling in a pitched-roof extension (the underside of the roof slope provides a higher, more dramatic ceiling)

*4. Level threshold not achieved*:

A level threshold between the extension and the garden patio is one of the most valued features of a kitchen extension — the 'inside-outside' blur. But it requires careful co-ordination between the extension floor level, the external patio level, and the existing ground level. If not planned from the beginning of the project, the result is a step up or down from extension to garden — which feels and looks like a compromise. Agree the level threshold strategy with the structural engineer and architect at the design stage.

**Typical costs for a kitchen extension in London (2025)**:

| Extension type and size | Typical all-in cost (London 2025) | |---|---| | Small single-storey rear extension 3m × 4m (12m²), flat roof, bi-fold doors, structural opening to existing house | £45,000–£65,000 | | Medium single-storey rear extension 4m × 5m (20m²), flat roof with rooflight, bi-fold doors, structural opening | £60,000–£90,000 | | Larger single-storey rear extension 6m × 5m (30m²), flat roof with large rooflight/lantern, bi-fold doors, full structural opening | £80,000–£120,000 | | Side return extension (enclosing side passage + rear infill, ~25–35m² total) | £65,000–£95,000 | | Wrap-around extension (side return + rear) | £80,000–£130,000 |

Note: All costs include structural opening/steelwork, insulation, electrics, underfloor heating (wet, linked to boiler), plastering and decoration, and drainage. Excludes kitchen unit supply and fit (budget £8,000–£30,000+ separately depending on specification). Second-fix materials (kitchen, tiling, flooring) are client-supplied under RCB's standard model.

Planning and professional fees: architect/designer £3,000–£8,000 (for planning drawings and Building Regs drawings); structural engineer £800–£2,000; planning fee (if required) £258; Building Regs fee £800–£1,500.