Dormer Types for London Victorian Terrace Loft Conversions: Box Dormer, Shed Dormer, L-Shaped, and Mansard

THE BOX DORMER (ALSO CALLED A FLAT-ROOF DORMER OR SQUARE DORMER — THE MOST COMMON DORMER TYPE ON LONDON VICTORIAN TERRACES — A VERTICAL-FACED DORMER WITH A FLAT ROOF THAT PROJECTS FROM THE MAIN ROOF SLOPE) IS THE DOMINANT DORMER TYPE USED IN LONDON LOFT CONVERSIONS FOR THE FOLLOWING REASONS: MAXIMUM HEAD HEIGHT AND FLOOR AREA (THE VERTICAL FRONT FACE OF THE BOX DORMER EXTENDS DIRECTLY UPWARD FROM THE FRONT OF THE DORMER — CREATING A FULL-HEIGHT VERTICAL WALL ON THE DORMER FRONT — THIS MAXIMISES THE USABLE FLOOR AREA INSIDE THE LOFT ROOM AS THE HEAD HEIGHT EXTENDS MUCH CLOSER TO THE FRONT EDGE OF THE DORMER THAN IN A PITCHED-ROOF DORMER); FLAT ROOF ENABLES LARGE FIXED OR OPENING WINDOWS (THE FLAT ROOF OF THE BOX DORMER ENABLES THE FRONT FACE TO BE FULLY GLAZED — OR FITTED WITH A LARGE FIXED WINDOW OR A FULL-WIDTH TILT-AND-TURN WINDOW — MAXIMISING NATURAL LIGHT INTO THE LOFT ROOM); STRUCTURAL SIMPLICITY RELATIVE TO MANSARD OR L-SHAPED DORMER (THE BOX DORMER STRUCTURE IS A TIMBER STUD FRAME ON THE FRONT FACE AND SIDES, WITH A FLAT ROOF OF C16 RAFTERS AT 400MM CENTRES OR POSI-JOISTS AT WIDER CENTRES WHERE LONGER SPANS ARE NEEDED — STRUCTURALLY MORE STRAIGHTFORWARD THAN THE STEEL-INTENSIVE MANSARD OR L-SHAPED DORMER); REAR ELEVATION — PERMITTED DEVELOPMENT: ON MOST LONDON VICTORIAN TERRACES, A BOX DORMER TO THE REAR SLOPE OF THE ROOF (THE ROOF SLOPE FACING THE REAR GARDEN — NOT THE STREET-FACING SLOPE) CAN BE BUILT UNDER PERMITTED DEVELOPMENT RIGHTS WITHOUT PLANNING PERMISSION (SUBJECT TO PD RULES — SEE BELOW) — MAKING IT THE MOST COMMONLY IMPLEMENTED DORMER OPTION IN LONDON WITHOUT REQUIRING PLANNING CONSENT. PD LIMITS FOR REAR DORMER ON A HOUSE (CLASS B — ADDITIONS AND ALTERATIONS TO THE ROOF — GPDO 2015 SCHEDULE 2 PART 1 CLASS B): MUST NOT EXTEND BEYOND THE PLANE OF THE EXISTING ROOF SLOPE (I.E. THE DORMER FLAT ROOF MUST NOT BE HIGHER THAN THE EXISTING RIDGE LINE); MUST NOT EXTEND BEYOND THE OUTERMOST EDGE OF THE ORIGINAL WALL OF THE MAIN HOUSE IN PLAN; SIDE WALLS OF THE DORMER MUST BE SET BACK AT LEAST 200MM FROM THE EDGES OF THE ORIGINAL ROOF (THE HIPS OR EXTERNAL WALL BELOW — SO A REAR DORMER ON A TERRACE MUST NOT ABUT THE PARTY WALL LINES); FRONT FACE MATERIALS MUST BE CLAD IN TILE, SLATE, OR METAL (NOT UPCV CLADDING); CLASS B PD DOES NOT APPLY IN CONSERVATION AREAS (ARTICLE 4 DIRECTIONS COMMONLY REMOVE CLASS B RIGHTS IN LONDON CONSERVATION AREAS — CHECK WITH THE LOCAL PLANNING AUTHORITY) OR ON LISTED BUILDINGS.

THE SHED DORMER (A DORMER WHERE THE ROOF OF THE DORMER SLOPES IN THE SAME DIRECTION AS THE MAIN ROOF BUT AT A SHALLOWER PITCH — CREATING A LONG CONTINUOUS SHALLOW-PITCH DORMER THAT TYPICALLY EXTENDS ACROSS THE FULL WIDTH OF THE REAR ROOF SLOPE) IS A LARGER-SCALE VERSION OF THE BOX DORMER — USED WHERE MAXIMUM FLOOR AREA AT THE REAR IS THE PRIMARY OBJECTIVE.

SHED DORMER (FULL-WIDTH REAR SHED DORMER — ALSO CALLED A REAR DORMER EXTENSION): EXTENDS ACROSS THE FULL WIDTH OF THE REAR ROOF SLOPE BETWEEN THE PARTY WALLS (OR SET BACK 200MM FROM PARTY WALLS UNDER PD); THE ROOF IS A SHALLOW-PITCH SINGLE-SLOPE (TYPICALLY 5-10° PITCH — FLAT MEMBRANE ROOFING APPROACH — SARNAFIL G410 OR EPDM RUBBER AS WITH ANY FLAT OR NEAR-FLAT ROOF); CREATES THE MAXIMUM POSSIBLE FLOOR AREA IN THE LOFT FOR A GIVEN ROOF GEOMETRY — THE FULL WIDTH OF THE HOUSE IS AVAILABLE AS USABLE LOFT FLOOR AREA (SUBJECT TO 200MM PD SETBACK AT EACH PARTY WALL SIDE); TYPICALLY REQUIRES MORE STRUCTURAL INTERVENTION THAN A NARROWER BOX DORMER — THE FULL-WIDTH FRONT PLATE AND STRUCTURAL CONNECTION TO THE EXISTING RIDGE/PURLINS REQUIRE STRUCTURAL ENGINEERING DESIGN; TYPICALLY REQUIRES PLANNING PERMISSION IN CONSERVATION AREAS OR IF THE SCALE EXCEEDS PD LIMITS; L-SHAPED DORMER (A COMBINATION OF A REAR BOX DORMER AND A REAR EXTENSION DORMER THAT WRAPS AROUND THE CORNER OF THE EXISTING ROOF — CREATING AN L-SHAPED PLAN FOOTPRINT): USED ON LONDON SEMI-DETACHED VICTORIAN TERRACES AND WIDER END-OF-TERRACE PROPERTIES WHERE BOTH THE REAR AND SIDE ROOF SLOPES CAN ACCOMMODATE DORMER PROJECTIONS; THE L-SHAPE DRAMATICALLY INCREASES TOTAL FLOOR AREA — COMBINING THE REAR BOX DORMER (ON THE MAIN REAR SLOPE) WITH A SIDE OR OUTRIGGER DORMER (ON THE SIDE OR RETURN SLOPE); SIDE DORMERS TYPICALLY REQUIRE PLANNING PERMISSION (NOT PD — THE GPDO PROHIBITS DORMERS ABOVE THE PRINCIPAL ELEVATION (FRONT) OR ON A SIDE ELEVATION FACING A HIGHWAY — SIDE DORMERS ALSO TYPICALLY REQUIRE PLANNING IF THEY EXCEED 200MM FROM THE EDGE OF THE ROOF); THE L-SHAPED DORMER IS MORE STRUCTURALLY COMPLEX AND COSTLY THAN A SIMPLE BOX DORMER — REQUIRES STRUCTURAL STEEL (TYPICALLY A UC COLUMN OR RIDGE BEAM AT THE L-JUNCTION) AND CAREFUL WATERPROOFING DETAILING AT THE INTERNAL CORNER OF THE L.

THE MANSARD DORMER (A DORMER WITH STEEPLY PITCHED SIDES (TYPICALLY 72-75° FROM HORIZONTAL) AND A SHALLOWER FLAT OR NEAR-FLAT ROOF — CLOSELY RELATED TO THE MANSARD ROOF FORM WHICH IS A ROOF TYPE WITH A DOUBLE PITCH ON ALL FOUR SIDES — LOWER SLOPE VERY STEEP, UPPER SLOPE NEARLY FLAT) IS THE HIGHEST-SPECIFICATION DORMER OPTION FOR LONDON VICTORIAN TERRACE LOFT CONVERSIONS — AND TYPICALLY THE MOST EXPENSIVE AND STRUCTURALLY COMPLEX.

MAN SARD DORMER ON REAR SLOPE (A REAR MANSARD DORMER — WHERE THE SLOPING CHEEKS OF THE MANSARD DORMER ARE SET AT THE CHARACTERISTIC 72-75° STEEP ANGLE — CLAD IN SLATE, LEAD, OR ZINC — WITH A NEAR-FLAT MEMBRANE ROOF ABOVE): CREATES THE SAME VERTICAL HEIGHT AS A BOX DORMER BUT WITH THE SLOPING CHEEKS GIVING AN ARCHITECTURAL FORM MORE SYMPATHETIC TO TRADITIONAL LONDON ARCHITECTURE — COMMONLY REQUIRED BY CONSERVATION AREA PLANNERS INSTEAD OF THE FLAT-FRONTED BOX DORMER AS A CONDITION OF PLANNING CONSENT; MANSARD ROOF CONVERSION (FULL MANSARD ROOF — REPLACING THE ENTIRE EXISTING PITCHED ROOF WITH A FULL MANSARD ROOF FORM — ALL FOUR SLOPES CONVERTED TO A MANSARD CONFIGURATION WITH 72-75° LOWER SLOPES AND A NEAR-FLAT TOP): THIS IS A MAJOR STRUCTURAL OPERATION — THE ENTIRE EXISTING ROOF STRUCTURE IS REMOVED AND REPLACED WITH A NEW STRUCTURAL FRAME (TYPICALLY USING LVLS — LAMINATED VENEER LUMBER — FOR THE RIDGE AND MAIN BEAMS — OR STRUCTURAL STEEL — WITH LIGHT TIMBER STUD FRAME INFILL) — THE RESULT IS ESSENTIALLY A NEW ADDITIONAL STOREY ADDED TO THE TOP OF THE HOUSE — THE MANSARD ROOF CONVERSION DRAMATICALLY INCREASES THE FLOOR AREA OF THE LOFT ROOM COMPARED TO A SIMPLE DORMER — BUT TYPICALLY REQUIRES PLANNING PERMISSION (IT ALTERS THE ENTIRE ROOFSCAPE OF THE PROPERTY — WHICH IS NOT WITHIN PD RIGHTS); MANSARD FRONT SLOPE (STREET-FACING MANSARD): IN MANY LONDON CONSERVATION AREAS, A FRONT MANSARD SLOPE (REPLACING THE ORIGINAL STREET-FACING PITCHED ROOF WITH A MANSARD CONFIGURATION) IS THE ONLY FORM OF LOFT EXTENSION THAT WILL BE GRANTED PLANNING PERMISSION — AS IT IS MORE SYMPATHETIC TO THE ORIGINAL VICTORIAN TERRACE ROOFSCAPE THAN A PROJECTING BOX DORMER ON THE FRONT SLOPE; PLANNING STATUS: FULL MANSARD ROOF CONVERSIONS AND FRONT MANSARD DORMERS ALWAYS REQUIRE PLANNING PERMISSION — THEY CANNOT BE DONE UNDER PERMITTED DEVELOPMENT (PD APPLIES ONLY TO REAR AND SIDE DORMERS THAT DO NOT ALTER THE FRONT ELEVATION).

THE STRUCTURAL SPECIFICATION FOR A DORMER ON A LONDON VICTORIAN TERRACE LOFT CONVERSION DEPENDS ON THE TYPE AND SIZE OF THE DORMER — BUT THE FOLLOWING ELEMENTS ARE COMMON TO MOST DORMER STRUCTURES.

DORMER FLOOR (THE STRUCTURAL FLOOR OF THE LOFT ROOM — THE FLOOR AT THE SAME LEVEL AS THE BOTTOM OF THE DORMER — TYPICALLY THE EXISTING CEILING JOIST LEVEL OF THE PROPERTY BELOW): EXISTING CEILING JOISTS WERE NOT DESIGNED AS FLOOR JOISTS AND TYPICALLY CANNOT CARRY RESIDENTIAL FLOOR LOADING (1.5 KN/M² IMPOSED LOAD PER BS EN 1991-1-1) WITHOUT REINFORCEMENT — A STRUCTURAL ENGINEER MUST SPECIFY THE FLOOR JOIST UPGRADE (TYPICALLY: NEW C16 OR C24 SAWN STRUCTURAL TIMBER FLOOR JOISTS AT 400MM CENTRES — 195MM OR 220MM DEEP — INSTALLED ALONGSIDE OR REPLACING THE EXISTING CEILING JOISTS — THE ENGINEER'S CALCULATIONS DETERMINE THE CORRECT DEPTH AND GRADE); STRUCTURAL BEAM AT HEAD OF STAIRCASE (THE STAIRCASE OPENING CUT THROUGH THE EXISTING FLOOR STRUCTURE REQUIRES A NEW STRUCTURAL TRIMMER BEAM — EITHER A TIMBER DOUBLED JOIST TRIMMER OR (FOR LARGER STAIRCASE OPENINGS) A SMALL STEEL BEAM — SPECIFIED BY THE STRUCTURAL ENGINEER); DORMER FRONT FACE (THE VERTICAL FRONT WALL OF A BOX DORMER): TIMBER STUD FRAME (TYPICALLY 89MM OR 140MM C16 SAWN TIMBER STUDS AT 400MM CENTRES — TREATED WITH ORGANIC SOLVENT PRESERVATIVE — SHEATHED WITH 12MM OSB3 ON THE EXTERIOR FACE — THEN BREATHER MEMBRANE (ROTHOBLAAS DIVA 80 OR MONARFLEX RSA 100) THEN EXTERNAL CLADDING); INSULATION TO DORMER CHEEKS AND FRONT FACE (PART L 2021 TARGET 0.15 W/M²K FOR NEW WALLS — ACHIEVABLE WITH: 140MM BETWEEN STUD + 25MM KINGSPAN K18 INSULATED PLASTERBOARD ON INTERNAL FACE — ACHIEVING U-VALUE APPROXIMATELY 0.15-0.20 W/M²K DEPENDING ON STUD FRACTION); DORMER FLAT ROOF (THE FLAT ROOF OF A BOX DORMER OR SHED DORMER): WARM ROOF SPECIFICATION — C16 SAWN TIMBER RAFTERS (100X50MM AT 400MM CENTRES SPANNING THE DORMER WIDTH — OR POSI-JOISTS FOR WIDER SPANS) + 18MM OSB3 DECK (GLUED AND NAILED T&G) + VCL + 100-120MM KINGSPAN THERMAROOF TR27 PIR INSULATION + GEOTEXTILE FLEECE + SARNAFIL G410-15EL PVC MEMBRANE (OR EPDM — FIRESTONE RUBBERCOVER 1.14MM) — WARM ROOF U-VALUE APPROXIMATELY 0.15-0.18 W/M²K; DORMER WINDOW SPECIFICATION: STANDARD DORMER FIXED OR CASEMENT WINDOW (TYPICALLY TIMBER OR ALUMINIUM — SMART ALITHERM 47 OR RATIONEL AURAPLUS OR TIMBER SOFTWOOD WINDOW WITH ACCOYA CILL) — DOUBLE OR TRIPLE GLAZED — SILL FLASHING (LEAD SOAKER FLASHING OR SARNAFIL FLASHING TAPE) CRITICAL FOR PREVENTING WATER INGRESS AT WINDOW CILL.

DORMER PLANNING STATUS (SEE ALSO SECTION 1 FOR BOX DORMER PD RULES): PERMITTED DEVELOPMENT (REAR DORMER ONLY — CLASS B — GPDO 2015 SCHEDULE 2 PART 1): BOX DORMER AND SHED DORMER TO THE REAR SLOPE ONLY — SET BACK 200MM FROM PARTY WALLS — NOT ABOVE RIDGE LINE — NOT ABOVE PRINCIPAL ELEVATION — MATERIALS CLAD IN TILE/SLATE/METAL — NOT IN CONSERVATION AREAS UNLESS ARTICLE 4 DIRECTION REMOVES CLASS B RIGHTS; PLANNING PERMISSION REQUIRED FOR: CONSERVATION AREA DORMER (EVEN REAR DORMER — IF ARTICLE 4 DIRECTION IN FORCE — WHICH IS VERY COMMON IN LONDON CONSERVATION AREAS); FRONT DORMER OR FRONT MANSARD; SIDE DORMER (SIDE ELEVATION FACING HIGHWAY); FULL MANSARD ROOF CONVERSION (ALTERS ROOFSCAPE); ANY DORMER EXCEEDING PD DIMENSIONAL LIMITS; PLANNING APPLICATION FEE: £258 HOUSEHOLDER APPLICATION (FROM DECEMBER 2023 — FEE INCREASED TO £258 FOR HOUSEHOLDER APPLICATIONS — PREVIOUSLY £206). DORMER COSTS (LONDON 2025 — APPROXIMATE RANGE — SUPPLY AND FIT — SPECIALIST LOFT CONVERSION CONTRACTOR — INCLUDING STRUCTURAL ENGINEERING, DORMER STRUCTURE, INSULATION, ROOFING MEMBRANE, AND WINDOW — EXCLUDING INTERNAL FINISHING, STAIRCASE, PLUMBING, AND ELECTRICAL): STANDARD REAR BOX DORMER (TYPICAL SIZE 3.0-4.0M WIDE X 1.8M HIGH — SINGLE BEDROOM LOFT CONVERSION): APPROXIMATELY £12,000-22,000 (STRUCTURE, ROOFING, WINDOW — BOX DORMER ONLY — NOT INCLUDING FULL LOFT CONVERSION INTERNAL FIT-OUT). FULL-WIDTH REAR SHED DORMER (FULL WIDTH OF HOUSE — TYPICALLY 4.5-6.0M WIDE — SINGLE DOUBLE BEDROOM LOFT CONVERSION): APPROXIMATELY £18,000-35,000 (STRUCTURE, ROOFING, FULL-WIDTH GLAZING OR WINDOWS — DORMER ONLY). L-SHAPED DORMER (REAR AND SIDE SLOPE — MORE COMPLEX — STEEL AT INTERNAL CORNER): APPROXIMATELY £25,000-50,000 (DORMER STRUCTURE AND ROOFING ONLY — NOT FULL LOFT CONVERSION FIT-OUT). MANSARD DORMER (REAR SINGLE MANSARD DORMER — SLATE-CLAD CHEEKS): APPROXIMATELY £18,000-35,000 (DORMER ONLY). FULL MANSARD ROOF CONVERSION (ENTIRE ROOF REPLACED — NEW STRUCTURAL FRAME — FULL MANSARD FORM — PLANNING REQUIRED): APPROXIMATELY £50,000-120,000+ (FULL MANSARD CONVERSION — TOTAL STRUCTURAL AND ROOFING WORKS ONLY — MAJOR OPERATION — HIGHLY VARIABLE WITH PROPERTY SIZE AND SPECIFICATION).