Windows for House Extensions: Types, Performance, and Specification

**Casement windows**:

The most common window type for modern extensions. A casement window has a frame fixed to the wall opening and one or more sashes (opening panes) hinged on a vertical edge (side-hung casement) or a horizontal edge (top-hung or bottom-hung casement). Available in uPVC, aluminium, and timber.

*Advantages*: Simple operation, good weather sealing, wide range of sizes and configurations; aluminium systems (Reynaers, Smart, Schueco) provide very slim sight lines that maximise the glazed area

*Typical use in extensions*: Fixed-size windows in the side or rear elevations of extensions where a simple, clean design is required

**Sash windows**:

A traditional London window type: a pair of sashes (upper and lower panes) that slide vertically past each other. Original Victorian sash windows are single-glazed timber; modern replacement sash windows are available in slim double-glazed timber (to match Conservation Area character) or in uPVC or aluminium.

*Use in extensions*: Where the extension is in a Conservation Area and the design must respond to the character of the original sash windows of the house; or where the extension faces the street and planning requires windows compatible with the street scene.

*Modern double-glazed timber sash suppliers*: Roseview, Ecoease, Bereco — all offer double-glazed slimline sash windows that achieve a reasonable match to original Victorian sash proportions and maintain Conservation Area compatibility.

**Fixed glazing panels**:

Fixed (non-opening) glazing panels are used where ventilation is not required from the window position and maximum light is the primary objective. Fixed panels have slightly better weather sealing than opening windows (no hinge hardware or moving seals to degrade) and can span larger areas without intermediate frames.

For an extension with a Juliet balcony, a full-height fixed glazed panel (floor to ceiling) with a safety rail at the appropriate height (1,100mm from finished floor level) provides views of the garden without the cost of a genuine balcony. Part K requires the glass to be safety-rated (toughened or laminated) below 800mm.

**Bifold doors**:

Bifold doors are a folding-stacking system where the doors fold in pairs or groups against one end of the opening when open, providing a wide unobstructed opening between the extension and the garden.

*Advantages*: Full-width opening creates a genuine 'inside-outside' connection; visually impactful; popular in London kitchen-dining extensions *Disadvantages*: The folded door stack takes space at the end of the opening; the multiple hinges and track system require regular maintenance; less thermally efficient than a sliding system for the same glazed area (more thermal breaks at the hinge joints); lead time typically 6–10 weeks for major suppliers *Main suppliers*: Origin (UK-manufactured, B2 certification, most trusted in the London market), Smart Architectural Aluminium, Schuco *U-value*: Quality bifold systems achieve U-values of 1.4–1.6 W/m²K (frame + glass composite) — marginally worse than a fixed panel but within Part L limits

**Sliding doors (lift-and-slide, parallel siding)**:

Sliding door systems have one or more large panes that slide parallel to the face of the building, passing behind or in front of a fixed panel. The panes 'lift' slightly off the track on a cam mechanism for opening, then 'slide' with minimal effort.

*Advantages*: No door stack within the opening; wider spans possible than bifold without an intermediate post; better thermal performance than bifold (fewer hinge thermal breaks); more secure (no exposed hinges) *Disadvantages*: The opening width is limited to approximately 50% of the total system width (one panel slides behind another); require very precise installation on a level track; higher cost than bifold for equivalent aperture *Main suppliers*: Schuco, Sky-Frame, Kawneer; premium systems from Swiss manufacturers (Sky-Frame) can achieve U-values as low as 0.6 W/m²K for triple-glazed frameless systems *U-value*: Aluminium sliding systems: 1.2–1.4 W/m²K for quality double-glazed; triple-glazed systems: 0.6–0.9 W/m²K

**Aluminium**:

Aluminium is the dominant material for windows and doors in modern London extensions. Aluminium sections are extruded with a thermal break (a polyamide strip interrupting the aluminium profile) to prevent cold bridging through the frame.

• *Advantages*:
• •Very slim sections — aluminium can achieve sightlines as narrow as 25–35mm, maximising the glazed area
• •Durable and virtually maintenance-free (powder coat finish lasts 20–30 years)
• •Wide colour range (any RAL colour available in powder coat)
• •Strong enough to span wide openings without intermediate posts
• •Can achieve very low U-values with triple glazing

• *Disadvantages*:
• •Higher initial cost than uPVC (typically 20–40% more for the same size)
• •Planning sensitivity in Conservation Areas — modern aluminium finishes may not match the character of original Victorian timber windows; some Conservation Area conservation officers accept slim aluminium windows in neutral colours; others require timber

*Planning position for aluminium in Conservation Areas*: In many London Conservation Areas, slim-profile aluminium windows are now accepted as a 'high quality contemporary material' rather than requiring timber — but this varies by borough and by Conservation Area character; always confirm with the planning authority.

**Timber**:

Timber windows have a natural character that is generally accepted in Conservation Areas. Softwood (Scandinavian pine) requires painting and maintenance every 5–10 years; hardwood (oak, meranti, iroko) is more durable and requires less frequent maintenance (oiling or painting every 10–15 years).

*Modern timber windows*: Engineered timber windows (multiple layers of timber bonded to prevent warping and movement) are significantly more stable than solid softwood windows. Brands such as Rationel (Danish engineered timber), Bereco, and Green & Abbott provide double-glazed timber windows with U-values of 1.0–1.4 W/m²K suitable for extensions in Conservation Areas.

*Cost*: Timber windows cost approximately 30–60% more than equivalent uPVC and 10–30% more than equivalent aluminium, depending on the quality specification.

**uPVC (unplasticised polyvinyl chloride)**:

uPVC windows are the most cost-effective glazing option and are widely used in back-of-house positions (utility rooms, rear bathrooms) or in outer London extensions where Conservation Area restrictions do not apply.

*Advantages*: Low cost; good thermal performance (modern A-rated uPVC windows achieve U-values of 1.2–1.6 W/m²K); minimal maintenance (no painting, clean with mild detergent) *Disadvantages*: Thicker sections than aluminium (sightlines typically 65–80mm, significantly more than aluminium at 25–35mm); limited colour options (white, brown, black, grey from stock; other RAL colours available but at premium); not accepted in most Conservation Areas

**Glazing specification — what U-values, gas fills, and coatings mean**:

*U-value*: The thermal transmittance of the glazing unit — lower is better. Part L requires windows in a new extension to achieve no worse than 1.6 W/m²K (as a stand-alone element). Modern standard double-glazed units with warm-edge spacer achieve 1.0–1.2 W/m²K; triple-glazed units achieve 0.6–0.9 W/m²K.

*Gas fill*: The void between the panes is filled with argon (double-glazing) or argon plus krypton (triple-glazing) to improve thermal performance — air-filled gaps are less effective.

*Low-E coating (low emissivity)*: A metallic coating on the inner surface of the outer pane (or outer surface of the inner pane) reduces radiant heat transfer through the glass. Standard in all modern double-glazed units. 'Hard coat' (pyrolytic) low-E is more durable but less effective than 'soft coat' (sputtered) low-E — specify soft coat for better thermal performance.

*Solar control glazing*: For south and west facing extensions with large glazed areas, solar control glass (a tinted or reflective coating that reduces solar gain) prevents summer overheating. The g-value (solar factor) indicates the proportion of solar energy transmitted — lower g-value = less solar gain. For a highly glazed south-facing extension, a g-value of 0.30–0.35 is typical for solar control glazing.

**Part L — energy performance**:

• Under Approved Document L (2021), for windows and doors in new extensions:
• •Maximum U-value: 1.6 W/m²K (centre-pane) as a limiting value for individual elements
• •Maximum total glazed area: 25% of new floor area added (the default; exceeding 25% requires an energy calculation — SAP or simplified method — showing that the overall extension meets the energy standard through enhanced insulation elsewhere)
• •Solar control: Where glazed area is high relative to floor area, and the extension is highly exposed to sun (south/west facing), overheating risk must be considered — either through solar control glass, external shading, or ventilation provision

**Part K — safety glazing**:

• Approved Document K specifies 'critical locations' where glazing must be safety glass (toughened or laminated to BS 6206 or BS EN 12600 Class 2):
• •All glazing below 800mm from floor level
• •All glazing in doors (within 300mm of a door on the side where the door opens)
• •All glazing in windows within 300mm of a door on the opening side at a height below 1,500mm
• •Glazing in a shower enclosure, bath screen, or wet area

Safety glazing breaks into blunt, small fragments (toughened) or holds together in a web when broken (laminated) rather than shattering into sharp shards.

**Part Q — security**:

Approved Document Q (2015, applying to new dwellings and new dwelling-houses including extensions that create new habitable rooms) requires that all easily accessible windows and doors (those accessible from ground level, or within 2m of a balcony or flat roof) resist attack for a minimum specified period — tested to BS PAS 24:2016.

Most mainstream aluminium and uPVC window and door systems from reputable suppliers (Origin, Smart, Schuco, Reynaers) are tested and certified to PAS 24 as standard. Confirm with the supplier before purchasing.

**Planning and Conservation Area considerations for glazing**:

*Permitted Development*: PD conditions require windows and doors in an extension to be of 'similar appearance' to the existing house. For a Victorian brick terrace, this typically means either timber sash windows (matching the originals) or slim aluminium casements in a neutral colour — both are typically accepted. Large contemporary bifold or sliding systems may require planning permission on a visible elevation.

*Conservation Areas*: Conservation Area character typically requires that windows visible from the public realm respond to the character of the area — timber sash proportions, traditional glazing bar arrangements, or appropriate contemporary materials. Confirm with the conservation officer before specifying aluminium systems on visible elevations in a Conservation Area.

*Overlooking conditions*: Planning officers routinely impose a condition on approved extensions that any window in a side elevation within 1m of the boundary, or any window overlooking a neighbouring property's garden, must be glazed with permanently obscured glass and fixed (non-opening). Confirm whether any such condition applies to your approval before window specification is finalised.