Designing a Kitchen for a London Rear Extension: Layout, Plumbing, Ventilation, and Glazing

**The most common kitchen layouts for London rear extensions**:

• *1. Single-run galley along the party wall*:
• All kitchen units and appliances are arranged along one wall — typically the party wall (shared wall with the neighbour). The remaining three walls and roof glazing provide the open-plan dining/living area. This layout works well for extension widths of 3.5–5.0m because:
• •The full depth of the extension is open and uncluttered
• •The kitchen does not project into the light zone below the roof lantern or bifold door
• •Services (plumbing waste; extract duct) can run vertically up the party wall — the shortest route to the existing drainage stack on the same wall
• •The extraction can be routed through the party wall or up through the ceiling without penetrating the flat roof waterproofing

• *2. L-shaped kitchen with island*:
• Kitchen units on two walls (typically the rear wall and party wall) with a freestanding or fixed island in the centre. Works for extension widths of 5.0–7.0m+. The island:
• •Must be coordinated with the structural grid (the island cannot be directly under the roof lantern — the roof lantern must be positioned to clear the island)
• •If plumbed (sink in the island), the waste pipe must route through the floor slab to a concealed drain below — this must be designed at foundation stage, not added later (waste pipes cannot be added to a cast concrete slab without core drilling, which is expensive and weakens the slab)
• •An island sink increases drainage complexity significantly and adds cost — specify island sinks only if they are a deliberate design intent, and confirm the drainage route before the slab is poured

*3. U-shaped kitchen along three walls*: Kitchen units wrapping three walls — two flanking walls and the rear wall. Used mainly for narrower extensions where the full perimeter is maximised for storage and worktop. The structural opening from the existing house into the extension must be positioned to allow access without interrupting the run of kitchen units.

*4. Full open-plan (kitchen, dining, and living in one continuous space)*: The entire extension plus the opened-up rear of the existing house becomes a single large kitchen-diner-living space. This requires a significant structural opening from the existing house into the extension — typically a 3.5–5.0m steel beam spanning the full width of the existing house rear wall, enabling the original rear wall to be removed or largely opened up. The structural beam is the critical cost item in this layout (see `structural-steelwork-guide`).

**The structural opening — connecting the extension to the existing house**:

• For all kitchen extension layouts, the key structural decision is the width and position of the opening from the existing house into the extension. This opening is created by removing the existing rear wall of the house (which is typically load-bearing) and installing a structural steel beam above to carry the floor/roof above. Common opening widths:
• •2.5–3.5m: standard for most London Victorian terraces — the opening extends from party wall to party wall, allowing a bifold or sliding door set spanning the full internal width
• •3.5–5.0m: possible only where the structural engineer confirms the beam size and bearing are adequate (the beam becomes very large and heavy at wider spans)
• •Full-width (both side walls removed): very occasionally; typically requires UC posts in corners to carry the load — specialist structural engineering required

• The position of the opening relative to the kitchen layout must be considered carefully:
• •An opening that is too narrow creates a pinch point — the kitchen feels disconnected from the dining and living areas beyond
• •An opening that is not aligned with the kitchen island creates a circulation conflict
• •The door set in the opening should slide or fold fully out of the plane of the wall when open — not project into the kitchen working space or the dining area

**Sink position and drainage — the most constrained decision in a kitchen extension**:

• The position of the kitchen sink is the most important plumbing decision in a kitchen extension because it determines:
• •Where the waste pipe runs (and whether it can connect to the soil stack without exceeding the Part H2 3.0m branch length limit)
• •Whether the drain must run through the concrete floor slab (island sink) or along the party wall (wall-mounted sink)
• •The water supply connection route (hot and cold supply pipes; less critical as they can run almost anywhere)

*Recommended sink position for a London extension*: **Against the party wall** — this places the sink as close as possible to the existing soil stack (which runs at the rear corner of the house on the party wall side in most London Victorian terraces). A short 40mm waste branch pipe runs along or within the party wall cavity, up to the existing stack or to a new stub stack run up the party wall, well within the Part H2 3.0m branch length limit.

*Island sink drainage*: An island sink requires the waste pipe to run through the floor. On a beam and block floor, this is achieved by routing the waste between the beam voids below the floor. On a concrete ground-bearing slab, the drain must be cast in at foundation stage (a 40mm drain channel or pre-formed recess in the slab) — adding it after the slab is poured requires expensive core drilling. The drain then connects underground (as a below-ground drain) to the main drainage run at the perimeter of the extension. Island sinks add approximately £500–£1,500 to the drainage cost and the kitchen fitting cost.

**Gas supply to the kitchen extension hob**:

• For gas hobs in a London extension, the gas supply must be extended from the existing meter/consumer unit to the extension. This is a Gas Safe registered work — only a Gas Safe registered installer can work on gas supplies. Key considerations:
• •The gas supply pipe should be sized for the appliance load (typically 15mm or 22mm copper for a domestic hob)
• •Gas pipes must not run through floor screed in enclosed situations (unless sleeved in an open-ended sleeve to allow gas to escape in the event of a leak)
• •The hob must be within reasonable distance of the gas supply entry point to minimise the pipe run length
• •Induction hob option: where gas pipework routing is complex, an induction hob (electric) eliminates the gas supply requirement entirely — at the cost of a slightly higher electricity tariff for cooking and a 32A or 40A ring circuit for the hob

**Ducted extract ventilation — the most commonly under-specified system in London kitchen extensions**:

Building Regulations Approved Document F requires minimum 30 l/s (litres per second) extract in a kitchen; 60 l/s if the cooker is within or adjacent to a cooking position (i.e., for any ducted cooker hood or downdraft extractor above or adjacent to a hob). This is a substantial extract rate — a typical recirculating hood (the type that filters the air and returns it to the kitchen without extracting it to outside) does NOT satisfy this requirement.

*Ducted extract requirements for a London kitchen extension*:

• A ducted extract system (cooker hood or downdraft extractor ducted to outside) must:
• •Achieve the minimum 60 l/s airflow at the hob position (typically the cooker hood or integrated downdraft extractor)
• •Be ducted to the outside of the building (not recirculated) via rigid ductwork (rectangular or circular)
• •Use rigid (not flexible) ductwork wherever the duct is within the building fabric (flexible ductwork reduces airflow; can kink; accumulates grease; not acceptable for the main duct run within the structure)
• •Terminate outside via a wall grille or roof extract (external termination)

*The extract duct route in a London kitchen extension — the critical design decision*:

The extract duct from the cooker hood must reach the outside of the building. In a single-storey rear extension, the route options are:

Option 1: Through the party wall — if the hob is adjacent to the party wall and the neighbour consents (rare; party wall matters apply). Not commonly used.

Option 2: Through the external rear wall above the bifold/sliding door — the duct runs horizontally from the hob to the external wall, exiting via a wall grille set into the brickwork above the door head. This route is generally the shortest for hobs on the party wall or rear wall of the extension. The duct must be insulated where it passes through the external wall to prevent condensation on the cold duct surface within the wall.

Option 3: Up and through the flat roof — the duct rises vertically from the hob, through the ceiling/roof structure, and exits via a rooftop termination cowl. This route is used where the external wall route is impractical (roof lantern directly above; conservation area planning restrictions on visible wall grilles). A roof penetration for the extract duct must be carefully waterproofed — the duct termination must be a purpose-made watertight deck penetration fitting, not just a cut hole in the EPDM membrane.

Option 4: Via the connection back to the existing house, through the existing house fabric, and out through an existing external air brick or via a new duct — the longest and most expensive route; used only where the other options are not possible.

**Conservation area planning implications for extract ventilation**:

A visible wall-mounted extract grille on the rear elevation of a property in a conservation area may require planning permission where it would be visible from a public highway. For rear extensions not visible from the highway, the grille is typically permitted development. For front or side elevations in conservation areas, a more discreet roof termination is preferred. Confirm with the planning officer for the specific conservation area.

**The 25% glazing limit under Building Regulations Part L**:

Building Regulations Approved Document L1B (Conservation of fuel and power, 2022) imposes a notional area limit on glazing in extensions — the total area of windows, roof windows, and glazed doors in an extension should not exceed 25% of the floor area of the extension, plus the area of any fixed windows or doors that become internal as a result of the extension.

• This rule has significant implications for the typical London open-plan kitchen extension, which often has:
• •A full-width bifold or sliding door across the rear of the extension
• •A roof lantern or rooflight
• •Side windows for additional natural light

• For a 25m² extension:
• •25% glazing limit = 6.25m² total glazing area
• •A bifold door set 2.5m wide × 2.1m high = 5.25m² of glazing — almost the entire allowed area
• •A 1.5m × 1.5m roof lantern = 2.25m² of glazing — combined with the bifold this is 7.5m², exceeding the limit by 1.25m²

This is why many London kitchen extension designs appear to exceed the 25% glazing limit. In practice, the Building Regulations provide a 'compensating' approach — if the glazing exceeds the 25% limit, the overall energy performance of the extension can still be demonstrated as compliant using a SAP calculation (energy model). A SAP calculation showing that the total heat loss through the extension envelope (including the excess glazing) is no worse than if the 25% limit had been met is an acceptable alternative compliance route. This requires an energy assessor to prepare the SAP calculation — typically costing £200–£500 for a straightforward single-extension calculation.

The key implication is that large bifold/sliding doors AND roof lanterns in the same extension virtually always require a SAP calculation as part of the Building Regulations submission. This is a normal part of the design process for contemporary London kitchen extensions — it should be budgeted for and commissioned at the design stage.

**Bifold doors vs. sliding doors vs. slide-and-fold patio doors — which to choose**:

• *Bifold doors*:
• •A stack of door panels that fold against each other as they open — panels fold to one side (or both sides for central opening)
• •Maximum practical opening width: approximately 5.0m for residential aluminium bifold
• •Open configuration: typically 90–95% of the opening is clear (the folded panel stack occupies one side of the opening)
• •Best for: maximum clear opening when fully open; popular choice for London kitchen extensions where a seamless inside-outside connection is the design intent
• •Cost: aluminium bifold 3000mm wide (3 panel): typically £3,500–£7,000 supply; 4000mm (4 panel): £5,000–£10,000 supply
• •Lead time: 6–10 weeks for aluminium bifold

• *Sliding doors (lift-and-slide or tilt-and-slide)*:
• •Panels slide horizontally on a track; one or two panels slide behind each other
• •Maximum clear opening when open: typically 50% for a 2-panel slide (one panel behind the other); up to 75% for a 3-panel slide-and-fold
• •Advantage over bifold: slimmer frame profiles possible; operates on a single smooth slide action; no panels projecting into the garden when open
• •Better thermal performance than bifold: the frame rebate depth and seal quality is typically higher for sliding systems, resulting in better weather tightness
• •Cost: similar to bifold for equivalent specification

• *Stacking slide (multi-panel sliding)*:
• •Multiple panels that all slide to one side and stack at the end of the run — creating a near full-width opening
• •Requires a deep 'pocket' or stacking zone at the end of the run — needs to be coordinated with the kitchen layout (the stack must not occupy space that is needed for kitchen units or circulation)

**Roof glazing and natural light in London kitchen extensions**:

• The roof lantern or rooflight is the signature element of most London single-storey kitchen extensions — providing natural daylight from above that transforms a previously dark rear kitchen into a bright, open space. Design considerations:
• •Position: centre of the kitchen/dining zone, clear of the structural opening beam above (the beam must be positioned at the edge of the roof light, not across it)
• •Size: limited by Part L 25% glazing rule (see above) and the structural capacity of the flat roof joists (heavier lanterns require structural engineer confirmation of joist sizing)
• •Solar control: rooflights face up — south-facing rooflights in particular can cause significant overheating in summer. Solar control glass (g-value ≤0.35) and/or opening vents for stack-effect cooling are strongly recommended for south-facing roof lanterns
• •Specification: see `roof-lantern-guide` for full glazing specification detail