Choosing Windows and Doors for Your Home Extension
The windows and doors you choose for your extension have a significant impact on: how much natural light you get; the thermal performance of the space; the acoustic performance; security; maintenance requirements; and the overall aesthetic. It's one of the higher-cost elements of an extension, and one where the right specification makes a meaningful difference. Here's how to navigate the options.
Glazing Performance
Modern window and door units are measured by their thermal transmittance (U-value). Building Regulations require a minimum performance for replacement windows and new openings in extensions — currently 1.6 W/m²K for windows under Part L. Better-performing units (A-rated double glazing at around 1.2–1.4 W/m²K, or triple glazing at 0.8 W/m²K or below) deliver meaningfully better thermal performance and are worth specifying in any well-insulated extension.
Solar gain is a related consideration: south-facing glazing in an extension can overheat the space in summer unless solar control glass (which limits solar heat transmission while maintaining light transmission) is specified.
Frame Materials
Aluminium. The most popular choice for extensions in London's design-conscious market. Slim sightlines, excellent durability, low maintenance, good thermal performance (in thermally broken profiles), and available in any RAL colour. Cost: medium-high. Suits contemporary and transitional designs well.
uPVC. The most common and lowest-cost frame material. Good thermal performance, maintenance-free, durable. Sightlines are thicker than aluminium. Suitable for more traditional or budget-conscious projects. Cost: low-medium.
Timber. Traditional and visually warm, particularly appropriate for period properties. Requires periodic maintenance (painting or oiling). Modern engineered timber or timber-aluminium composite systems (timber inside, aluminium outside) reduce the maintenance burden. Cost: medium-high.
Door Types for Extensions
Bifold doors. Multiple panels that fold back to fully open the rear wall. Maximum opening width; panels fold to one or both sides. Excellent for garden connection. Require clearance zone for folded panels. Sightlines slightly thicker when open than slide-and-fold alternatives.
Sliding doors. Two or more panels that slide parallel to each other. Thinner frame profiles than bifolds, sleeker appearance. One panel typically always covers part of the opening (50% in a two-panel system). Available in very large formats.
Pivot doors. A single panel that rotates on a central pivot point. A strong architectural statement for a wide single opening. Works best as a front door or feature entrance rather than a rear garden connection.
French doors. Traditional two-panel outward-opening doors. Narrower maximum opening than bifolds or sliders. Well-suited to traditional or Victorian-influenced designs.
Budget Guide
Rooflights: £600–£2,500 per unit depending on size and specification. Bifold doors: £2,000–£7,000+ per set. Sliding doors: £3,000–£10,000+ per set. Standard aluminium windows: £800–£2,500 per unit. Budget for supply only — installation is additional.
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