Kitchen Extension Design Guide for London Victorian Terraces: Layout, Glazing, Structure, and Services

**Decision 1 — The structural opening between existing kitchen and extension**:

In the majority of London Victorian terraces, the ground-floor rear wall of the existing house (the rear elevation wall facing the garden) is a load-bearing wall — either a solid external wall, or a cavity wall with a structural element. The extension is built immediately behind this wall, and the connection between the existing kitchen (or dining room) and the new extension space is made by removing a section of the rear wall and installing a structural steel beam (RSJ — rolled steel joist) to carry the load from above.

• *Structural opening design decisions*:
• •**Full-width opening vs. partial opening**: a full-width opening (wall-to-wall, or close to it) creates the most open, flowing connection between the existing house and the extension — the two spaces become effectively one room. A partial opening creates a more defined threshold between the two spaces. For a kitchen extension intended to create an open-plan kitchen-diner, a full-width opening is almost always the better design choice.
• •**Beam exposure or concealment**: the RSJ beam can be boxed in (surrounded by plasterboard and finished as a flat ceiling soffit) or exposed (left as a visible steel beam in the ceiling plane). Exposed steel beams in kitchen extensions are increasingly popular in London contemporary interior design — they provide a raw industrial aesthetic contrast with a kitchen fitted below. Agree the beam exposure preference with the structural engineer at design stage, as exposed beams require different surface treatment (shot-blasted; painted; or intumescent fire protection coating) than concealed beams.
• •**Beam depth and ceiling height**: the RSJ size required depends on the span (width of the opening) and the load above. A typical 3–4m span opening in a London terrace might require a 203mm or 254mm deep universal beam (UB) — which reduces the headroom in the opening zone by the beam depth. This must be accounted for in the ceiling height design: if the existing ceiling is 2.4m, a 254mm beam reduces the headroom at the beam to approximately 2.15m — which is acceptable but must be checked.
• •**Multiple opening elements (bifold/slide-and-fold door panel within the structural opening)**: where bifold or sliding doors are specified within the structural opening, the opening must be designed to accommodate the door frame/threshold assembly as well as the structural steel. Coordinate the structural engineer's opening specification with the bifold/sliding door supplier's frame dimensions.

• *Typical structural opening costs (London 2025)*:
• •Structural engineer design: £500–1,500
• •Demolition and making good: £500–1,500
• •RSJ beam supply and install (3–4m span; propped): £1,500–4,000 (inclusive of temporary propping; structural connection; fire protection)
• •Building Regulations approval for structural opening: included in the extension Building Control application

**Decision 2 — Roof form: flat roof vs. pitched roof for a London kitchen extension**:

The large majority of London Victorian terrace kitchen extensions use a flat roof (or very low-pitch hidden behind a parapet). A small proportion use a lean-to pitched roof (pitched up from the existing rear elevation, with the ridge against the existing wall). The design choice has major implications for headroom, glazing, drainage, and cost.

• *Flat roof — why it is the default for London kitchen extensions*:
• •**Maximises internal height**: a flat roof extension can achieve the same headroom (typically 2.4–2.7m) across the full depth of the extension — there is no reduction in height toward the garden end (as there would be with a lean-to pitched roof)
• •**Enables rooflights**: a flat roof is the natural location for rooflights — fixed or opening glass panels set into the roof plane that flood the kitchen extension with natural daylight from above. Rooflights are one of the most valuable design features of a London kitchen extension — they dramatically improve natural light in the middle of the extension where a wall window cannot reach
• •**Simple drainage**: flat roof drainage via internal or external outlets is well understood and straightforward
• •**Part B fire boundary**: a flat roof within 1m of the party boundary must use AA-rated roofing membranes and fire-rated glass rooflights — manageable requirements
• •**Cost**: flat roof construction is slightly less expensive than an equivalent pitched roof in most London configurations

• *Lean-to pitched roof — when to consider it*:
• •Conservation area planning requirements: some London conservation areas have policies that prefer pitched roofs on extensions visible from the public realm (though this is more relevant to side extensions and front extensions than to rear kitchen extensions)
• •Client preference: some clients prefer the appearance or tradition of a pitched roof
• •Avoiding roof drainage into the property: a pitched lean-to roof drains away from the house (down the pitch toward the garden) rather than requiring internal roof drainage

*Flat roof construction specification for London kitchen extensions*: The standard flat roof specification for a contemporary London kitchen extension is a warm flat roof: 1. Structural deck: 18–22mm tongue-and-groove WBP plywood on C24 timber joists (or engineered joists — I-joists or LVL — for longer spans or higher loads) 2. Vapour control layer (VCL): a polyethylene or foil-faced VCL on the warm side of the insulation (below the insulation), taped at all joints 3. PIR insulation boards: 140–180mm PIR (lambda 0.022 W/mK) to achieve Part L 2021 Ur ≤ 0.18 W/m²K, laid in two layers with staggered joints above the structural deck and VCL 4. Waterproof membrane: EPDM (single-ply rubber membrane, fully adhered — most common for London kitchen extensions; 20+ year lifespan); GRP fibreglass; or hot-melt bituminous waterproofing 5. Roof drainage: internal roof outlets (drain through the extension ceiling and wall) or external edge-mounted outlets (overflow weir at edge) 6. Rooflights: set into the roof build-up in framed upstands (curbs), sealed with manufacturer's flashing kit

**Decision 3 — Glazing strategy: rooflights; bifold doors; sliding doors; and the balance of fixed vs. openable**:

*Rooflights — the most important source of natural light in a London kitchen extension*:

In a typical London Victorian terrace, the rear kitchen extension is constrained on both sides by the party walls, and the only wall available for windows or doors is the rear (garden-facing) elevation. This means that the side walls and the front connection to the existing house have no glazing, and natural light in the middle of the extension (far from the rear elevation glazing) depends almost entirely on rooflights.

• The two main rooflight options for a London kitchen extension:
• •**Flat glass rooflights (individual units)**: typically 600mm × 900mm to 1,000mm × 2,000mm fixed or opening flat glass rooflights set into the roof plane. Fixed-only rooflights are less expensive and have a cleaner appearance; opening rooflights (electrically operated; with sensor to close in rain) provide natural ventilation from above — particularly useful for summer cooling in a kitchen extension where the main bifold doors face south. Popular brands: Vitral flat roof windows; Velux flat roof windows; Fakro flat roof windows. Typical cost (1,000 × 1,000mm, double-glazed, fixed): £400–800 supply.
• •**Roof lanterns / glass roof structures**: a roof lantern (a three-dimensional glazed structure, typically aluminium-framed, projecting above the roof plane) provides overhead glazing in a more architectural statement. Roof lanterns are popular in higher-specification London kitchen extensions. They provide more headroom under the glazed section than a flat rooflight and create a strong visual focal point. Typical cost (1,000 × 2,000mm double-glazed aluminium roof lantern): £2,000–5,000 supply.

*Bifold doors vs. sliding doors — garden connection glazing*:

The rear (garden-facing) elevation of the extension is almost always fully glazed, with the connection to the garden being the primary design feature of the space. The two most common systems are bifold doors and large-format sliding doors (sometimes called slide-and-fold, or slim-section aluminium sliders).

| Feature | Bifold doors | Sliding doors | |---|---|---| | Opening width (max clear open) | Up to 90% of aperture width (panels fold and stack) | Typically 50% of aperture width (one half slides behind the other) or up to 100% (pocket sliders where the door slides into the wall) | | Aperture options | 2-panel up to 6+ panel; folding to one side or both | 2-panel (most common); 4-panel; or multi-panel | | Profile sight lines | Wider profiles when panels fold (stack of panel edges visible when open) | Slimmer profiles; less visual obstruction at edges | | Air tightness (closed) | Typically lower than equivalent sliding door (more seals; more junctions) | Better air tightness; better weather performance | | Typical cost (3m × 2.1m aperture, aluminium, double-glazed low-E) | £3,500–7,000 supply | £4,000–8,000 supply | | Planning (conservation area) | Aluminium bifold or slider usually acceptable on rear elevations | As bifold |

*Recommendation for most London kitchen extensions*: bifold doors where maximum opening (full connection to garden) is the priority; aluminium slim-section sliding doors where weather performance, air tightness, and slimmer sightlines are priorities or where the extension is on a particularly exposed (north; east) elevation.

**Underfloor heating in a London kitchen extension — the default specification**:

As described in the `heating-system-extension-guide`, wet underfloor heating (UFH) in the concrete slab of a single-storey kitchen extension is the standard, preferred heating specification for almost all contemporary London kitchen extensions. The practical specification for a kitchen extension UFH system:

*Slab and UFH sequence*: 1. Hardcore/compacted fill below slab 2. DPC membrane (damp proof course — 1200 gauge polythene) 3. 150mm concrete slab (C25 or C30 mix; mesh-reinforced) 4. DPM lap up walls (damp proof membrane — 1200 gauge polythene, lapped up to DPC in walls) 5. 100mm PIR insulation below UFH screed (Part L 2021 requires this; also improves UFH efficiency) 6. UFH pipework (16mm PE-RT; manifold-connected; zones per room) 7. Sand:cement screed (65mm minimum above pipe) or anhydrite liquid screed (45mm minimum above pipe) 8. Final floor finish (porcelain tile; natural stone; engineered timber — see above for floor temperature limits with timber)

*The UFH manifold position in a kitchen extension*: the UFH manifold (the flow/return header with zone valves and balancing valves for each circuit) is typically housed in a manifold cupboard or in an accessible service void. For a kitchen extension, position the manifold where it can be easily accessed for commissioning and servicing — typically in a utility room, under-stair cupboard, or dedicated manifold cupboard. The manifold must be accessible after the kitchen is fitted — do not locate it behind fitted kitchen units with no access panel.

**Kitchen extraction in a London extension — the most commonly under-designed element**:

Kitchen extraction — removing the cooking fumes, steam, and smells from the kitchen — is one of the most commonly under-designed elements in a London kitchen extension. Building Regulations Part F requires a minimum extract rate of 60 litres/second (l/s) adjacent to the hob (or a recirculating cooker hood). However, a larger open-plan kitchen-diner extension generates significant cooking odours and steam that require effective extraction to prevent the smells from permeating the living space above and adjacent.

*Extraction duct routing options in a kitchen extension*:

| Route | Pros | Cons | |---|---| | Through the flat roof (vertical duct through roof deck + cap) | Direct, short route; effective | Penetration through waterproof membrane requires careful detailing; fire-rated penetration at roof deck | | Through the external wall (horizontal duct through rear wall or side elevation) | Avoids roof penetration | Longer duct; bends reduce flow efficiency; visible external grille on wall | | Through the existing house wall into existing kitchen extract route | Uses existing route | Only where existing extract duct route is accessible and of adequate diameter | | Recirculating cooker hood (no duct; internal activated carbon filter) | No structural penetration required | Filters require regular replacement; does not remove moisture or heat; not ideal for heavy cooking |

• *Extraction duct specification for a London kitchen extension*:
• •Use 150mm circular duct (minimum for 60 l/s flow rate at reasonable velocity) — do not use 100mm round or 100mm × 150mm flat duct, which are too restrictive for effective cooker hood performance at rated flow rates
• •Duct length: keep as short as possible; each 90° bend adds the equivalent of approximately 2m of straight duct (resistance) — minimise bends
• •Insulate the duct where it passes through unheated spaces (loft; above flat roof deck) to prevent condensation in the duct
• •External termination cap: use a backdraught shutter and rain cap at the external termination — not an open-end duct

**Services planning for a kitchen extension — what to design into the extension before the slab is poured**:

The most expensive and disruptive services problems in a completed kitchen extension are those that were not planned before the slab was poured and the walls were built. The following services must be planned and installed at first-fix stage (before the screed is poured):

• *1. Drainage — waste connections from kitchen sink, dishwasher, and any utility sink*:
• •The kitchen sink waste pipe (typically 40mm diameter) must be connected to the existing drainage system — either via a new connection to the soil stack, or via a drain running through the floor slab to an underground connection
• •Kitchen waste pipes under the slab run in ducts or sleeves (not bare pipes cast into the concrete — they must be accessible for rodding in case of blockage)
• •For an extension with a kitchen island sink: a drain must run under the slab from the island position to the wall connection — plan this before the slab is poured, not after

• *2. Electrical first-fix — socket positions; lighting; extraction fan connection*:
• •Extension slab and screed stage: install conduits in the slab floor for any services that cross the floor (power runs to island units; in-floor heating controls; floor-mounted sockets)
• •Wall first-fix: all socket positions; switched spur positions for appliances; underfloor heating thermostat positions; extractor fan wiring
• •Rooflights with electric opening mechanism: provide a 230V switched spur at the rooflight position in the ceiling

• *3. Boiler and heating connections*:
• •First-fix heating pipes (flow and return) from the boiler to the UFH manifold
• •Hot water supply to kitchen sink
• •Coordinate boiler pipe positions with kitchen unit layout — pipes running behind a bank of kitchen units must be accessible via access panels or must be re-routed to a location that does not conflict with unit positions and the unit kick-board line

• *4. External water tap and outdoor lighting*:
• •Install a garden tap connection at first-fix stage (external bib tap, frost-protected stop valve on the supply) — run the supply pipe under the extension floor or through the extension wall before the screed is poured
• •Garden lighting and power: run external armoured cable (SWA — steel wire armoured) from the consumer unit to the garden positions at first-fix stage — burying it under the extension slab before the screed is poured is far easier than excavating after completion

**The four main kitchen layout options for a London terrace extension**:

*Layout 1 — Galley kitchen in extension (run of units along one wall)*:

A galley kitchen layout (kitchen units along one side wall of the extension, with the opposite side wall free as a dining or seating area) is the simplest and most space-efficient layout for narrower London extension configurations — where the extension is 3–4m wide and 4–6m deep. Advantages: maximises the open circulation space; leaves one side wall clear for a dining table; places all kitchen services (sink; dishwasher; oven; hob; extractor) in one run (reducing plumbing and drainage complexity). Disadvantage: only one working surface run; less storage than an L-shaped or island configuration.

*Layout 2 — L-shaped kitchen in extension (units on two walls)*:

An L-shaped kitchen (units along the rear/garden-facing wall AND one side wall) is the most common layout in a London kitchen extension where the rear glazing is bifold or sliding doors across the full width. The 'L' of kitchen units sits against the side wall (which is a solid party wall — no windows) and wraps around the kitchen area, leaving the centre of the extension as an open dining/social space. Advantages: more storage than a single run; divides the extension into distinct kitchen and dining zones without a physical partition; allows the cooker/hob to be against the side wall with the extractor duct routed through the side wall. Note: the key challenge of an L-shaped layout with bifold doors across the rear elevation is that the rear wall of units must be set back from the bifold door threshold to allow the doors to open and stack inward without blocking access to the units.

*Layout 3 — Island kitchen in extension*:

A kitchen island (a freestanding or fixed central run of units in the middle of the extension floor area, usually with the hob and/or sink in the island, and bar seating on the garden/dining side) is the most popular premium specification for larger London kitchen extensions (25m²+ floor area). Islands require: (a) adequate floor area (minimum approximately 900mm clear circulation space on all sides of the island — typically requires 4m+ extension width); (b) a drainage run under the slab from the island sink position to the wall (must be planned before the slab is poured); (c) an extraction duct from the island hob position to the external wall or roof (island cooker hoods — ceiling-mounted hood above the island hob; or downdraft extractors in the hob unit itself — are the two options for island extraction without a wall-mounted cooker hood). Island cooker hoods are striking design features and are increasingly popular in London kitchen extensions. Cost for a ceiling-mounted island extractor hood: typically £600–3,000 supply (Elica; Novy; Bora cooktop with integrated downdraft — Bora Classic or Pro is the premium integrated solution).

*Layout 4 — Peninsula kitchen in extension*:

A peninsula layout is similar to an island but with one end of the central unit connected to the side wall or existing kitchen wall — a three-sided accessible worktop rather than four-sided. The peninsula creates a semi-enclosed kitchen space and a breakfast bar on the open side (facing the dining/garden area). Advantages over a full island: easier drainage (the peninsula connects to the side wall, so sink waste can run through the wall without a sub-slab drain); slightly less floor area required than a full island. Used in extensions that are wide enough for a peninsula but narrower than the minimum recommended for an island.

**Connection between the extension and the existing house — level threshold considerations**:

One of the most commonly overlooked design details in a London kitchen extension is the relationship between the existing house floor level and the new extension floor level — and the connection to the rear garden.

• *Extension floor level relative to existing kitchen floor level*:
• •The new extension concrete slab must accommodate: the DPC membrane; the insulation layer; the UFH screed; and the final floor finish. Total build-up typically 300–450mm above the existing hardcore level. The existing kitchen floor level is typically 150–200mm above the external ground level (adjacent to the garden), making the new extension floor slab achievable at the same finished floor level as the existing kitchen with some care in setting out the slab formation level.
• •Where the extension is level with the existing kitchen: a flat threshold at the structural opening (ideally: a discreet aluminum threshold bar that accommodates the flooring transition) allows a seamless flow from the existing kitchen into the extension — the floor finish can run continuously through both spaces with no step. This 'level threshold' design is highly desirable and adds significantly to the sense of space and connection between the two areas.
• •Where a step is unavoidable (if the existing kitchen floor level cannot be matched): a single 100–150mm step up or down into the extension is functional, but a step at the threshold of a kitchen extension is a tripping hazard (particularly at night, or for elderly occupants) and should be avoided where possible by adjusting the slab formation level at design stage.

• *Extension floor level relative to rear garden level*:
• •Most London Victorian terraces have a rear garden that is 100–200mm lower than the existing ground floor of the house. The extension floor level (which matches the existing house floor) is therefore elevated relative to the garden — a step down to the garden is typically required at the bifold/sliding door threshold.
• •Part M (Access and facilities for disabled people): where reasonably achievable, a level or near-level threshold from the extension to the garden is preferable. A threshold drainage channel immediately inside or outside the bifold/sliding door can accommodate a low-profile threshold seal while managing any rainwater ingress at the door frame. Premium bifold and sliding door systems have very low-profile threshold bars (20–30mm) that are close to level with the external paving — minimising the trip risk.
• •External patio level: the patio or deck in the rear garden immediately outside the bifold/sliding doors is typically raised slightly (by 50–100mm above the general garden level) to be close to the extension floor level, with a shallow step from the patio to the main garden — creating a graduated connection from the extension, to the patio, to the garden.