Glazing for London Home Extensions in 2025: Bifold Doors, Sliding Doors, Roof Lights, Aluminium vs Timber, U-values, Costs

Bifold doors vs sliding doors, aluminium vs timber vs steel Crittal-style frames, and triple glazed vs double glazed specification for London home extensions in 2025: BIFOLD DOORS vs SLIDING DOORS: THE MAIN CHOICE IN LONDON EXTENSIONS: BIFOLD DOORS: panels fold accordion-style and stack to one or both sides of the opening; advantages: the ENTIRE OPENING AREA CAN BE CLEARED when fully open (no fixed side panels, unlike most sliding systems); maximum connection between inside and outside on a warm day; CAN SPAN LARGE OPENINGS (typically 1.5m-7.0m, with 3-4 panel bi-folds the most popular for London terraces); disadvantages: REQUIRE CLEAR FLOOR SPACE to swing open (the folded panels project approximately 200-300mm in front of the wall plane when open); the fold-and-stack mechanism creates a WIDE STACK of panels when open (three panels stacking: approximately 450-600mm wide); LOW-THRESHOLD OPTION (10-20mm sill, suitable for wheeled or walking access) available from premium manufacturers (Schuco, Origin, Smart Architectural Aluminium); POPULAR BIFOLD CONFIGURATIONS FOR LONDON EXTENSIONS: 3-pane bifold: 2.1-3.0m wide; 4-pane bifold: 2.8-4.0m; 5-pane bifold: 3.5-5.0m; SECURITY: look for PAS24 ENHANCED SECURITY rating (the specification that satisfies Part Q of the Building Regulations — RESISTANCE TO ATTACK); all new external doors in dwellings must meet Part Q; SLIDING DOORS (SLIDING-FOLDING OR LIFT-AND-SLIDE): panels slide horizontally on a track, with one or two panels fixed (non-opening) and one or two panels opening; a LIFT-AND-SLIDE SYSTEM (used by Schuco, Reynaers, Vitrocsa) lifts the panel slightly off the track before sliding — provides a very tight weather seal when closed (much better than older rolling-track sliding systems); advantages: PANELS STACK BEHIND EACH OTHER (only the width of one panel is used when open, rather than the accordion stack of a bifold); in a narrow side passage or courtyard, this is a significant advantage; HIGHER WEATHER PERFORMANCE when closed (lift-and-slide) compared to bifolds; WIDER MAXIMUM SPANS (a Schuco ASS 70.HI lift-and-slide can span up to 10m+ as a multi-panel system); THINNER SIGHTLINES on premium systems (Vitrocsa: 12mm sightline — minimal frame visual interruption); MINIMAL THRESHOLD (3-8mm on premium lift-and-slide systems); disadvantages: IN A STANDARD 2-PANEL CONFIGURATION, 50% OF THE OPENING IS ALWAYS GLASS (one panel is fixed); in a corner or wrap-around extension, a CORNER SLIDING DOOR (where two sliding panels meet at the corner with no corner post) creates a dramatic floor-to-ceiling corner glass effect; CORNER SLIDING DOORS: Schuco, Reynaers, and Origin all offer corner sliding systems; the corner mullion is eliminated when the doors slide open; a significant structural steel frame above is required to carry the loads across the corner opening; ALUMINIUM vs TIMBER vs STEEL CRITTAL-STYLE: ALUMINIUM FRAMES (MOST POPULAR FOR LONDON EXTENSIONS): POWDER-COATED ALUMINIUM is the dominant material for bifold and sliding doors in London extensions in 2025; ADVANTAGES: INHERENTLY STRONG (thin sightlines possible because aluminium is stronger than timber for equivalent cross-section); THERMAL BREAK (a polyamide thermal break runs through the section to prevent cold bridging through the aluminium — look for THERMALLY BROKEN ALUMINIUM for external use); LOW MAINTENANCE (powder coat lasts 25+ years with correct RAL colour; repaint is possible); WIDE COLOUR RANGE (RAL powder coat — anthracite grey RAL 7016, black RAL 9005, white RAL 9010, and custom colours); DURABILITY (aluminium does not rot, warp, or twist — critical in London's damp climate); POWDER COAT STANDARD: BS EN 12206-1:2004 (aluminium powder coating standard); MANUFACTURERS AND BRANDS: PREMIUM: Schuco (German — benchmark quality; ADS 65 HD, ADS 70 HD bifold; ASS 70.HI sliding); Reynaers (Belgian — CS 86-HI, CP 130 sliding; popular in London); Vitrocsa (Swiss — minimal sightlines); GOOD MID-RANGE: Origin (UK-made — O-90 bifold, OB-49 sliding); Smart Architectural Aluminium (UK — Visoglide sliding, Alitherm bifold); TIMBER FRAMES: ADVANTAGES: BETTER THERMAL PERFORMANCE (timber has lower conductivity than aluminium — timber frames have better inherent U-values without requiring a thermal break); TRADITIONAL APPEARANCE (appropriate for conservation areas and listed buildings where aluminium would be refused); CARBON BENEFIT (timber is a carbon sink — lower embodied carbon than aluminium); DISADVANTAGES: MAINTENANCE REQUIRED (timber must be painted or stained every 5-10 years and must be protected from standing water and prolonged damp); HIGHER COST for equivalent performance to aluminium; WARPING RISK in timber if not maintained and if the joinery is not properly sealed end-grain at all times; TIMBER SPECIES: ACCOYA (acetylated pine — highly durable, 25-year guarantee, low movement); HARDWOOD (e.g. Iroko, Sapele — durable, appropriate for conservation area replacement); ENGINEERED TIMBER (glulam or engineered wood cores with timber face — more stable than solid timber); STEEL CRITTAL-STYLE FRAMES: CRITTAL (the original company — Crittal Windows, established 1884) produces steel W20 range windows and doors; also many competitors (Winson, Concept Windows, Mondrian); HOT-ROLLED STEEL SECTIONS provide the FINEST SIGHTLINES POSSIBLE (typically 25-35mm, compared with 60-90mm for aluminium bifolds); the INDUSTRIAL, WAREHOUSE-STYLE AESTHETIC is extremely popular in London industrial-loft conversions and arts-and-crafts style extensions; ADVANTAGES: finest sightlines; authentic industrial character; longevity (decades with correct maintenance); DISADVANTAGES: THERMAL PERFORMANCE IS POOR (steel has very high thermal conductivity; thermal break versions exist but sightlines become wider — some of the advantage over aluminium is lost; standard non-thermally-broken Crittal-style: U-value approximately 3.0-5.0 W/m²K — far above the Part L limit); therefore: Crittal-style steel MUST be paired with a HIGH-PERFORMANCE LOW-E DOUBLE GLAZED UNIT to achieve acceptable overall unit U-values; or use in a LISTED BUILDING where Part L requirements have a relaxation; thermally broken Crittal: approximately 1.5-2.5 W/m²K; HIGH MAINTENANCE (steel must be painted every 5-10 years and any rust treated immediately); WEIGHT (steel is heavier than aluminium — requires heavier ironmongery and installation); TRIPLE vs DOUBLE GLAZING: DOUBLE GLAZING (TWO PANES, ARGON-FILLED CAVITY): centre-pane U-value approximately 1.0-1.4 W/m²K (low-E coating); TRIPLE GLAZING (THREE PANES, TWO ARGON-FILLED CAVITIES): centre-pane U-value approximately 0.5-0.8 W/m²K (two low-E coatings); overall unit U-value (including frame) approximately 0.8-1.2 W/m²K (depending on frame); TRIPLE GLAZING ADVANTAGES: significantly lower heat loss through the glass (important for a heavily glazed rear extension — a 4m wide bifold in a London extension is approximately 4×2.1 = 8.4m² of glass; reducing the U-value from 1.4 to 0.8 saves approximately 50 W per °C of temperature difference — meaningful in winter); IMPROVED ACOUSTIC PERFORMANCE (triple glazing reduces sound transmission); warmer glass surface temperature (improves comfort near large glass areas — less cold radiation); TRIPLE GLAZING DISADVANTAGES: higher cost (approximately 20-30% above double glazing); heavier (glass panels are heavier — impacts hardware specification); slightly reduced light transmission (each additional pane and coating reduces solar heat gain and visible light transmittance slightly — G-value typically 0.35-0.50 for triple vs 0.55-0.70 for double); SOLAR CONTROL GLASS: for south- or west-facing glazed extensions (rear extensions in a north-south orientated terrace): SOLAR CONTROL LOW-E COATINGS (e.g. Saint-Gobain Planitherm ULTRA N, Pilkington K Glass) reduce the SOLAR HEAT GAIN COEFFICIENT (G-value, also called SHGC) — reducing summer overheating; a solar control unit might have G-value approximately 0.25-0.40 (rejecting 60-75% of incident solar energy while admitting light); PART L (BUILDING REGULATIONS THERMAL PERFORMANCE): maximum U-value for new doors/windows in extensions: approximately 1.4-1.6 W/m²K overall (frame + glass) for windows and doors; as part of a whole extension energy package; DOUBLE GLAZED LOW-E ARGON-FILLED UNITS IN THERMALLY BROKEN ALUMINIUM FRAMES typically achieve approximately 1.4-1.6 W/m²K overall (minimum Part L compliance); TRIPLE GLAZED UNITS IN THERMALLY BROKEN ALUMINIUM FRAMES achieve approximately 0.8-1.2 W/m²K (exceeds Part L); FENSA (FENESTRATION SELF-ASSESSMENT SCHEME) / CERTASS REGISTRATION: glazing contractors registered with FENSA or CERTASS can self-certify that replacement windows and doors comply with Part L and Part Q — they issue a compliance certificate; NON-REGISTERED CONTRACTORS: must use a Building Regulations application for the glazing work.

Roof lights, roof lanterns, conservation area glazing rules, secondary glazing, and glazing costs for London home extensions in 2025: ROOF LIGHTS AND ROOF LANTERNS: ROOF LIGHTS (FLAT ROOF LIGHTS): FLAT ROOFLIGHTS installed into a flat roof extension are one of the most effective ways to BRING NATURAL LIGHT DEEP INTO A LONDON TERRACE (which may be dark at the rear because neighbouring properties block low-angle light from the sides and rear); SPECIFICATION: ALUMINIUM-FRAMED SELF-CLEANING TRIPLE GLAZED UNITS: the best specification for a permanent flat rooflight in a London extension; self-cleaning glass (Pilkington Activ, Saint-Gobain Bioclean — photocatalytic and hydrophilic coating) reduces maintenance frequency (rain naturally washes off loosened dirt); triple glazed: U-value approximately 1.0-1.4 W/m²K; MANUFACTURERS: Korniche (aluminium, slim section, UK-made — extremely popular in London extensions), Vision AGI (structural glass — minimal sightlines, suitable for frameless or near-frameless installations), Fixed Flat Rooflights (various UK manufacturers), Velux Flat Roof Windows (modular, easy to specify, widely stocked by builders merchants); MINIMUM KERB HEIGHT: 150mm above the finished flat roof surface (prevents water ingress at low falls and maintains the waterproof membrane connection); MINIMUM OPENING AREA RULE (BUILDING REGULATIONS PART L AND PART F): the aggregate area of rooflights and windows in a room affects: (1) the room's ability to have adequate natural ventilation (Part F — top-hung opening sections or opening lights needed); (2) the thermal performance of the extension (more glass = more heat loss — a fully glazed roof in a flat roof extension will perform significantly worse thermally than a partially glazed roof unless triple glazed; Part L compliance should be assessed as a whole); VELUX ROOF WINDOWS (IN A PITCHED ROOF): as noted in the roofing guide: installed flush with the slope of a pitched roof; EDW flashing kit for natural slate; EDL for low-pitch applications; minimum pitch 15°; ROOF LANTERNS: a ROOF LANTERN is a glazed pyramid-like structure raised above the flat roof surface, admitting light from all four sides and the apex; very popular in London kitchen extensions as the feature element of an open-plan kitchen-diner; ALUMINIUM-FRAMED ROOF LANTERNS: Korniche Roof Lantern, Atlas Roof Lantern, Leka Systems Roof Lantern, Clement Conservation Roof Lanterns; sizes: typically 1.0m × 1.5m to 3.0m × 4.0m or larger; SPECIFICATION POINTS: THERMALLY BROKEN ALUMINIUM FRAME; TRIPLE GLAZED GLASS PANELS (reduce heat loss through the large glazed area); SELF-CLEANING GLASS (Pilkington Activ or equivalent — essential for roof lanterns as cleaning a roof lantern requires specialist access); UPSTAND: the roof lantern sits on an UPSTAND (a kerb above the flat roof) — minimum 150mm high; the upstand is built as part of the flat roof structure and is waterproofed and flashed by the roofer before the lantern is installed; OPENING SECTIONS: some roof lanterns include an ELECTRIC OPENING SECTION (a remote-controlled ventilation panel that opens the lantern to allow hot air to escape in summer — solar-controlled and rain-sensor shutting); highly recommended for SOUTH-FACING kitchen extensions where the roof lantern admits significant solar gain in summer; CONSERVATION AREA GLAZING RULES: in a CONSERVATION AREA, glazing choices are scrutinised by the LPA'S CONSERVATION OFFICER; GENERAL RULES: FRONT ELEVATIONS: glazing visible from the street is most strictly controlled; UPVC windows are generally refused (the appearance differs from original timber or steel); ALUMINIUM (slimline, dark colour) may be accepted in some conservation areas but is generally less preferred than timber; TIMBER SASH WINDOWS (on original openings): like-for-like replacement is normally PD; changing from timber to aluminium requires planning; REAR EXTENSIONS (LEAST VISIBLE): glazing on rear extensions in conservation areas is less strictly regulated; contemporary aluminium bifold or sliding doors on rear extensions are typically acceptable where the extension design is approved; SOME CONSERVATION AREAS (PARTICULARLY LISTED BUILDING CONSERVATION AREAS): the conservation officer may prefer TIMBER or STEEL CRITTAL-STYLE frames, consistent with the character of the original building; LISTED BUILDINGS: LISTED BUILDING CONSENT for any glazing change; the conservation officer and Historic England will guide appropriate choices; typically timber or original material required; aluminium or UPVC is almost never accepted on a listed building; SECONDARY GLAZING: SECONDARY GLAZING is an INNER FRAME fitted behind the existing window (on the internal face of the reveal) — it is separate from the original window and creates an additional air gap; applicable where the ORIGINAL WINDOW CANNOT BE REPLACED (conservation area; listed building; windows that are original fabric); advantages: significantly improves U-value of the combined system (original + secondary glazing); ACOUSTIC PERFORMANCE is excellent with a 150-200mm air gap (Rw up to 40-48 dB as noted in the windows guide); no planning permission required (it is internal and does not change the external appearance); SELECTAGLAZE is the leading UK manufacturer; GLAZING COSTS IN LONDON IN 2025: ALUMINIUM BIFOLD DOORS: 3-pane, 2.1m wide: approximately £1,800-£4,500 supply; 4-pane, 3.0m wide: approximately £2,500-£6,500 supply; 5-pane, 4.0m wide: approximately £3,500-£9,000 supply; installation approximately £500-£1,500 additional; ALUMINIUM LIFT-AND-SLIDE SLIDING DOOR: 2-pane, 2.4m wide (1 sliding, 1 fixed): approximately £2,000-£5,000 supply; TIMBER BIFOLD DOORS: approximately 30-50% more than aluminium equivalent; CRITTAL-STYLE STEEL DOORS AND WINDOWS (NON-THERMALLY BROKEN): approximately £600-£1,200/m² supply (expensive per m² — sightlines are fine but cost per area is high); ALUMINIUM FLAT ROOFLIGHT (600×600 to 1500×1500mm, triple glazed): approximately £800-£2,500 supply; ALUMINIUM ROOF LANTERN (1.5m × 1.5m, triple glazed, self-cleaning glass): approximately £1,500-£4,000 supply; larger lanterns (3m × 2m): approximately £3,000-£7,000+ supply; VELUX ROOF WINDOW (PITCHED ROOF, 78×140cm including flashings): approximately £400-£900 supply; SECONDARY GLAZING (SELECTAGLAZE, PER WINDOW, SUPPLY + INSTALL): approximately £350-£700 standard; approximately £600-£1,200 acoustic; INSTALLATION ALLOWANCE FOR GLAZING IN AN EXTENSION: typically £500-£2,000 per opening depending on size, height, and lintel work.