Windows and Doors for a London Home Extension: Options, Costs and Planning Rules

Bifold doors are the most popular glazed opening element for the rear wall of a London ground-floor extension. They fold flat against the wall when fully open, creating a wide unobstructed opening to the garden and connecting the indoor and outdoor spaces visually and practically. Frame materials: aluminium bifold doors are the standard choice for a modern London extension. Aluminium has much narrower sight lines than uPVC (25-35mm aluminium profile vs 70-80mm uPVC profile), is stronger and more durable, and is available in a wide range of powder-coated colours (RAL and BS colour range). Thermally broken aluminium (an insulating polyamide bridge between the inner and outer aluminium profiles) is the standard specification for UK domestic bifold doors — it significantly reduces cold bridging at the frame and improves the U-value of the unit. uPVC bifold doors are the budget option — wider sight lines but significantly lower cost. Wood-framed bifold doors (hardwood Accoya, softwood Sapele) are an option for period properties where the aluminium profile appearance is not appropriate. Opening configurations: bifold doors are specified by the number of panels and the direction of the primary opening. For a 2.4m-3m wide opening: 3 panels (master door plus 2 folding panels) is standard. For a 3m-4m wide opening: 4 panels. For a wider opening: 5-6 panels. The master door (the panel with the full-height handle and the main locking mechanism) typically opens first as a standard door and then the remaining panels fold flat. Threshold options: standard aluminium bifold threshold — a small raised threshold that requires a step up or step down between internal and external levels. Low-threshold and flush-threshold options — a rebated threshold that sits almost flush with the internal and external floor surfaces — are available at additional cost (£500-£1,000 extra) and provide a fully accessible, flat transition between inside and outside. Cost of bifold doors for a London extension (supply and install): standard aluminium bifold 2.4m wide × 2.1m high (3 panels, thermally broken, standard powder coat): £3,200-£5,500. 3m wide × 2.1m high (3-4 panels): £3,800-£6,500. 3.6m wide × 2.1m high (4-5 panels): £4,500-£8,000. Premium aluminium bifold (Schuco ADS 75, Reynaers CS 68, Sunflex SF55): £5,500-£12,000 for a 3m unit. LVL (structural timber) concealed frame bifold: £6,000-£12,000 for 3m. uPVC bifold (budget option): £2,000-£4,500 for 3m.

Aluminium sliding doors: inline aluminium sliding doors (panels that slide behind each other on parallel tracks) are an alternative to bifolds where: the clear opening width is less important than a cleaner sightline profile when closed; the adjacent side wall does not allow bifold panels to fold flat; or a more contemporary flush-floor aesthetic is preferred. Inline sliding vs lift-and-slide: standard inline sliding doors are the most common and affordable type. Lift-and-slide systems (the panel lifts off the threshold seal by a cam mechanism when the door is opened and lowers again when closed) provide significantly better weatherproofing and thermal performance because the door seals properly against the threshold and jambs when in the closed and locked position. Lift-and-slide is the preferred system for any exposed or south-facing opening where wind-driven rain may be a concern. Cost: standard aluminium inline sliding (2 panels, 2.4m wide): £2,500-£4,500. Lift-and-slide (2 panels, 2.4m wide): £4,000-£7,000. Premium lift-and-slide (Schuco ASS 70, 77 or AWS 65): £5,500-£12,000 for 3m. Fixed picture windows: where the design calls for a large glazed opening but not a wide opening door, fixed-frame picture windows provide maximum glazed area at minimum cost and with the best thermal and acoustic performance (a fixed pane has no moving parts, no drainage channels, and no threshold gaps, giving better overall performance than a door of the same glazed area). Fixed aluminium picture windows (2m × 1.2m, thermally broken): £800-£2,000 supply and install. Roof lanterns: a roof lantern (a raised glazed element on a flat extension roof) is the most effective way to bring natural light deep into a ground-floor extension plan — it illuminates the centre of the plan, away from the perimeter glazing. Glass type for roof lanterns: toughened and laminated safety glass is mandatory (the lantern is overhead glazing — it must not fall or shatter in a way that endangers occupants below). Low-emissivity (low-e) glass coating significantly reduces heat loss and solar gain through the lantern. Self-cleaning glass coating reduces the frequency of cleaning on a surface that is difficult to access. Cost: standard aluminium roof lantern (1.2m × 0.9m): £1,200-£2,500 supply and install. 2m × 1m roof lantern: £2,000-£4,500. Premium structural glazing roof lantern (IQ Glass, Origin, Lamilux flat roof window): £4,000-£12,000. Structural glazed wall (frameless or near-frameless): a fully glazed rear wall using a structural silicone-bonded or near-frameless aluminium system provides the maximum visual connection between inside and outside with minimal framing. Cost: £8,000-£20,000+ for a 3m wide full-height structural glazed wall, depending on system and specification.

Side windows in a London extension: side windows (windows on the flank walls of an extension, facing the neighbour's property) require careful design for both planning and privacy reasons. Under permitted development rules, side windows in a ground-floor or first-floor extension on a side elevation must be obscure-glazed and non-opening (or with opening restricted so that no one can look directly into the neighbour's property). In practice, for a ground-floor rear extension, a side window above 1.7m high may have a restricted opening but a clear lower portion — but for any side window that overlooks a neighbour's garden or window, obscure glazing is both a planning requirement and good neighbourly practice. Obscure glazing levels: obscure glass is rated by obscurity level 1 (slight obscuring — not suitable for privacy) to level 5 (fully opaque — only light transmission, no image). Level 3 (which obscures while allowing light transmission) is the standard specification for side windows in planning conditions. Conservation area and listed building glazing rules: in conservation areas and listed buildings, the planning authority typically has requirements for glazing design that go beyond the standard PD or planning application rules: Conservation area glazing materials: uPVC windows are typically refused in conservation areas — most LPA character appraisals specify 'timber' or 'metal' frames for window replacements in conservation areas. Aluminium can be acceptable if the profile and colour are appropriate. New window sizes and proportions: new windows in a conservation area extension should match the proportions and rhythm of the existing windows (the same height-to-width ratio, the same sill and head heights). Flat-roof extension glazing: a flat-roof extension in a conservation area may be required to have a raised parapet wall with limited glazing rather than a full-height glazed rear wall, depending on the specific LPA policy. Permission for conservation area window replacements: in most London conservation areas, replacing or adding windows on the front elevation requires planning permission (under Article 4 Direction removing the PD right to replace windows without planning consent). Rear extension windows typically do not require planning permission in conservation areas if the extension itself has consent, but the condition of consent typically specifies the materials.

Building Regulations Part L (Conservation of Fuel and Power) sets minimum thermal performance standards for all new glazing installed in a new extension. The maximum U-value for new glazing in a domestic extension is 1.4 W/m²K for the whole window unit (frame + glass combined). This is the minimum standard — and all standard specification double-glazed units (4mm toughened glass + 16mm argon-filled cavity + low-e coating + 4mm toughened glass, in a thermally broken aluminium frame) achieve approximately 1.0-1.4 W/m²K depending on the exact specification. The areas of glazing in a new extension are also subject to the 'notional building' calculation in Part L — the total glazed area of the new extension must not exceed 25% of the new floor area unless the non-glazed elements compensate for the additional heat loss. For example, a 20m² single-storey rear extension has a notional glazed area allowance of 5m². If the bifold door is 3m × 2.1m (6.3m²) plus a 1.2m × 0.9m roof lantern (1.1m²) — total 7.4m² of glazing — this exceeds the 25% notional limit. The Part L SAP calculation (carried out by an energy assessor or by the Building Control body) must confirm that the total heat loss of the extension envelope still meets the Part L target despite the excess glazing, compensated by the high insulation levels in the walls, roof, and floor. In practice, most standard London single-storey extensions with bifold doors and a roof lantern can be made Part L compliant by achieving high insulation levels in the walls (PIR cavity insulation) and roof (100mm PIR warm roof) — the highly insulated opaque elements compensate for the excess glazing heat loss.