Window Installation in London Extensions: Sequence, Building Regulations, and What to Specify

**The main window types used in London residential extensions**:

• *uPVC windows*:
• The most widely installed window type in London residential extensions (particularly rear and side extensions where visibility from the street is limited). uPVC is durable, low maintenance, and available from most window manufacturers in standard sizes with short lead times.
• •Typical lead time from standard manufacturer: 7–14 working days
• •Custom sizes (non-standard modules): 10–21 working days
• •U-value (whole window): typically 1.2–1.4 W/m²K for a standard double-glazed uPVC window (well within the Part L notional U-value of 1.6 W/m²K for replacement windows; 2.0 W/m²K for new extensions in most calculations)
• •Cost: from approximately £250–£600 per casement window (standard size) supply only; installed: £350–£900 per window

• *Aluminium windows*:
• Increasingly the preferred choice for contemporary London extensions — slim sight lines; powder-coated colour finishes (RAL and BS colour range); thermally broken frames (essential for residential use — without a thermal break the aluminium frame becomes a significant cold bridge).
• •Typical lead time: 4–8 weeks for standard aluminium windows; 6–12 weeks for bespoke or large formats
• •U-value (whole window, thermally broken aluminium): typically 1.2–1.6 W/m²K depending on glazing specification and frame size
• •Cost: significantly higher than uPVC — from approximately £600–£1,500 per casement window supply only; installed: £800–£2,200 per window; large sliding/fixed lights can be considerably higher

• *Timber windows*:
• Used in conservation areas where planning requires traditional materials; also used in high-specification Victorian terrace refurbishments where matching the original window style is important.
• •Timber softwood (painted): traditional Victorian sash or casement; requires painting every 5–7 years; specialist slim double-glazed units available in authentic timber sash frames
• •Timber engineered hardwood: more durable than softwood; natural colour or stained; longer maintenance interval
• •Lead time: 4–8 weeks for standard; 8–12 weeks for bespoke sash or heritage profiles

**Window energy performance and Part L**:

• For a new extension, Approved Document L1B (Conservation of fuel and power) requires that windows and doors in the extension achieve a minimum energy performance:
• •Whole window U-value: maximum 1.6 W/m²K (from June 2022 Part L update)
• •Energy rating: minimum window energy rating of Band B (WER B), under the British Fenestration Rating Council (BFRC) scheme — as an alternative to the U-value route

Most modern double-glazed windows from reputable UK manufacturers meet these requirements. For extensions in exposed locations (south-facing with significant solar gain potential), g-value (solar energy transmittance) is also relevant — a lower g-value (0.35–0.45) reduces summer overheating risk but slightly reduces passive solar gain in winter.

**Glazing specification for the insulated glass units (IGU/DGU)**:

• The glazed area is typically a greater proportion of the window's total area than the frame, so the IGU specification has the largest impact on thermal performance:
• •Standard DGU (double glazed unit): 4mm outer glass / 12mm argon-filled cavity / 4mm inner low-emissivity (low-e) glass — centre-pane U-value approximately 1.1–1.2 W/m²K; whole-window U-value with uPVC frame approximately 1.4 W/m²K
• •Better DGU: 4mm / 16mm argon / 4mm low-e — centre-pane U-value approximately 1.0 W/m²K
• •TGU (triple glazed unit): 4mm / 12mm / 4mm / 12mm / 4mm — centre-pane U-value approximately 0.7 W/m²K; whole-window U-value approximately 0.8–1.0 W/m²K; better thermal performance but heavier (impacts on hinge and hardware specification) and higher cost (approximately 30–40% more than DGU)

**Safety glazing requirements (Building Regulations Part K, now incorporated into Part N for England)**:

• Toughened or laminated safety glazing is required in 'critical locations' where impact is foreseeable:
• •Any glass less than 800mm above finished floor level (including low-level glazed panels below windows)
• •Any glass within 300mm of a door in the area between 1500mm and 300mm above floor level
• •All glass in doors and side panels within 300mm of door edges between 1500mm and 300mm above floor level
• •Glazed doors (throughout)
• •Rooflights (all glazing in a rooflight)

For a rear extension with floor-to-ceiling glazed panels (below a high-level fixed light or sliding door), the glass must be toughened or laminated. Standard toughened glass (toughened safety glass to BS EN 12150) shatters into small, blunt fragments; laminated glass (two panes bonded with an interlayer) holds together when broken. For overhead glazing (rooflights), laminated is preferred — it retains broken glass rather than dropping shards.

**Part B — Means of escape windows**:

In a dwelling, every habitable room on the first or second floor that does not have direct access to a protected staircase escape route must have an escape window (also called a 'fire egress window') — a window that opens sufficiently to allow occupants to climb out in a fire emergency.

• Minimum dimensions for an escape window under Part B:
• •Clear opening area: minimum 0.33m² (e.g., 600mm × 550mm = 0.33m²)
• •Clear opening height: minimum 450mm
• •Clear opening width: minimum 450mm
• •Bottom of clear opening: maximum 1100mm above finished floor level (so a person can reach through and climb out without difficulty)

For a loft conversion, the Part B escape window requirements apply to the loft bedroom unless: the staircase from the loft to the ground floor is enclosed in a 30-minute fire-resistant enclosure AND fire doors are provided. See the `building-regulations-part-b-guide` for the full loft conversion Part B specification.

For a ground-floor extension (single-storey rear extension), escape windows are not typically required — the extension is at ground level and the room has direct external access through the glazed doors or opening windows. However, if the extension includes accommodation above (a two-storey extension) the first-floor rooms must comply.

**Part F — Ventilation and trickle vents**:

• From 2022 Part F updates (and the Building Regulations Approved Document F 2021), all replacement windows and new windows in extensions must include trickle ventilators unless the building is demonstrated to have equivalent background ventilation provision:
• •Minimum trickle vent equivalent area (EA): 8,000mm² EA for a kitchen; 4,000mm² EA for habitable rooms
• •Equivalent area is not the same as geometric area — check with the trickle vent manufacturer for the EA rating of their product
• •Trickle vents are typically located in the window frame head, or occasionally in the transom
• •Conservation areas: slot ventilators (less visible) are available as an alternative to standard trickle vent profiles

**Part M — Accessibility and threshold heights**:

• Building Regulations Part M (Access to and use of buildings) — Category 1 (Visitable Dwellings) requires that the extension meets minimum accessibility standards for visitability:
• •External threshold: step-free access to the extension from the outside should be provided where practicable — the Part M guidance requires that the finished floor level of the extension is within 15mm of the external threshold level (a 'level threshold' or 'accessible threshold')
• •The threshold height affects the detail at the bottom of sliding or bifold doors — flush thresholds typically require a recessed threshold drain or a concealed drainage channel to prevent water ingress at the level threshold
• •For patio doors on a ground-floor rear extension, a 'visitable' entrance with a maximum 15mm upstand is the Building Regulations standard — often achieved with an aluminium threshold ramp

**Cavity wall window installation — the correct detail for London extension cavity walls**:

The vast majority of London residential extensions are built with cavity masonry external walls (outer leaf facing brick or render block; 100mm cavity with partial or full-fill insulation; inner leaf 100mm blockwork). Installing a window into a cavity wall requires attention to:

• *1. Window frame size and reveal position*:
• The window frame must be ordered to the correct size for the opening. A common rule:
• •Frame width = opening width minus 10–20mm (5–10mm tolerance each side)
• •Frame height = opening height minus 10–20mm

The window is typically positioned at the outer face of the inner leaf (the inner third of the cavity wall) — this keeps the window frame within the insulated zone and minimises the cold bridge at the reveal. Some architects position the window in the middle of the cavity — more airtight detail but requires a custom cavity closer with frame tie-in.

• *2. Cavity closers*:
• At the sides, top, and bottom of every window opening, the cavity between the inner and outer leaf is closed with a proprietary cavity closer (insulated plastic or masonry closer) that:
• •Closes the cavity (prevents air and water penetration)
• •Provides thermal insulation at the reveal to reduce the cold bridge at the window junction
• •Provides a key for the internal plaster to return into the reveal
• •Provides a seal for the window frame to fix and seal against

*3. DPC (damp proof course) at cill*: At the base of the window, a DPC (lead-core or polythene DPC) is required under the window cill to prevent water ingress at the sill. The DPC must extend into the outer leaf and be turned up to form a back damp and up into the cavity closer on each side — a continuous water-shedding detail. Without a DPC at the cill, water penetrating behind the cill will saturate the blockwork below the window over time.

• *4. Lintel*:
• The structural lintel above the window opening carries the masonry above. For a cavity wall extension:
• •Combined metal lintel (e.g., Catnic CN71A or Birtley range): a proprietary galvanised steel lintel that spans both the inner and outer leaf simultaneously, with an insulated infill section to reduce the cold bridge at the lintel. Most London extension builders specify a combined cavity lintel from a standard manufacturer's range
• •Separate lintels (one for inner leaf, one for outer leaf): more flexible for very wide spans or unusual loadings; requires two lintels at different heights (the inner leaf lintel carries more load and is typically deeper)

• *5. Airtightness sealing*:
• After the window frame is installed and fixed, the gap between the frame and the reveals/cill/head must be filled and sealed to achieve airtightness:
• •Expanding foam: fills the structural gap between frame and opening; provides some additional air sealing
• •Compriband (pre-compressed foam tape): specifically designed for window frame air sealing; applied to the frame before fitting; the tape expands to fill and seal the gap between frame and opening; vapour-open (allows moisture to escape) while air-tight and weather-tight
• •Breather membrane tape or liquid-applied airtightness membrane: applied over the frame-to-reveal joint on the inside of the frame; part of the continuous airtightness layer of the building

For modern London extensions targeting SAP/DER (SBEM) compliance and good EPC ratings, airtightness at windows is a critical detail. Poorly sealed windows are the single largest source of uncontrolled air leakage in new extensions.

**The window installation sequence in a London extension build programme**:

Windows are installed at the 'weathertight' stage of the build — after the roof structure and waterproof membrane are complete, but before plastering or plasterboarding begins:

1. Weeks 1–5: foundations and structure 2. Week 5: roof structure complete 3. Week 6: roof waterproofing (EPDM/flat roof membrane or roof tiles) 4. **Week 7: windows and external doors installed** (making the building weathertight and secure) 5. Week 7–8: first-fix M&E (plumbing and electrical first-fix) 6. Weeks 8–9: insulation and plasterboard 7. Week 10 onwards: plastering; second-fix; decorating

• Windows must be on site and ready to install at Week 7 — a common programme risk is delayed window delivery (particularly for aluminium windows with 6–8 week lead times). Always order windows as soon as the design is frozen (not when the roof is going on). A delayed window delivery can hold up the entire build programme because:
• •The extension remains unweathertight — work inside (first-fix plumbing and electrics) cannot start in wet conditions
• •The site is less secure without glazed windows
• •Heating (which dries the building out before plastering) cannot be commissioned without airtight windows in place

**Window installation costs for London extensions (2025)**:

• *Supply cost — uPVC double-glazed windows (per window, standard residential sizes 900×1200mm to 1200×1500mm)*:
• •Casement opener: £250–£500 supply
• •Fixed light: £150–£350 supply
• •uPVC sliding patio door (1800×2100mm): £600–£1,200 supply
• •uPVC bifold door (3 or 4 panels; 2400mm wide): £1,500–£3,000 supply

• *Supply cost — aluminium thermally broken windows*:
• •Casement opener: £500–£1,200 supply
• •Fixed light: £350–£800 supply
• •Aluminium sliding patio door: £1,500–£3,500 supply
• •Aluminium bifold door (4 or 5 panels; 3000mm wide): £3,500–£8,000 supply

• *Installation (fitting) labour costs — per opening, London residential*:
• •Standard window (up to 1500mm width): £150–£350 per window
• •Large fixed light or patio door: £250–£500 per door set
• •Bifold or sliding system: £400–£800 per set

**Common window installation mistakes in London extensions**:

*1. Ordering windows too late (aluminium especially)*: The single most common programme risk. Aluminium windows with bespoke powder coat colours or non-standard sizes routinely take 6–8 weeks. Order as soon as the structural engineer has confirmed the lintel specification and the planning permission is in place — not when the roof goes on.

*2. Wrong frame dimension (building to drawing tolerances vs. actual site opening)*: On-site masonry openings are rarely exactly as drawn — they are typically 5–15mm wider or taller than the drawing dimension. The window frame must be sized to the actual opening, not the drawing dimension. A window surveyor should measure the actual opening after the masonry is complete before placing the order — or the frame must be ordered with sufficient tolerance to allow for adjustment.

*3. Omitting trickle vents*: Some window installers omit trickle vents to simplify the installation or because the client did not specify them. This is a Building Regulations non-compliance (Part F 2021) for new-build windows in extensions. Building Control will require trickle vents to be retrofitted before issuing the completion certificate.

*4. No DPC at cill level*: Omitting the DPC under the cill is a common defect in London extension construction — particularly for contractors who are not specialist window fitters. Without a DPC, the cill-level masonry becomes saturated with rain over time, staining the brickwork below and eventually causing internal damp at the base of the window.

*5. Gap not sealed (airtightness)*: Leaving the frame-to-reveal gap unsealed (or sealed only with expanding foam which shrinks over time) is a significant airtightness failure. Use Compriband tape or airtightness membrane tape to seal the frame perimeter on the internal face, as part of the continuous air barrier layer.