Bifold & Sliding Doors for London Extensions: The Complete Guide 2025

THE CHOICE BETWEEN BIFOLD AND SLIDING DOORS (ALSO CALLED LIFT-AND-SLIDE OR INLINE SLIDING SYSTEMS) IS THE FIRST DECISION A HOMEOWNER MUST MAKE. EACH HAS DISTINCT ADVANTAGES:

BIFOLD DOORS (FOLDING SLIDING SYSTEM): EACH PANEL FOLDS AGAINST THE ADJACENT PANEL AND THE ENTIRE DOOR STACK MOVES TO ONE OR BOTH SIDES OF THE APERTURE, LEAVING THE OPENING COMPLETELY CLEAR. FOR AN APERTURE OF 3-5 METRES, A 3 OR 4-PANEL BIFOLD LEAVES A COMPLETELY OPEN REAR WALL. THE DOOR PANELS FOLD OUTWARD OR INWARD DEPENDING ON CONFIGURATION. STANDARD PANEL WIDTH: 700-1000MM. FRAME FACE WIDTH: 92-107MM DEPENDING ON SYSTEM.

ADVANTAGES OF BIFOLDS: — COMPLETE CLEARANCE OF THE OPENING WHEN FULLY FOLDED — EXCELLENT FOR KITCHEN-DINERS WHERE INDOOR-OUTDOOR FLOW IS THE PRIORITY — CAN INCORPORATE A TRAFFIC DOOR (SINGLE PANEL THAT OPENS LIKE A CONVENTIONAL DOOR) FOR EVERYDAY ACCESS WITHOUT OPENING THE FULL SYSTEM — WIDE VARIETY OF PANEL CONFIGURATIONS (2+1, 3+0, 4+0, 3+3 ETC)

DISADVANTAGES: — TAKES UP FLOOR SPACE WHEN OPEN (STACKED PANELS INTRUDE INTO THE ROOM OR TERRACE) — HEAVIER PANELS WITH MULTIPLE PIVOT POINTS — MORE MECHANICAL COMPONENTS — MAXIMUM PRACTICAL SINGLE-PANEL WIDTH: ~1,000MM FOR ALUMINIUM

SLIDING DOORS (LIFT-AND-SLIDE OR INLINE): PANELS SLIDE HORIZONTALLY ON PRECISION TRACKS. IN A LIFT-AND-SLIDE SYSTEM (SCHUCO ASS 70.HI, REYNAERS MASTERSLIDE, SMARTSYSTEMS VISOGLIDE PLUS), A HANDLE ROTATION LIFTS THE PANEL OFF THE SILL SEAL AND ONTO PRECISION ROLLERS FOR EFFORTLESS SLIDING. INLINE SLIDING SYSTEMS SLIDE WITHOUT LIFTING.

ADVANTAGES OF SLIDING: — MUCH WIDER INDIVIDUAL PANELS (UP TO 3,000MM WIDE AND 3,500MM TALL FOR STRUCTURAL GLAZING SYSTEMS) — SLIMMER VISIBLE FRAME LINES (FRAME FACE 50-70MM IN TOP SYSTEMS VERSUS 92-107MM FOR BIFOLD) — OPENING TRAVELS BEHIND FIXED PANES — NO INTRUSION INTO FLOOR SPACE — TECHNICALLY MORE SUITABLE FOR VERY LARGE APERTURES (6M+ WIDE) — SCHUCO ASS 70.HI: PASSIVE HOUSE CERTIFIED SLIDING DOOR, U=0.79 W/M2K WITH TRIPLE GLAZING

DISADVANTAGES: — NEVER FULLY CLEARS THE APERTURE (TYPICALLY 50% OF APERTURE IS OPENABLE, 50% REMAINS FIXED) — PANELS BEHIND EACH OTHER CAN LIMIT ACCESS WIDTH

THE SINGLE MOST IMPORTANT TECHNICAL ELEMENT OF A LARGE BIFOLD OR SLIDING DOOR INSTALLATION IS THE STRUCTURAL STEEL BEAM THAT SPANS THE OPENING ABOVE THE DOORS. WITHOUT AN ADEQUATELY SIZED BEAM, THE MASONRY OR STRUCTURAL ELEMENT ABOVE THE APERTURE HAS NO SUPPORT — THE RESULT WOULD BE STRUCTURAL FAILURE.

BEAM SIZING FOR TYPICAL LONDON REAR EXTENSION APERTURES: — 2.4M WIDE APERTURE (3-PANEL BIFOLD), 1 STOREY ABOVE: UB 152X89X16 OR UC 152X152X23 — COMPACT BEAM, OFTEN SPECIFIED WHERE CEILING HEIGHT IS CONSTRAINED — 3.0M WIDE APERTURE (4-PANEL BIFOLD), 1 STOREY ABOVE: UB 203X133X25 S275 — THE MOST COMMONLY INSTALLED BEAM SIZE IN LONDON REAR EXTENSION OPENINGS — 4.5M WIDE APERTURE (6-PANEL BIFOLD OR LARGE SLIDING), 1 STOREY ABOVE: UB 305X127X37 OR UB 305X165X40 S275 — 6.0M+ WIDE APERTURE (LARGE SLIDING, DOUBLE-PANEL), 1 STOREY + ROOF: UB 406X178X54 OR UB 457X152X60 S275 — SUBSTANTIAL SECTION, MAY REQUIRE BOXED STEEL

PADSTONE REQUIREMENTS: ENGINEERING BRICK PADSTONES (CLASS B ENGINEERING BRICK, 215X215X102MM MINIMUM) OR REINFORCED CONCRETE PADSTONES AT EACH END OF THE BEAM BEARING POINT. PADSTONES SPREAD THE CONCENTRATED BEARING LOAD FROM THE BEAM END INTO THE SUPPORTING MASONRY WITHOUT LOCAL CRUSHING.

TEMPORARY WORKS: DURING INSTALLATION, ACROW PROPS AND STRONGBOYS (STEEL WALL PROPS THAT GRIP THE WALL ABOVE THE PROPOSED OPENING) MUST SUPPORT THE MASONRY ABOVE WHILE THE OPENING IS FORMED AND THE BEAM IS INSTALLED. NEVER REMOVE LOAD-BEARING MASONRY WITHOUT PROPER TEMPORARY WORKS IN PLACE.

BUILDING CONTROL: ALL STRUCTURAL OPENINGS FOR BIFOLD AND SLIDING DOORS REQUIRE BUILDING REGULATIONS APPROVAL (FULL PLANS APPLICATION OR BUILDING NOTICE). THE STRUCTURAL ENGINEER'S CALCULATIONS MUST BE SUBMITTED WITH THE APPLICATION. BUILDING CONTROL WILL INSPECT THE OPEN STEELWORK BEFORE IT IS COVERED.

THE ALUMINIUM BIFOLD AND SLIDING DOOR MARKET IN THE UK IS DOMINATED BY A FEW KEY SYSTEMS THAT ARE SPECIFIED BY ARCHITECTS AND CONTRACTORS ACROSS THE LONDON MARKET:

ORIGIN OB-49 BIFOLD (UK MANUFACTURER, ALTON, HAMPSHIRE): THE MOST WIDELY RECOGNISED AND SPECIFIED BIFOLD SYSTEM IN THE UK RESIDENTIAL MARKET. 107MM OUTER FRAME, THERMALLY BROKEN (POLYAMIDE THERMAL BREAK). U-VALUE 1.6 W/M2K WITH STANDARD DOUBLE GLAZED UNIT; IMPROVED TO 1.4 W/M2K WITH PILKINGTON OPTITHERM S3 PLUS TRIPLE GLAZED UNIT. POWDER-COATED IN 150+ STANDARD COLOURS AND METALLIC FINISHES. 20-YEAR ORIGIN GUARANTEE. HARDWARE: ORIGIN INLINE SHOOTBOLT SYSTEM (MULTI-POINT LOCKING, PAS 24:2016 CERTIFIED). POPULAR LONDON COLOURS: ANTHRACITE GREY RAL 7016, TRAFFIC WHITE RAL 9016, BLACK RAL 9005. COST: £1,000-£1,500/M2 SUPPLY ONLY FOR ORIGIN OB-49.

SMART ALITHERM 600 BIFOLD (SMART SYSTEMS, UK): COMPARABLE TO ORIGIN AT SLIGHTLY LOWER PRICE POINT. THERMALLY BROKEN, 92MM FRAME FACE. AVAILABLE THROUGH APPROVED FABRICATORS ACROSS LONDON. COST: £800-£1,200/M2 SUPPLY ONLY.

SMART VISOGLIDE PLUS (INLINE SLIDING SYSTEM): SMART'S PREMIUM INLINE SLIDING SYSTEM. 70MM FRAME FACE (SIGNIFICANTLY SLIMMER THAN BIFOLD). PANELS UP TO 3000MM WIDE. STANDARD DOUBLE GLAZED: U=1.6 W/M2K. GOOD MID-MARKET SLIDING OPTION.

SCHUCO ASS 70.HI (LIFT-AND-SLIDE PREMIUM): GERMAN PREMIUM SLIDING SYSTEM. PASSIVE HOUSE CERTIFIED (U=0.79 W/M2K WITH TRIPLE GLAZING). PANEL WEIGHTS TO 400KG. MAXIMUM PANEL WIDTH 3,000MM. THE SPECIFIER'S CHOICE FOR ULTRA-PERFORMANCE PROJECTS AND LARGE-FORMAT GLAZING. COST: £1,800-£2,800/M2 SUPPLY ONLY.

RAYNAERS MASTERSLIDE: BELGIAN PREMIUM LIFT-AND-SLIDE. SPECCED ON ULTRA-CONTEMPORARY LONDON REFURBISHMENTS AND ARCHITECT-LED EXTENSIONS. COST: £1,500-£2,200/M2 SUPPLY ONLY.

RALF BREMERMANN / RATIONEL ALDAS (SCANDINAVIAN TIMBER-ALUMINIUM COMPOSITE): TIMBER INTERNAL FACE, ALUMINIUM EXTERNAL FACE. POPULAR IN CONSERVATION EDGE AND HIGH-SPEC RESIDENTIAL WHERE TIMBER WARMTH IS DESIRED INTERNALLY. VELFAC 200 RANGE IS A COMPARABLE SYSTEM. COST: £1,200-£2,000/M2 SUPPLY ONLY.

ALL REPLACEMENT WINDOWS AND DOORS IN ENGLAND MUST MEET MINIMUM ENERGY PERFORMANCE STANDARDS UNDER APPROVED DOCUMENT L (CONSERVATION OF FUEL AND POWER). FOR LARGE GLAZED DOORS, THE REQUIRED WHOLE-UNIT U-VALUE IS 1.4 W/M2K OR BETTER FOR REPLACEMENT WORKS.

GLAZING SPECIFICATION: — DOUBLE GLAZED UNIT (DGU): STANDARD SPECIFICATION FOR MOST LONDON BIFOLD AND SLIDING INSTALLATIONS. PILKINGTON OPTITHERM S3 PLUS LOW-E COATED GLASS WITH 16MM ARGON-FILLED SPACER: CENTRE-PANE U=1.0 W/M2K; WHOLE UNIT U~1.4-1.6 W/M2K. — TRIPLE GLAZED UNIT (TGU): PREFERRED FOR HIGH-PERFORMANCE AND PASSIVE HOUSE PROJECTS. PILKINGTON OPTITHERM S3 PLUS X2 (BOTH PANES COATED): CENTRE-PANE U=0.6 W/M2K; WHOLE UNIT U~0.8-1.2 W/M2K. ADDS 10-20KG PER PANEL AND REQUIRES STRONGER HARDWARE. — PRIVACY AND SPECIALIST GLASS: PILKINGTON ACTIV (SELF-CLEANING, PARTICULARLY USEFUL FOR OVERHEAD SECTIONS); PILKINGTON OPTIKLEAR (ANTI-REFLECTIVE COATING, RECOMMENDED FOR SOUTH-FACING LARGE GLAZED AREAS TO REDUCE GLARE); PILKINGTON SUNCOOL (SOLAR CONTROL, REDUCES SOLAR GAIN — RECOMMENDED FOR SOUTH-FACING EXTENSIONS TO REDUCE OVERHEATING)

SOLAR OVERHEATING RISK: LARGE SOUTH-FACING BIFOLD DOORS CREATE A SIGNIFICANT SOLAR GAIN RISK — THE SAME GLASS THAT LETS IN LOVELY AUTUMN AND WINTER LIGHT CAN MAKE A SOUTH-FACING KITCHEN-DINER UNBEARABLY HOT IN SUMMER. MITIGATION OPTIONS: EXTERNAL ALUMINIUM LOUVRES OR VENETIAN BLIND SYSTEM; SOLAR CONTROL GLASS (PILKINGTON SUNCOOL 70/40 OR GUARDIAN SOLAR); INTERNAL BLINDS WITHIN THE GLAZING CAVITY (DUETTE BLINDS); EXTERNAL FIXED PERGOLA OR CANOPY CREATING SHADING WHEN THE SUN IS HIGH (SUMMER) WHILE ALLOWING LOW-WINTER-SUN PENETRATION.

TOTAL COST GUIDE FOR BIFOLD AND SLIDING DOORS IN LONDON EXTENSIONS:

SUPPLY AND FIT COSTS (INCLUDING STEEL BEAM, BUILDING REGULATIONS, AND INSTALLATION):

3-PANEL BIFOLD DOOR, 2.4M WIDE, STANDARD DOUBLE GLAZED (ORIGIN OB-49): — DOOR SUPPLY: £2,400-£3,600 — STEEL BEAM (UB 203X133X25, 3.0M): £400-£600 — STRUCTURAL ENGINEER: £400-£700 — BUILDING CONTROL: £300-£450 — INSTALLATION (BEAM, DOOR, SILLS, MAKING GOOD): £800-£1,500 — TOTAL: £4,300-£6,850

4-PANEL BIFOLD DOOR, 3.2M WIDE, DOUBLE GLAZED (SMART ALITHERM 600): — DOOR SUPPLY: £2,600-£3,800 — STEEL BEAM (UB 254X146X31, 3.8M): £500-£800 — STRUCTURAL ENGINEER + BUILDING CONTROL: £700-£1,200 — INSTALLATION: £1,000-£1,800 — TOTAL: £4,800-£7,600

6-PANEL BIFOLD DOOR, 4.8M WIDE, TRIPLE GLAZED (ORIGIN OB-49): — DOOR SUPPLY: £7,200-£10,800 — STEEL BEAM (UB 305X165X40, 5.4M): £700-£1,200 — STRUCTURAL ENGINEER + BUILDING CONTROL: £800-£1,400 — INSTALLATION: £1,500-£2,500 — TOTAL: £10,200-£15,900

LIFT-AND-SLIDE PREMIUM (SCHUCO ASS 70.HI, 4M WIDE, TRIPLE GLAZED): — DOOR SUPPLY: £7,200-£11,200 — BEAM + STRUCTURAL + BUILDING CONTROL: £1,500-£2,600 — INSTALLATION: £1,500-£2,500 — TOTAL: £10,200-£16,300

GENERAL PRINCIPLES: — ALWAYS ALLOW 10-15% CONTINGENCY ON GLAZED DOOR PROJECTS — APERTURE SIZES OFTEN VARY SLIGHTLY ONCE THE WALL IS OPENED — ORDER DOORS ONLY AFTER THE APERTURE IS FULLY FORMED AND MEASURED — NEVER FROM A DRAWING DIMENSION ALONE — SPECIFY SILL HEIGHT CAREFULLY — BIFOLD SILLS COLLECT WATER AND MUST FALL AWAY FROM THE INTERNAL FLOOR FINISH BY A MINIMUM 15MM STEP OR REBATED THRESHOLD