Dormer Windows for London Victorian Terraces: Box Dormer, L-Dormer, and Rooflight Comparison

THE CHOICE OF DORMER TYPE FOR A LONDON VICTORIAN TERRACE LOFT CONVERSION INVOLVES BALANCING PLANNING ACCEPTABILITY, AVAILABLE HEAD HEIGHT AND FLOOR AREA, DAYLIGHT, COST, AND THE RELATIONSHIP WITH THE EXISTING BUILDING FORM.

BOX DORMER (FLAT-ROOF DORMER — THE DOMINANT DORMER TYPE IN LONDON VICTORIAN TERRACE LOFT CONVERSIONS): A BOX DORMER IS A RECTANGULAR PROJECTION FROM THE ROOF SLOPE WITH VERTICAL CHEEKS (SIDE WALLS), VERTICAL FRONT FACE (TYPICALLY FULLY GLAZED — CASEMENT WINDOW OR LARGE FIXED GLAZED PANEL), AND A FLAT ROOF. THE BOX DORMER MAXIMISES: INTERNAL HEADROOM — THE FULL CEILING HEIGHT OF THE BOX DORMER VOLUME IS VERTICAL (2.3M OR MORE IS ACHIEVABLE, SUBJECT TO THE PITCH OF THE ORIGINAL ROOF AND DORMER RIDGE HEIGHT), PROVIDING MAXIMUM USABLE FLOOR AREA. WINDOW AREA — THE FULL FRONT FACE OF THE BOX DORMER CAN BE GLAZED, DELIVERING HIGH LEVELS OF DAYLIGHT. THE BOX DORMER IS THE MOST COST-EFFECTIVE DORMER FORM PER UNIT OF HABITABLE FLOOR AREA CREATED.

L-SHAPED DORMER (SOMETIMES CALLED MANSARD-REAR DORMER — WRAPPING REAR AND SIDE): THE L-SHAPED DORMER EXTENDS THE BOX DORMER CONCEPT TO WRAP AROUND ONE SIDE OF THE ROOF — PARTICULARLY USEFUL IN LONDON VICTORIAN TERRACES WITH A SIDE RETURN (WHERE THE GROUND FLOOR SIDE EXTENSION, OR THE ABSENCE OF ONE, CREATES A LOWER ROOF LEVEL AT THE SIDE OF THE PROPERTY). THE L-SHAPED DORMER PROVIDES: MAXIMUM HABITABLE VOLUME — COMBINING THE REAR DORMER VOLUME WITH A SIDE EXTENSION OF THE LOFT ALLOWS LARGER BEDROOMS, AN ADDITIONAL BATHROOM, OR AN OPEN-PLAN LOFT SPACE. THE SIDE ELEMENT OF THE L IS TYPICALLY A SEPARATE FLAT-ROOF DORMER ON THE SIDE ELEVATION — PLANNING FOR THIS IS MORE COMPLEX THAN A PURELY REAR DORMER.

ROOFLIGHT (VELUX CONSERVATION OR FAKRO FLAT-ROOF WINDOW — FLUSH WITH ROOF SLOPE): A ROOFLIGHT (ALSO CALLED ROOF WINDOW) SITS FLUSH WITH THE EXISTING ROOF SLOPE — NO DORMER PROJECTION. VELUX CONSERVATION WINDOWS (GK24 0059 LOOK-ALIKE OR GGU FK06 CONSERVATION WINDOW — GREY POWDER-COAT FRAME FLUSH WITH SLATE OR PLAIN TILE): THE CONSERVATION ROOFLIGHT IS THE PLANNING-ACCEPTABLE CHOICE FOR LONDON VICTORIAN TERRACES IN CONSERVATION AREAS WHERE THE PLANNING AUTHORITY RESTRICTS DORMER PROJECTIONS ON FRONT OR VISIBLE SLOPES.

DORMER WINDOW PLANNING RULES FOR LONDON VICTORIAN TERRACES ARE GOVERNED BY THE GPDO 2015 CLASS B (ADDITIONS TO THE ROOF) AND BY SUPPLEMENTARY PLANNING GUIDANCE ISSUED BY LONDON BOROUGHS FOR THEIR CONSERVATION AREAS.

PERMITTED DEVELOPMENT — CLASS B GPDO 2015 (ROOF ADDITIONS): A REAR DORMER ON A LONDON VICTORIAN TERRACE MAY BE PERMITTED DEVELOPMENT (NO PLANNING APPLICATION REQUIRED) IF: THE DORMER DOES NOT EXTEND BEYOND THE PLANE OF THE ORIGINAL ROOF SLOPE ON THE PRINCIPAL (FRONT) ELEVATION — I.E. NO FRONT DORMERS UNDER PD (ONLY VELUX-STYLE ROOFLIGHTS FLUSH WITH THE FRONT ROOF SLOPE ARE PERMITTED UNDER CLASS C). THE DORMER DOES NOT EXCEED THE HEIGHT OF THE EXISTING ROOF RIDGE — THE FLAT ROOF OF A BOX DORMER MUST BE BELOW THE RIDGE HEIGHT. NO SIDE-FACING WINDOWS WITHIN 10M OF A BOUNDARY AT SECOND-FLOOR LEVEL OR ABOVE (CLASS B.2(d)).

CONSERVATION AREA IMPACT ON DORMER PLANNING IN LONDON: LONDON BOROUGH CONSERVATION AREAS OFTEN HAVE SUPPLEMENTARY PLANNING DOCUMENTS (SPDS) OR ARTICLE 4 DIRECTIONS THAT REMOVE OR RESTRICT PD RIGHTS FOR DORMER WINDOWS ON PROPERTIES WITHIN THE CONSERVATION AREA. THE MAIN CONSERVATION AREA RESTRICTIONS THAT AFFECT DORMER WINDOW PLANNING IN LONDON: ARTICLE 4 DIRECTION (REMOVING CLASS B PD RIGHTS): MEANS THAT ANY DORMER ADDITION (EVEN A SMALL REAR BOX DORMER) REQUIRES A HOUSEHOLDER PLANNING APPLICATION IN THE AFFECTED CONSERVATION AREA. PLANNING AUTHORITY PREFERENCES IN LONDON CONSERVATION AREA DORMER APPLICATIONS: DORMERS SHOULD BE SUBSERVIENT TO THE MAIN ROOF (SET BACK FROM THE EAVES AND SET IN FROM THE SIDES OF THE ROOF SLOPE). DORMER CHEEKS (SIDE WALLS) SHOULD BE IN LEAD, ZINC, OR BLACK-STAINED TIMBER CLADDING — NOT TILE HANGING OR RENDER (WHICH LOOKS ALIEN ON A VICTORIAN ROOF SLOPE). DORMER WINDOW FRAMES SHOULD BE IN TIMBER OR POWDER-COATED ALUMINIUM (BLACK OR DARK GREY) — NOT WHITE UPVC (WHICH IS WIDELY REFUSED IN LONDON CONSERVATION AREA DORMER PLANNING APPLICATIONS). DORMER ROOF SHOULD BE IN LEAD OR ZINC (NOT ASPHALT FELT — WHICH IS INADEQUATE FOR A DORMER FLAT ROOF IN LONDON).

THE DORMER WINDOW FRAME MATERIAL AND PROFILE ARE VISIBLE FROM THE STREET (FOR REAR DORMERS, FROM NEIGHBOURING GARDENS AND PROPERTIES) AND ARE A KEY CONSIDERATION IN BOTH PLANNING ACCEPTABILITY AND LONG-TERM PERFORMANCE.

TIMBER DORMER WINDOWS (THE MOST PLANNING-SYMPATHETIC CHOICE FOR LONDON VICTORIAN TERRACE CONSERVATION AREA LOFT CONVERSIONS): RATIONEL ALDAS TIMBER-ALUMINIUM WINDOWS: DANISH-MANUFACTURED TIMBER-ALUMINIUM COMPOSITE WINDOW — INTERIOR EUROPEAN OAK (OR PINE) FRAME WITH ALUMINIUM OUTER CASING (RAL 7016 ANTHRACITE OR RAL 9005 BLACK). SCHUECO TIMBER 85 OR SCHUECO AWS 75 BS (ALUMINIUM — THE PREMIUM CHOICE FOR CONTEMPORARY LONDON DORMER WINDOW REPLACEMENT): HIGH-PERFORMANCE THERMALLY BROKEN ALUMINIUM FRAMING — UF = 1.6 W/M²K (FRAME U-VALUE). COMPATIBLE WITH TRIPLE GLAZING (1.0 W/M²K OVERALL WINDOW U-VALUE). JELD-WEN BEDFORD TIMBER CASEMENT: A MORE AFFORDABLE TIMBER CASEMENT OPTION — AVAILABLE IN PAINTED SOFTWOOD OR HARDWOOD (MERANTI).

CRITTALL / STEEL-LOOK WINDOWS (INCREASINGLY POPULAR FOR LONDON LOFT DORMER APPLICATIONS — THE CONTEMPORARY INDUSTRIAL AESTHETIC): CRITTALL WINDOWS W20 SERIES (STEEL — THE ORIGINAL STEEL WINDOW BRAND): ULTRA-SLIM SIGHTLINES (20MM FACE WIDTH), HERITAGE-CORRECT FOR PRE-WAR LONDON PROPERTIES. THERMALLY BROKEN VERSION (W20 TF): IMPROVED THERMAL PERFORMANCE — UW APPROXIMATELY 2.0 W/M²K (SINGLE PANE EQUIVALENT). THERMALLY BROKEN STEEL FRAMES CANNOT ACHIEVE THE PART L 2021 NOTIONAL 1.4 W/M²K TARGET (WHOLE WINDOW) WITHOUT SECONDARY OR TRIPLE GLAZING — FOR A CONSERVATION AREA LONDON LOFT DORMER WHERE CRITTALL AESTHETICS ARE DESIRED, AN EXEMPTION UNDER TECHNICAL FEASIBILITY OR BUILDING CONTROL AGREEMENT IS OFTEN APPROPRIATE. STEEL LOOK (ALUMINIUM — ALTERNATIVE TO REAL STEEL): ALUMINIUM STEEL-LOOK WINDOWS (SMAL ALUMINIUM STEEL LOOK — SLIMLINE BLACK ALUMINIUM MIMICKING CRITTALL PROFILE) ARE A LOWER-COST ALTERNATIVE TO REAL STEEL — BETTER THERMAL PERFORMANCE (THERMALLY BROKEN ALUMINIUM, UW ≈ 1.4 W/M²K WITH TRIPLE GLAZING) AND LOWER MAINTENANCE.

THE STRUCTURAL MODIFICATIONS REQUIRED FOR A DORMER LOFT CONVERSION IN A LONDON VICTORIAN TERRACE ARE SIGNIFICANT — AND OFTEN MORE COMPLEX THAN FOR A SIMPLE ROOFLIGHT-ONLY CONVERSION — BECAUSE CUTTING THROUGH THE EXISTING ROOF STRUCTURE TO INSERT THE DORMER REQUIRES THAT THE ORIGINAL STRUCTURAL FUNCTION OF THE RAFTERS BEING REMOVED IS REDISTRIBUTED TO NEW STRUCTURAL ELEMENTS.

LONDON VICTORIAN TERRACE ROOF STRUCTURE: THE TYPICAL VICTORIAN TERRACE IN LONDON HAS A PITCHED ROOF (32° TO 42° PITCH — MOST LONDON STOCK-BRICK TERRACES ARE 40° PITCH) WITH: RAFTERS: 50X100MM SAWN SOFTWOOD AT 400MM CENTRES (CUT AND PITCHED FROM THE WALL PLATE TO THE RIDGE BOARD). COLLAR TIES (HORIZONTAL TIE BETWEEN OPPOSING RAFTERS AT APPROXIMATELY MID-RAFTER HEIGHT). PURLINS: HORIZONTAL TIMBERS SPANNING BETWEEN GABLE WALLS (OR PARTY WALL PILASTERS) THAT PROVIDE INTERMEDIATE SUPPORT TO THE RAFTERS AT MID-SPAN.

DORMER STRUCTURAL STRATEGY: WHEN INSTALLING A BOX DORMER ON THE REAR SLOPE OF A LONDON VICTORIAN TERRACE: THE RAFTERS TO BE REPLACED BY THE DORMER CHEEKS AND FRONT FACE ARE CUT BACK AND SUPPORTED ON TRIMMER TIMBERS (DOUBLED OR TREBLED RAFTERS AT EACH SIDE OF THE DORMER OPENING). THE FLAT ROOF STRUCTURE OF THE BOX DORMER IS CONSTRUCTED FROM STRUCTURAL TIMBER (NEW FLAT ROOF JOISTS — TYPICALLY 200X50MM OR 220X57MM C24 AT 400MM CENTRES, SPANNING FROM THE REAR WALL PLATE TO THE FRONT FACE OF THE DORMER). A STRUCTURAL ENGINEER CALCULATION IS REQUIRED FOR ALL DORMER LOFT CONVERSIONS IN LONDON VICTORIAN TERRACES — BUILDING CONTROL WILL NOT APPROVE THE WORKS WITHOUT STRUCTURAL CALCULATIONS SIGNED BY A REGISTERED STRUCTURAL ENGINEER (MIStructE OR MICE). PARTY WALL ACT: FOR A DORMER THAT INVOLVES CUTTING THROUGH OR MODIFYING THE PARTY WALL (THE WALL SHARED WITH THE NEIGHBOURING TERRACE) — WHICH IS TYPICAL FOR L-SHAPED DORMERS AND FOR DORMERS THAT SPAN THE FULL WIDTH OF THE ROOF SLOPE — A PARTY WALL NOTICE UNDER SECTION 2 OF THE PARTY WALL ETC. ACT 1996 (WORKS TO A PARTY WALL) IS REQUIRED.

THE FLAT ROOF OF A BOX DORMER IN A LONDON VICTORIAN TERRACE MUST BE WATERPROOFED TO A HIGH STANDARD — IT IS A CRITICAL WEATHER INTERFACE AND FAILURE OF THE DORMER FLAT ROOF IS ONE OF THE MOST COMMON CAUSES OF WATER INGRESS INTO LONDON LOFT CONVERSIONS.

FLAT ROOF MEMBRANE OPTIONS FOR LONDON VICTORIAN TERRACE DORMER ROOFS: GRP (GLASS-REINFORCED PLASTIC — FIBREGLASS): THE MOST COMMON FLAT ROOF WATERPROOFING FOR LONDON DORMER FLAT ROOFS. PRODUCT: MASTIC ASPHALT ALTERNATIVE — NOT USED ON NEW LONDON DORMER FLAT ROOFS (REQUIRES SPECIALIST HOT-APPLIED APPLICATION). SIKA SARNAFIL G410-15EL (SINGLE-PLY PVC MEMBRANE — 1.5MM): ONE OF THE LEADING SINGLE-PLY ROOF MEMBRANES FOR LONDON DORMER FLAT ROOF APPLICATIONS. APPLIED BY WELDING SHEETS TOGETHER (HOT-AIR WELDING — AIR TEMPERATURE UP TO 400°C AT THE WELD). MECHANICALLY FIXED TO OSB SUBSTRATE (SARNAVAP 1000E VCL BELOW OSB). COLOUR: PREDOMINANTLY LIGHT GREY OR DARK GREY (ANTHRACITE). 20-YEAR GUARANTEE (SIKA SARNAFIL WARRANTY ON SARNAFIL ROOF SYSTEMS INSTALLED BY SIKA-TRAINED CONTRACTORS — REQUIRES SIKA-APPROVED CONTRACTOR). FIRESTONE RUBBERCOVER EPDM (1.14MM — THE LOWEST-COST HIGH-QUALITY DORMER FLAT ROOF WATERPROOFING): EPDM RUBBER MEMBRANE — SINGLE-PLY, FULLY BONDED TO SUBSTRATE WITH FIRESTONE BONDING ADHESIVE. UV-STABLE BLACK RUBBER — DOES NOT REQUIRE PAINTING OR COATING. 25-YEAR LIFE EXPECTANCY. £25 TO £45 PER M² SUPPLY AND INSTALL. IKO ARMOURPLAN PVC (1.2MM — COMPETITOR TO SARNAFIL): SIMILAR PERFORMANCE TO SARNAFIL — PVC SINGLE PLY, HOT-AIR WELDED. IKO-ACCREDITED CONTRACTORS IN LONDON: £30 TO £55 PER M² SUPPLY AND INSTALL.

DORMER CHEEK CLADDING: LEAD (THE TRADITIONAL AND PLANNING-PREFERRED CHOICE FOR LONDON CONSERVATION AREA DORMER CHEEKS): CODE 4 LEAD SHEET (1.80MM THICK) — BS EN 12588 ROLLED LEAD SHEET. ZINC (RHEINZINK PRISM BLUE-GREY OR VM ZINC ZINC NATUREL): AN ALTERNATIVE TO LEAD FOR DORMER CHEEKS IN LONDON — LIGHTER, FASTER TO INSTALL, SIMILAR PLANNING ACCEPTABILITY TO LEAD IN MOST LONDON CONSERVATION AREAS.

DORMER LOFT CONVERSION COSTS IN LONDON VARY SIGNIFICANTLY BY DORMER TYPE, SIZE, SPECIFICATION, AND THE NUMBER OF ROOMS CREATED IN THE NEW LOFT SPACE.

FULL BOX DORMER LOFT CONVERSION (LONDON VICTORIAN TERRACE — REAR ONLY, CREATING 1 BEDROOM + 1 BATHROOM + STAIRCASE): STRUCTURAL WORKS (RIDGE BEAM, TRIMMER TIMBERS, DORMER FRAME): £8,000 TO £16,000. FLAT ROOF (DORMER + STAIRCASE VOID — APPROX 15M² — EPDM OR SARNAFIL): £1,500 TO £3,500. DORMER CHEEKS (LEAD OR ZINC — APPROX 10M²): £1,500 TO £3,500. DORMER WINDOW (TIMBER CASEMENT OR ALUMINIUM — FULL FRONT FACE GLAZING): £1,500 TO £5,000. INSULATION (BETWEEN AND BELOW RAFTERS — KNAUF DRITHERM + PIR BOARD TO ACHIEVE 0.15 W/M²K): £1,500 TO £3,500. PLASTERBOARD AND PLASTERING: £1,500 TO £3,000. ELECTRICAL AND PLUMBING (LOFT BATHROOM — UPVC SOIL STACK EXTENSION, WC, BASIN, SHOWER): £4,000 TO £8,000. STAIRS (LOFT STAIRCASE — QUARTER-TURN WITH GLASS BALUSTRADE): £3,000 TO £6,000. DECORATING AND FLOORING: £1,500 TO £3,000. TOTAL REAR BOX DORMER LOFT CONVERSION — LONDON (CONTRACTOR-LED, EXCLUDING DESIGN FEES AND VAT): £25,000 TO £55,000. PLANNING APPLICATION (CONSERVATION AREA — HOUSEHOLDER): £258 (PLANNING FEE) + £1,000 TO £3,000 ARCHITECT DRAWING FEES. STRUCTURAL ENGINEER CALCULATIONS: £800 TO £2,500. PARTY WALL SURVEYOR (IF REQUIRED): £800 TO £2,500.