Bi-Fold Doors for London Victorian Terrace Extensions: Origin, Schüco, and Structural Specification

THE QUALITY AND SPECIFICATION OF THE BI-FOLD DOOR SYSTEM IS A CRITICAL COMMERCIAL AND AESTHETIC DECISION — THE DOOR SYSTEM IS THE MOST VISIBLE AND FREQUENTLY USED ELEMENT OF THE REAR EXTENSION AND WILL DEFINE THE QUALITY OF THE SPACE FOR THE LIFETIME OF THE EXTENSION.

ORIGIN OB-49 (ORIGIN GLOBAL — HENLEY-ON-THAMES — THE UK'S LARGEST MANUFACTURER OF ALUMINIUM BI-FOLD DOORS): THE MOST WIDELY SPECIFIED THERMALLY BROKEN ALUMINIUM BI-FOLD DOOR SYSTEM IN THE UK RESIDENTIAL MARKET: FRAME: 107MM OUTER FRAME DEPTH (THERMALLY BROKEN ALUMINIUM — POLYAMIDE THERMAL BREAK — PREVENTS COLD BRIDGING THROUGH THE FRAME). LEAF WIDTH: INDIVIDUAL DOOR PANELS UP TO 1,000MM WIDE (SO A 4-PANEL CONFIGURATION CAN ACHIEVE A 3,600-4,000MM CLEAR OPENING). GLASS: ORIGIN OB-49 IS SUPPLIED WITH PILKINGTON OPTITHERM S3 PLUS GLASS (LOW-E, TRIPLE-COATED — U-VALUE 1.0 W/M²K FOR THE GLASS UNIT). OVERALL WINDOW ENERGY RATING: TYPICALLY A+ OR A RATED. HARDWARE: STAINLESS STEEL MULTI-POINT LOCKING (SHOOT BOLTS TOP AND BOTTOM OF EACH PANEL PLUS CENTRAL HANDLE LOCK — HIGH SECURITY). COLOUR: POWDER-COATED IN ANY RAL COLOUR (MOST POPULAR FOR LONDON VICTORIAN TERRACE EXTENSIONS: ANTHRACITE GREY RAL 7016, TRAFFIC BLACK RAL 9017, SIGNAL WHITE RAL 9003). SUPPLIED AND INSTALLED THROUGH ORIGIN TRADE NETWORK (APPROVED INSTALLERS NATIONWIDE). PRICE: APPROXIMATELY £650-900 PER SQUARE METRE OF DOOR (SUPPLY AND FIT — LONDON — STANDARD CONFIGURATION).

SCHÜCO ASS 70.HI (SCHÜCO INTERNATIONAL — GERMAN ENGINEERING — HIGH-PERFORMANCE THERMALLY BROKEN ALUMINIUM BI-FOLD/LIFT-AND-SLIDE SYSTEM): ASS 70.HI: A PREMIUM SCHÜCO SYSTEM WITH A VERY NARROW BOTTOM TRACK (FLUSH-TO-FLOOR DESIGN — THE TRACK SITS FLUSH WITH THE FLOOR LEVEL, ALLOWING A SEAMLESS INSIDE-OUTSIDE TRANSITION — VERY DESIRABLE FOR KITCHEN-GARDEN CONNECTIONS IN LONDON EXTENSIONS). U-VALUE: OVERALL WINDOW U-VALUE DOWN TO 0.8 W/M²K (WITH TRIPLE GLAZING) — EXCEEDS PART L 2021 REQUIREMENTS. PRICE: APPROXIMATELY £900-1,400 PER SQUARE METRE (SUPPLY AND FIT — LONDON — PREMIUM SPECIFICATION).

ALUK 58BF (ALUK GROUP — A MAINSTREAM THERMALLY BROKEN ALUMINIUM BI-FOLD SYSTEM — THE BUDGET-CONSCIOUS ALTERNATIVE TO ORIGIN AND SCHÜCO): FRAME: 58MM OUTER FRAME DEPTH. GLASS: SUPPLIED WITH STANDARD DOUBLE-GLAZED UNIT (4-16-4 ARGON FILLED LOW-E — U-VALUE 1.4-1.6 W/M²K). PRICE: APPROXIMATELY £450-650 PER SQUARE METRE (SUPPLY AND FIT — LONDON). SMART ARCHITECTURAL ALUMINIUM (SMART SYSTEMS — ANOTHER MAINSTREAM ALUMINIUM BI-FOLD SYSTEM — VISOGLIDE PLUS — ALSO POPULAR IN THE LONDON EXTENSION MARKET AT A SIMILAR PRICE TO ALUK).

STEEL-FRAMED BI-FOLD (IQ GLASS OR BESPOKE STEEL FABRICATORS — FOR THE HIGHEST-END EXTENSION PROJECTS WHERE ULTRA-SLIM FRAME PROFILE IS REQUIRED — THE ENTIRE BI-FOLD FRAME IS FABRICATED FROM MILD STEEL, POWDER-COATED — FRAME SIGHT LINE CAN BE AS THIN AS 25-35MM VS 107MM FOR ORIGIN): MAXIMUM VISUAL OPENNESS. PRICE: £2,000-4,000 PER SQUARE METRE (SUPPLY AND FIT).

THE STRUCTURAL LINTEL OR RSJ BEAM THAT SPANS OVER THE BI-FOLD DOOR OPENING IS ONE OF THE MOST IMPORTANT — AND MOST FREQUENTLY UNDER-SPECIFIED — ELEMENTS OF A BI-FOLD DOOR INSTALLATION IN A LONDON VICTORIAN TERRACE REAR EXTENSION.

WHY THE LINTEL MATTERS: THE BI-FOLD DOOR OPENING IS TYPICALLY THE WIDEST STRUCTURAL SPAN IN THE EXTENSION — TYPICALLY 3.0-4.5M CLEAR — AND THE LINTEL CARRIES THE FULL WEIGHT OF THE ROOF STRUCTURE AND FIRST FLOOR CONSTRUCTION ABOVE (IF THE EXTENSION HAS AN UPPER FLOOR) OR THE FLAT ROOF STRUCTURE (IF SINGLE-STOREY — THE MOST COMMON CASE FOR A LONDON VICTORIAN TERRACE REAR EXTENSION). A LINTEL THAT IS UNDERSPECIFIED FOR THE SPAN AND LOAD WILL DEFLECT — CAUSING CRACKING ABOVE THE OPENING, DOOR ALIGNMENT PROBLEMS (THE BI-FOLD PANELS WILL NO LONGER SLIDE AND FOLD SMOOTHLY IF THE FRAME HAS DISTORTED DUE TO LINTEL DEFLECTION — A VERY COMMON DEFECT IN POORLY-SPECIFIED EXTENSIONS), AND POTENTIALLY STRUCTURAL CRACKING OF THE WALL ABOVE.

STEEL UNIVERSAL BEAM (UB) SPECIFICATION FOR BI-FOLD DOOR OPENING IN A LONDON VICTORIAN TERRACE SINGLE-STOREY REAR EXTENSION (FLAT ROOF — CONCRETE FLAT ROOF OR WARM FLAT ROOF): BI-FOLD OPENING UP TO 3.0M CLEAR: TYPICALLY A UB 152X89X16 OR UB 203X133X25 (S275 STEEL — BS EN 10025-1 — THE STANDARD STRUCTURAL STEEL GRADE FOR UK RESIDENTIAL CONSTRUCTION) — STRUCTURAL ENGINEER TO CALCULATE ACTUAL REQUIRED SECTION. BI-FOLD OPENING 3.0M TO 4.5M CLEAR: TYPICALLY A UB 254X146X31 OR UB 305X127X37 (S275) — STRUCTURAL ENGINEER CALCULATION REQUIRED — AS THE SPAN INCREASES, THE BEAM SIZE AND WEIGHT INCREASES SIGNIFICANTLY — CRANE OR SPECIALIST LIFTING MAY BE REQUIRED FOR BEAMS OVER 300KG. PADSTONES (THE BEARING PAD ON WHICH THE END OF THE STEEL BEAM RESTS ON THE MASONRY): PRECAST CONCRETE PADSTONES (215X215X100MM FOR UB 203X133, 327X215X150MM FOR UB 254-305 — STANDARD PRECAST CONCRETE PADSTONE SIZES) OR CLASS A ENGINEERING BRICK PADSTONES (IBSTOCK BLUE CLASS B ENGINEERING BRICK, MINIMUM 3 COURSES EQUIVALENT BEARING) — THE PADSTONE DISTRIBUTES THE POINT LOAD OF THE BEAM END OVER SUFFICIENT MASONRY AREA TO AVOID LOCAL CRUSHING OF THE BRICK.

ACROW PROPS (TEMPORARY PROPPING DURING BEAM INSTALLATION): THE WALL ABOVE THE PROPOSED BEAM MUST BE TEMPORARILY PROPPED (ACROW PROPS ON SPREADER BOARDS) BEFORE THE OPENING IS CUT — THE PROPS REMAIN IN POSITION UNTIL THE BEAM IS INSTALLED AND THE PADSTONES HAVE CURED — TYPICALLY 24-48 HOURS MINIMUM. NUMBER OF PROPS: MINIMUM ONE PROP PER 600MM OF WALL ABOVE THE OPENING (FOR A 3.0M OPENING = 5 ACROW PROPS MINIMUM AT 600MM CENTRES — BOTH SIDES OF THE WALL).

THE THRESHOLD DETAIL — THE JUNCTION BETWEEN THE INTERNAL FLOOR LEVEL AND THE EXTERNAL LEVEL AT THE BASE OF THE BI-FOLD DOOR — IS ONE OF THE MOST IMPORTANT PRACTICAL AND AESTHETIC DETAILS IN THE BI-FOLD DOOR INSTALLATION, AND GETTING IT WRONG CAUSES PERSISTENT PRACTICAL PROBLEMS (FLOODING FROM RAINWATER INGRESS, DIFFICULT STEP FOR ACCESSIBILITY, VISIBLE STRUCTURAL TRACK INTERRUPTING THE FLOOR TRANSITION).

FLUSH THRESHOLD (THE IDEAL THRESHOLD DETAIL FOR A LONDON VICTORIAN TERRACE REAR EXTENSION): WHERE THE INTERNAL FLOOR LEVEL IS AT THE SAME HEIGHT AS THE EXTERNAL TERRACE OR PATIO LEVEL — ALLOWING THE BI-FOLD TRACK TO BE SET FLUSH WITH BOTH SURFACES — CREATING A SEAMLESS INSIDE-OUTSIDE TRANSITION. THIS REQUIRES: INTERNAL FLOOR BUILD-UP TO BE DESIGNED SO THAT THE TOP SURFACE OF THE INTERNAL FINISHED FLOOR (TYPICALLY 65MM CEMFLOOR + 150MM STRUCTURAL SLAB + INSULATION + DPM = TOTAL BUILD-UP APPROXIMATELY 300-400MM ABOVE SLAB BASE) ALIGNS WITH THE EXTERNAL TERRACE LEVEL. HANSON CEMFLOOR (OR EQUIVALENT LIQUID SCREED) WITH UNDERFLOOR HEATING EMBEDDED = THE MOST COMMON INTERNAL FLOOR BUILD-UP FOR A LONDON REAR EXTENSION WITH BI-FOLD DOORS AND UFH. ACO THRESHOLD DRAIN (ACO DESIGN DRAIN — 70MM WIDE STAINLESS STEEL LINEAR DRAIN — SET IN THE THRESHOLD ZONE JUST INSIDE THE DOOR TRACK): ALLOWS A FLUSH THRESHOLD WHILE PROVIDING A WATER-MANAGEMENT PATH FOR RAINWATER THAT DRIVES PAST THE DOOR SEALS IN HEAVY RAIN — ESSENTIAL FOR A TRULY FLUSH THRESHOLD WHERE RAINWATER ENTRY IS A CONCERN. SCHÜCO ASS 70.HI AND ORIGIN OB-49 BOTH OFFER A FLUSH SILL TRACK PROFILE (SITS AT FLOOR LEVEL — MINIMAL TRIP HAZARD) — COMPARED WITH SOME BUDGET SYSTEMS THAT HAVE A HIGHER-PROFILE TRACK THAT CREATES A SIGNIFICANT TRIP RISK.

RAISED THRESHOLD (WHERE A FLUSH THRESHOLD IS NOT ACHIEVABLE DUE TO FLOOR LEVEL DIFFERENCES OR STRUCTURAL CONSTRAINTS): A RAISED STEP AT THE THRESHOLD (TYPICALLY 100-150MM STEP UP FROM EXTERNAL TO INTERNAL) IS COMMON WHERE THE INTERNAL FLOOR IS HIGHER THAN THE EXTERNAL GROUND LEVEL (E.G. WHERE THE EXISTING KITCHEN FLOOR IS RAISED ABOVE EXTERNAL GROUND LEVEL — COMMON IN LONDON VICTORIAN TERRACES WHERE THE GROUND FLOOR INTERNAL LEVEL IS TYPICALLY 300-600MM ABOVE EXTERNAL GROUND LEVEL AT THE REAR). THE STEP CREATES A MOISTURE AND FLOOD BUFFER — BUT REDUCES ACCESSIBILITY AND DISRUPTS THE VISUAL CONTINUITY. ACCESSIBILITY CONSIDERATION: BUILDING REGULATIONS APPROVED DOCUMENT M (ACCESS) — FOR NEW EXTENSIONS ON EXISTING DWELLINGS — REQUIRES LEVEL OR MINIMAL-STEP THRESHOLDS WHERE REASONABLY PRACTICABLE.

BI-FOLD DOOR INSTALLATIONS IN A LONDON VICTORIAN TERRACE REAR EXTENSION TYPICALLY FALL INTO ONE OF THREE PLANNING SCENARIOS:

SCENARIO 1 — PERMITTED DEVELOPMENT (PD): MOST SINGLE-STOREY REAR EXTENSIONS TO LONDON VICTORIAN TERRACES (NOT IN CONSERVATION AREAS WHERE ARTICLE 4 DIRECTIONS APPLY — AND NOT LISTED BUILDINGS) CAN BE BUILT UNDER PERMITTED DEVELOPMENT (GPDO 2015 CLASS A — SCHEDULE 2 PART 1) WITHOUT PLANNING PERMISSION, PROVIDED: SINGLE STOREY — MAXIMUM 4M PROJECTION BEYOND THE REAR WALL OF THE ORIGINAL HOUSE (OR 3M IF SEMI-DETACHED — BUT LONDON VICTORIAN TERRACES ARE TYPICALLY TERRACED = 4M LIMIT OR 6M UNDER THE NEIGHBOUR CONSULTATION SCHEME). MAXIMUM EAVES HEIGHT 3M AND OVERALL HEIGHT 4M. NO EXTENSION ON DESIGNATED LAND (AONB, SSSI, WHS) OR LISTED BUILDING. BI-FOLD DOORS IN A PD EXTENSION: THE BI-FOLD DOOR OPENING IN A PD EXTENSION CAN BE ANY SIZE — THE PD RESTRICTION IS ON THE OVERALL EXTENSION SIZE, NOT THE DOOR SIZE. PRIOR APPROVAL IS REQUIRED UNDER THE NEIGHBOUR CONSULTATION SCHEME FOR SINGLE-STOREY EXTENSIONS OF 4-6M (3-4M FOR SEMI-DETACHED).

SCENARIO 2 — CONSERVATION AREA (ARTICLE 4 DIRECTION): IN A LONDON CONSERVATION AREA WITH AN ARTICLE 4 DIRECTION REMOVING PD RIGHTS FOR REAR EXTENSIONS: PLANNING PERMISSION IS REQUIRED FOR THE ENTIRE REAR EXTENSION (NOT JUST THE BI-FOLD DOORS). THE COUNCIL DESIGN GUIDANCE FOR THE CONSERVATION AREA WILL SPECIFY REQUIREMENTS FOR THE REAR ELEVATION TREATMENT — BI-FOLD DOORS ARE GENERALLY ACCEPTABLE IN MOST LONDON CONSERVATION AREA GUIDANCE AS A CONTEMPORARY ELEMENT IN A CLEARLY NEW EXTENSION — BUT THE DESIGN AND MATERIALS OF THE OVERALL EXTENSION MUST BE SYMPATHETIC.

PARTY WALL ACT: FOR A REAR EXTENSION TO A LONDON VICTORIAN TERRACE: SECTION 6 PARTY WALL NOTICE (EXCAVATION WITHIN 3M OF NEIGHBOUR'S FOUNDATIONS): LIKELY TO BE REQUIRED FOR THE EXTENSION FOUNDATIONS (PARTICULARLY IF THE EXTENSION IS CLOSE TO THE REAR BOUNDARY WALL — WHICH IS TYPICALLY A PARTY STRUCTURE). SECTION 2 NOTICE (WORKS ON OR NEAR THE PARTY WALL — THE SIDE BOUNDARY WALLS): IF THE EXTENSION WALLS ADJOIN OR ARE BUILT OFF THE PARTY WALL, SECTION 2 NOTICE IS REQUIRED. SECTION 1 NOTICE (NEW WALL ON THE LINE OF JUNCTION): IF A NEW WALL IS BEING BUILT ON THE BOUNDARY, SECTION 1 NOTICE. AWARD AND SCHEDULE OF CONDITION STRONGLY RECOMMENDED FOR ALL EXTENSION WORKS NEAR NEIGHBOURS.

THE THERMAL PERFORMANCE OF BI-FOLD DOORS IN A LONDON VICTORIAN TERRACE REAR EXTENSION IS GOVERNED BY BUILDING REGULATIONS APPROVED DOCUMENT L (CONSERVATION OF FUEL AND POWER — VOLUME 1 DWELLINGS — 2021 EDITION) AND BY THE SAP ENERGY ASSESSMENT FOR THE EXTENSION.

U-VALUE REQUIREMENTS (PART L 2021): WINDOWS AND DOORS (INCLUDING BI-FOLD DOORS) IN A NEW EXTENSION: MAXIMUM OVERALL WINDOW U-VALUE: 1.4 W/M²K (OVERALL — INCLUDING FRAME — NOT JUST CENTRE-PANE). THIS IS THE MINIMUM REQUIREMENT — THE SPECIFYING ENGINEER OR SAP ASSESSOR MAY REQUIRE A BETTER U-VALUE TO COMPENSATE FOR THE HIGH PROPORTION OF GLAZING IN THE EXTENSION (PARTICULARLY WHERE THE BI-FOLD DOORS FORM A LARGE PROPORTION OF THE REAR ELEVATION — WHICH IS COMMON IN A LONDON VICTORIAN TERRACE REAR EXTENSION WHERE THE BI-FOLD MAY OCCUPY 60-80% OF THE REAR WALL AREA). ORIGIN OB-49: OVERALL WINDOW U-VALUE APPROXIMATELY 1.4-1.6 W/M²K (WITH STANDARD DOUBLE GLAZING) OR DOWN TO 1.1 W/M²K (WITH PILKINGTON OPTITHERM LOW-E). SCHÜCO ASS 70.HI: OVERALL WINDOW U-VALUE DOWN TO 0.8 W/M²K (WITH TRIPLE GLAZING) — EXCEEDS PART L 2021 REQUIREMENT COMFORTABLY. SOLAR GAIN: FOR A SOUTH-FACING REAR EXTENSION (THE MOST COMMON ORIENTATION FOR A LONDON VICTORIAN TERRACE — REAR FACING SOUTH OR SOUTH-WEST), SOLAR GAIN THROUGH LARGE BI-FOLD GLAZING CAN CAUSE SUMMERTIME OVERHEATING — AN IMPORTANT CONSIDERATION IN SAP ASSESSMENT AND IN COMFORT. SOLUTIONS: SOLAR CONTROL GLASS (PILKINGTON ACTIV BLUE — REDUCES G-VALUE — THE PROPORTION OF SOLAR ENERGY TRANSMITTED — FROM 0.68 FOR STANDARD CLEAR GLASS TO 0.33 FOR SOLAR CONTROL GLASS) OR EXTERNAL SOLAR SHADING (INTEGRAL VENETIAN BLIND IN THE GLAZED ROOF OR PERGOLA ABOVE THE PATIO).

BI-FOLD DOOR COSTS IN LONDON (2025) FOR A STANDARD LONDON VICTORIAN TERRACE REAR EXTENSION (SINGLE-STOREY, 3.6M WIDE REAR OPENING — 4-PANEL BI-FOLD SYSTEM).

DOOR SYSTEM (SUPPLY AND FIT — LONDON — 3.6M WIDE X 2.1M HIGH — 4-PANEL): ALUK 58BF OR SMART VISOGLIDE (BUDGET — STANDARD DGU): £3,500 TO £5,500. ORIGIN OB-49 (STANDARD — THERMALLY BROKEN — LOW-E DGU — ANTHRACITE GREY RAL 7016 OR TRAFFIC BLACK — THE MOST COMMON SPECIFICATION IN LONDON VICTORIAN TERRACE EXTENSIONS): £5,500 TO £8,000. SCHÜCO ASS 70.HI (PREMIUM — FLUSH TRACK — TRIPLE GLAZING — GERMAN ENGINEERING): £8,000 TO £14,000. STEEL-FRAMED BESPOKE (IQ GLASS OR BESPOKE STEEL FABRICATOR — ULTRA-SLIM FRAME — HIGHEST-END SPECIFICATION): £15,000 TO £30,000+.

STRUCTURAL STEEL RSJ LINTEL (UB 254X146X31 — S275 — FOR A 3.6M SPAN — SUPPLY, DELIVER, LIFT INTO POSITION, PADSTONES, PROPPING): £1,500 TO £3,500 (LABOUR-INTENSIVE — BEAM LIFT REQUIRES 2-3 OPERATIVE TEAM AND CRANE OR ACROW PROPPING SYSTEM).

ACO THRESHOLD DRAIN (70MM STAINLESS STEEL LINEAR DRAIN — SUPPLY AND FIT — 3.6M LENGTH): £300 TO £600.

HANSON CEMFLOOR INTERNAL SCREED (FOR UFH + SCREED FLOOR IN EXTENSION — APPROXIMATELY 20M²): £2,000 TO £4,000 SUPPLY AND PUMP.

TOTAL BI-FOLD DOOR + STRUCTURAL LINTEL + THRESHOLD DRAIN (STANDARD SPECIFICATION — ORIGIN OB-49 + UB 254 LINTEL + ACO DRAIN): £8,000 TO £13,000 (EXCLUDING EXTENSION STRUCTURE, FOUNDATION, ROOF, FLOOR, AND INTERNAL FINISHES — DOOR AND STRUCTURAL ELEMENTS ONLY).