Windows and Doors for London Home Extensions and Renovations

Sash windows are the defining window type of London's Victorian and Edwardian housing stock — and their treatment during renovation is one of the most contested decisions in London conservation area planning. Two broad approaches: Restore the original sash windows: draught-proof and repair the original timber box sash frames; replace glass with double-glazed units where the reveals and frame depth allow (slim-profile double glazing units — typically 6mm gap — can be fitted into original sash frames where sufficient frame depth exists); replace glass with secondary glazing (an inner pane fixed inside the existing frame) where the original frame cannot accept double glazing without alteration. Restoration approach advantages: retains the original fabric and character of the property (important in conservation areas); avoids planning permission in most cases (maintenance and repair of like-for-like does not require planning permission); often more cost-effective than full replacement in a period property with sound original frames; original old-growth timber sash frames are durable and frequently outlast replacement uPVC. Replace with new sash windows: new timber sash windows (traditional box-sash construction, typically with slim double-glazed units or vacuum glazing); new aluminium sash windows (conservation-area acceptable profiles available from Slimlite and similar manufacturers); new uPVC sash windows (acceptable in non-conservation areas where cost is a priority; generally not acceptable in London conservation areas). Conservation area rules for windows: in London conservation areas (the majority of the Victorian terrace areas of inner and outer London), replacement windows in the front elevation visible from the public realm typically require planning permission. The Borough's conservation area appraisal and Article 4 Direction will specify what is and is not acceptable. Common rules: like-for-like replacement (same frame material, profile, and dimensions) may or may not be permitted development depending on the Article 4 Direction; replacement of original timber sash with uPVC is almost universally refused in London conservation areas; replacement of original timber sash with aluminium sash in matching profile may be acceptable in some London Boroughs. Cost: sash window restoration (draught-proofing, cord replacement, minor repairs, secondary glazing): £300-£800 per window. New quality timber sliding sash (double-glazed): £1,200-£3,000 per window supply and install. New aluminium slim-frame sash (conservation specification): £1,500-£4,000 per window supply and install.

Glazed bifold and large sliding doors are one of the defining design elements of a contemporary London rear extension — connecting the kitchen-diner or living room to the garden or rear yard and maximising natural light. Bifold doors: a series of door panels (typically 3-6 panels) that fold back against the wall to create a wide opening. Benefits: extremely wide clear opening possible when fully open (4-6m clear span with panels folded back); very popular in London contemporary extension design; available in aluminium (most durable and thermally efficient with thermally broken frames), timber (premium aesthetic, requires maintenance), or composite (timber interior, aluminium exterior). Technical considerations: requires space to stack the panels when open (panels fold to one or both ends of the opening); sill height must be considered for level-access to the garden (a low cill or threshold strip to ensure a practical level transition and minimise thermal bridging at the floor/door junction); thermal performance is critical for a rear extension — U-value for a London extension door set should be 1.0 W/m²K or better. Sliding doors: large sliding panels that glide horizontally across the opening without folding into the internal space. Types: traditional lift-and-slide (panels lift slightly from the sill on a mechanism before sliding — very smooth operation; typical panel sizes up to 3m wide per leaf); inline sliding (panels slide in the same plane — typically in two or three tracks); Vitrocsa-type minimalist sliding (frameless or near-frameless glazed panels for a maximum glass, minimum frame aesthetic). Benefits over bifold: no stacking of panels in the interior space; cleaner sight lines when closed; typically better air tightness performance; larger glass panels possible (a 3m x 2.4m single sliding panel). The most popular choice for London contemporary extensions where maximum glazing and a clean frameless aesthetic is desired. Cost: Aluminium bifold doors (3 panels, 3m wide, thermally broken): £3,000-£6,000 supply and install. Aluminium bifold doors (5 panels, 5m wide): £6,000-£12,000. Aluminium sliding doors (2-panel, 2.4m wide): £4,000-£8,000. Minimalist sliding doors (Vitrocsa-type, large format): £10,000-£25,000+ for a 4-5m opening. Structural: both bifold and large sliding door systems require a structural steel or deep engineered timber beam above the opening to carry the load of the structure above — this must be calculated by a structural engineer and is in addition to the door cost.

Roof windows, skylights, and glazed rooflights are one of the most impactful tools for introducing natural light into a London extension — particularly the rear extension, which is typically built in the shadow of the main house. Types: Rooflight (fixed): a fixed, non-opening glazed panel in a flat or pitched roof. The simplest and most cost-effective way to introduce light into a London extension. Standard flat-roof rooflights (Conservation Rooflights, Velux Modular Skylights) can be installed at minimal pitch (1.5° minimum). Manually opening rooflight: a hinged rooflight that can be opened for ventilation — important in a kitchen extension or bathroom where mechanical ventilation is not available. Electric opening rooflight: an electrically operated rooflight with a rain sensor (auto-closes in rain) — increasingly standard in London extension rooflights. Walk-on rooflight: a structurally reinforced rooflight that can be walked on from above — used where the rooflight is at terrace or garden level. Roof lantern: a glazed box raised above the roof level, typically with a central ridge and sloping glass panels on four sides — provides light from four directions and a feature architectural element in a kitchen-diner flat-roof extension. Extremely popular in London Victorian terrace extensions. Frameless minimal rooflight: a near-frameless large-format glazed panel (TrimLight, Glassolutions, Sky-Frame) — the most expensive and most architecturally refined rooflight option. Conservation area rules for roof windows: in London conservation areas, additions to roof slopes visible from the public realm typically require planning permission. Rooflights on the rear slope of a terrace roof are generally permitted development (they are not visible from the street). Rooflights on a flat-roof extension at first-floor level visible from the street may require planning permission. Always check with the London Borough. Cost: Standard fixed flat-roof rooflight (600x900mm): £400-£800 supply and install. Electric opening rooflight (1000x1500mm): £1,000-£2,500 supply and install. Roof lantern (2000x3000mm): £3,000-£8,000 supply and install. Minimalist frameless rooflight (large format, specialist): £5,000-£20,000+ per unit.

The front door is one of the most visible elements of a London terrace property — seen from the street and forming part of the character of the wider terrace row and conservation area. Front door design in London conservation areas: replacing the original timber front door of a London Victorian or Edwardian terrace typically requires planning permission in a conservation area (Article 4 Directions commonly remove the PD right for front door replacement in the principal elevation). Like-for-like replacement (new timber door in the same configuration, profile, and with similar glazing pattern and ironmongery) may or may not be permitted development — check with the London Borough. Front door options for London Victorian terraces: Original restored timber front door: the best approach where the original door survives and is structurally sound — restoration, stripping, re-painting, and ironmongery replacement. Reproduction Victorian timber front door: a new timber door in the authentic period style — panelled, glazed upper quarter, matching ironmongery (period knocker, letterbox, pull handle, black cast iron lock case). Appropriate in conservation areas where the original door has been lost. Cost: £800-£3,000 supply; £200-£600 installation. Contemporary timber front door: a contemporary flush or minimally-detailed timber door — suitable for a modern extension entrance or for a property in a non-conservation area that has already lost its period character. Contemporary aluminium front door: thermally broken aluminium front doors are increasingly popular for London properties — excellent thermal performance, low maintenance, available in RAL colour range (including the very popular anthracite grey and urban grey favoured in 2025). Cost: aluminium contemporary front door supply and install: £1,500-£4,500. Hardware and ironmongery: the front door hardware (knocker, letterbox, pull, lock) makes a significant visual difference. Period brass, polished chrome, satin chrome, and matt black are all popular in London 2025. Period reproductions (Cheshire Hardware, Carlisle Brass, From the Anvil) provide authentic period detailing.

The three main frame materials for windows and doors in London homes each have different characteristics in terms of aesthetics, thermal performance, maintenance requirements, and cost. Aluminium (thermally broken): the dominant premium window and door frame material for London contemporary extensions and new-build. Key characteristics: extremely slim sightlines (profiles of 15-25mm versus 60-100mm for timber); high strength-to-weight ratio; excellent thermal performance when thermally broken (polyamide thermal break between inner and outer aluminium sections — prevents heat conduction through the frame); very durable and low maintenance; available in any RAL colour; recycled content available. Leading manufacturers: Schüco, WICONA, AluK, Smart Systems, Reynaers, Origin. Cost: aluminium windows are typically 2-4x the cost of uPVC but deliver significantly better aesthetics and thermal performance. Supply cost per m² opening area: £400-£900. Timber: the original and traditional window frame material for London Victorian and Edwardian properties. Advantages: natural warmth and character; appropriate in conservation areas; excellent thermal performance (timber is a natural insulator); high recyclability. Disadvantages: requires maintenance (repainting or re-oiling every 5-7 years); higher cost than uPVC; limited availability of slim contemporary profiles for modern applications. Most appropriate for: heritage and conservation area applications in London; high-end contemporary applications where the warmth of timber is desired. Cost: supply per m² opening area: £500-£1,200 (quality engineered timber). uPVC: the most cost-effective window frame material. Advantages: low maintenance (no painting required); good thermal performance with multi-chamber profiles; widely available. Disadvantages: bulkier profiles than aluminium; not acceptable in most London conservation areas for visible elevations; limited longevity (30-40 years versus 60+ for aluminium or timber); difficult to recycle. Appropriate for: rear or non-visible applications in non-conservation areas in London where budget is the primary constraint. Supply cost per m² opening area: £150-£350.