Balcony and Roof Terrace Construction in London: Structural, Waterproofing, Planning, and Material Guide

PLANNING PERMISSION IS REQUIRED FOR ALMOST ALL BALCONIES AND ROOF TERRACES IN LONDON — THIS IS ONE OF THE MOST FREQUENTLY MISUNDERSTOOD PLANNING RULES IN LONDON RESIDENTIAL RENOVATION.

WHY BALCONIES REQUIRE PLANNING PERMISSION: UNDER THE TOWN AND COUNTRY PLANNING (GENERAL PERMITTED DEVELOPMENT) ORDER 2015 (AS AMENDED), ADDING A BALCONY TO A HOUSE — INCLUDING A JULIETTE BALCONY WITH A FULL DOOR OPENING — IS EXPRESSLY NOT PERMITTED DEVELOPMENT. THE GPDO SPECIFICALLY EXCLUDES FROM PERMITTED DEVELOPMENT RIGHTS ANY ADDITION OF A BALCONY, VERANDAH, OR RAISED PLATFORM. THIS MEANS THAT: A FULL BALCONY ON AN EXTENSION OR LOFT DORMER REQUIRES A HOUSEHOLDER PLANNING APPLICATION. A JULIETTE BALCONY (FRENCH DOORS OPENING TO A RAILING WITHOUT A PROJECTING FLOOR PLATFORM) ALSO REQUIRES PLANNING PERMISSION IN MOST LONDON BOROUGHS.

ROOF TERRACES: A ROOF TERRACE — WHERE AN EXISTING ROOF IS MADE ACCESSIBLE BY ADDING STEPS AND A BALUSTRADE — ALSO REQUIRES PLANNING PERMISSION. THIS IS BECAUSE A ROOF TERRACE CHANGES THE USE OF THE ROOF FROM AN INACCESSIBLE STRUCTURE TO AN ACCESSIBLE AMENITY SPACE, AND BECAUSE THE OVERLOOKING OF ADJACENT PROPERTIES THAT RESULTS IS A MATERIAL PLANNING CONSIDERATION.

PLANNING CONSIDERATIONS FOR LONDON BALCONY AND ROOF TERRACE APPLICATIONS: OVERLOOKING AND LOSS OF PRIVACY: THE PRIMARY PLANNING OBJECTION TO BALCONIES AND ROOF TERRACES IN LONDON. BALCONIES AND ROOF TERRACES ON REAR ELEVATIONS OVERLOOK ADJOINING GARDENS AND REAR ROOMS. LONDON BOROUGH PLANNERS WILL ASSESS THE DEGREE OF OVERLOOKING AGAINST THE SEPARATION DISTANCES BETWEEN THE BALCONY AND NEIGHBOURING HABITABLE ROOMS AND GARDENS. TYPICAL SEPARATION DISTANCE EXPECTATION IN LONDON: 12 TO 21 METRES FROM A BALCONY TO THE REAR WINDOWS OF THE NEIGHBOURING PROPERTY. IN DENSELY BUILT INNER LONDON TERRACES, THIS SEPARATION IS RARELY ACHIEVABLE — MAKING REAR BALCONIES AND ROOF TERRACES HARD TO OBTAIN PLANNING PERMISSION FOR IN AREAS SUCH AS ISLINGTON, HACKNEY, SOUTHWARK, AND LAMBETH.

WHERE PLANNING PERMISSION IS OBTAINED (OR WHERE THE TERRACE IS ON A FLAT ROOF OF AN EXTENSION AND STRUCTURAL WORKS ARE PART OF A NEW EXTENSION BUILDING REGULATIONS APPLICATION), THE STRUCTURAL REQUIREMENTS FOR A ROOF TERRACE IN A LONDON CONTEXT ARE SIGNIFICANT.

DESIGN LOADING FOR ROOF TERRACES: UNDER BS EN 1991-1-1 (EUROCODE 1 — ACTIONS ON STRUCTURES), ACCESSIBLE ROOF TERRACES ARE CLASSIFIED AS IMPOSED LOAD CATEGORY C1 (AREAS WITH TABLES — RESTAURANTS, CAFES) OR A (DOMESTIC) DEPENDING ON USE. FOR A PRIVATE RESIDENTIAL ROOF TERRACE: IMPOSED LOAD: 1.5 KN/M² (MINIMUM, DOMESTIC). FOR A ROOF TERRACE ALSO USED FOR PLANTERS, FURNITURE, AND GATHERINGS: 2.0 TO 3.0 KN/M² SHOULD BE CONSIDERED. THE EXISTING FLAT ROOF OF A LONDON VICTORIAN TERRACE EXTENSION (TYPICALLY DESIGNED ONLY FOR MAINTENANCE ACCESS LOADING OF 0.75 KN/M²) IS VERY UNLIKELY TO BE STRUCTURALLY ADEQUATE FOR TERRACE USE WITHOUT SIGNIFICANT STRUCTURAL UPGRADE. A STRUCTURAL ENGINEER MUST ASSESS AND DESIGN THE ROOF TERRACE STRUCTURE.

TYPICAL STRUCTURAL UPGRADE FOR A LONDON FLAT ROOF TERRACE CONVERSION: WHERE AN EXISTING FLAT ROOF IS TO BECOME A ROOF TERRACE, THE FOLLOWING STRUCTURAL UPGRADES ARE TYPICAL: REPLACE EXISTING ROOF JOISTS WITH LARGER SECTION (E.G. REPLACE 100MM X 50MM AT 400MM CENTRES WITH 150MM X 47MM C24 AT 400MM CENTRES OR 200MM X 47MM C24 — CONFIRMED BY STRUCTURAL ENGINEER). ADD NOGGINGS BETWEEN JOISTS. FIX 22MM STRUCTURAL PLYWOOD DECK (T&G — POSI-JOIST OR EQUIVALENT STRUCTURAL DECK). REVIEW SUPPORT WALLS FOR INCREASED LOAD PATH. STRUCTURAL COSTS: £5,000 TO £15,000 DEPENDING ON EXISTING STRUCTURE AND REQUIRED UPGRADE.

THE WATERPROOFING SYSTEM UNDER A ROOF TERRACE MUST WITHSTAND FOOT TRAFFIC, PLANTER LOADS, UV EXPOSURE (IF NOT FULLY COVERED BY DECKING), AND THE CONSTANT MOISTURE ENVIRONMENT OF AN OUTDOOR DECK IN LONDON'S CLIMATE.

IBC (INFORMATION ON A MASTIC ASPHALT SYSTEM — COMMON IN LONDON MANSION BLOCK AND PERIOD FLAT ROOF TERRACE WATERPROOFING): MASTIC ASPHALT LAID AT 3-COAT THICKNESS TO BS 988 (20MM MINIMUM) IS A TRADITIONAL UK FLAT ROOF TERRACE WATERPROOFING MATERIAL — DURABLE, SEAMLESS, AND WELL PROVEN IN LONDON RESIDENTIAL APPLICATIONS. HOWEVER, MASTIC ASPHALT IS THERMOPLASTIC — IT SOFTENS IN VERY HOT WEATHER (RARE IN LONDON BUT NOT IMPOSSIBLE) AND IS NOT COMPATIBLE WITH DIRECT FOOT TRAFFIC WITHOUT A PROTECTION LAYER (PAVING OR DECKING OVER).

SARNAFIL G410 PVC SINGLE-PLY (THE RECOMMENDED MODERN SPECIFICATION FOR LONDON ROOF TERRACES): SARNAFIL G410 1.5MM HOT-AIR WELDED SINGLE-PLY PVC MEMBRANE, MECHANICALLY FIXED OR FULLY ADHERED ON INSULATED DECK, IS THE LEADING SPECIFICATION FOR LONDON ROOF TERRACE WATERPROOFING. SARNAFIL IS UV-STABLE, CHEMICALLY RESISTANT TO PLANTER SOIL AND WATER, AND CAN ACCEPT PEDESTAL-MOUNTED DECKING DIRECTLY ON THE MEMBRANE SURFACE (SARNAFIL'S WARRANTY IS MAINTAINED WITH PEDESTAL INSTALLATION). SARNAFIL G410 WITH SARNAPAD PROTECTION LAYER (WHERE DIRECT FOOT TRAFFIC IS EXPECTED WITHOUT DECKING — E.G. TILED TERRACE) PROVIDES A FULLY PROTECTED MEMBRANE SYSTEM.

FIRESTONE EPDM: FIRESTONE EPDM 1.5MM IS APPROPRIATE FOR SMALLER LONDON ROOF TERRACES (UP TO 30 M²) WHERE THE SINGLE-PIECE SEAM-FREE ADVANTAGE IS PARTICULARLY VALUABLE. FIRESTONE RUBBERCOVER WALKWAY PADS (BONDED ANTI-SLIP EPDM STRIPS) ALLOW DIRECT FOOT TRAFFIC ON THE MEMBRANE IN MAINTENANCE ZONES.

THE SURFACE FINISH OF A LONDON ROOF TERRACE IS THE DESIGN ELEMENT THAT MOST AFFECTS THE LOOK AND FEEL OF THE SPACE.

COMPOSITE DECKING ON PEDESTALS (THE MOST POPULAR CHOICE FOR LONDON ROOF TERRACES): COMPOSITE DECKING (WOOD-PLASTIC COMPOSITE WPC — COMPOSITE PRIME, TREX TRANSCEND, MILLBOARD WEATHERED OAK) ON ADJUSTABLE HEIGHT PEDESTALS IS THE MOST WIDELY INSTALLED ROOF TERRACE SURFACE IN LONDON. THE PEDESTAL SYSTEM (BUZON OR ETERNO IVICA ADJUSTABLE HEIGHT PEDESTALS IN HEIGHTS FROM 15MM TO 600MM) ALLOWS THE DECK TO BE LEVELLED ACROSS AN UNEVEN ROOF AND RAISED TO MATCH INTERNAL FLOOR LEVELS AT DOOR THRESHOLDS. COMPOSITE DECKING ADVANTAGES: LOW MAINTENANCE (NO OILING, NO PAINTING, RESISTANT TO MOISTURE AND UV). AVAILABLE IN HARDWOOD-EFFECT COLOURS (TEAK, CHARCOAL, GREY-BROWN). 25 TO 30-YEAR PRODUCT WARRANTIES FROM LEADING BRANDS.

PORCELAIN PAVING ON PEDESTALS (THE PREMIUM CHOICE FOR HIGH-END LONDON ROOF TERRACES): LARGE FORMAT PORCELAIN TILES (600MM X 600MM, 900MM X 900MM, 1200MM X 600MM — MINIMUM 20MM THICKNESS FOR OUTDOOR PEDESTALLED USE — KANDLA GREY, PORCELANOSA PRADA MARFIL, OR ATLAS CONCORDE BOOST) ON ADJUSTABLE PEDESTALS CREATE A LUXURIOUS, EASY-CLEAN ROOF TERRACE SURFACE. THE MINIMUM 20MM TILE THICKNESS IS REQUIRED FOR OUTDOOR TILE USE ON PEDESTALS — STANDARD 10-12MM INTERNAL TILES WILL CRACK UNDER FOOT TRAFFIC AND THERMAL MOVEMENT. BUZON PEDESTAL SPACER TABS PROVIDE SELF-DRAINING JOINTS BETWEEN TILE PANELS.

HARDWOOD TIMBER DECKING ON PEDESTALS: BANGKIRAI (YELLOW BALAU), CUMARU, OR GARAPA HARDWOOD DECKING (25MM THICK X 90MM OR 140MM WIDE) ON PEDESTALS PROVIDES A NATURAL, WARM AESTHETIC FOR LONDON ROOF TERRACES — PARTICULARLY POPULAR FOR OLDER LONDON PERIOD CONVERSIONS WHERE A NATURAL MATERIAL IS PREFERRED. REQUIRES OILING ANNUALLY TO MAINTAIN COLOUR. TIMBER SUPPLIER: TIMBER HOLDINGS LONDON, ARBOR FOREST PRODUCTS.

A BALUSTRADE (GUARDRAIL) IS REQUIRED BY BUILDING REGULATIONS (APPROVED DOCUMENT K — PROTECTION FROM FALLING) AT ALL EDGES OF A ROOF TERRACE OR BALCONY WHERE THERE IS A DROP OF MORE THAN 600MM.

BUILDING REGULATIONS REQUIREMENTS FOR BALUSTRADES (APPROVED DOCUMENT K): MINIMUM HEIGHT: 1100MM FOR ROOF TERRACES (BALCONIES AND ROOF TERRACES WITH A DROP OF MORE THAN 600MM — HIGHER THAN THE 900MM REQUIREMENT FOR INTERNAL STAIRS). THE BALUSTRADE MUST BE CAPABLE OF RESISTING A HORIZONTAL FORCE OF 740 N/M (UNIFORMLY DISTRIBUTED). CHILD SAFETY: THE BALUSTRADE MUST NOT HAVE HORIZONTAL RAILS OR OPENINGS THAT COULD BE CLIMBED BY YOUNG CHILDREN — MAXIMUM 100MM OPENING IN ANY DIRECTION.

GLASS BALUSTRADE (THE PREMIUM CHOICE FOR LONDON ROOF TERRACES AND BALCONIES): FRAMELESS TOUGHENED GLASS BALUSTRADE (12MM OR 15MM FULLY TOUGHENED LAMINATED SAFETY GLASS, FIXED IN STAINLESS STEEL BASE SHOES OR A CONTINUOUS BASE CHANNEL) IS THE MOST WIDELY SPECIFIED BALUSTRADE FOR HIGH-END LONDON ROOF TERRACES. GLASS BALUSTRADES PRESERVE VIEWS AND NATURAL LIGHT. KEY SPECIFICATION: FULLY TOUGHENED LAMINATED SAFETY GLASS (PVB INTERLAYER — SO THAT IF THE GLASS BREAKS, THE LAMINATED PANELS HOLD TOGETHER RATHER THAN FRAGMENTING). STAINLESS STEEL BASE CHANNELS: GRADE 316 STAINLESS FOR COASTAL OR POLLUTED URBAN ENVIRONMENTS (LONDON'S AIR QUALITY JUSTIFIES 316 GRADE). COST: £350 TO £750 PER LINEAR METRE SUPPLY AND INSTALL.

SQUARE-SECTION STEEL BALUSTRADE (MID-RANGE, POWDER-COATED): 50MM X 50MM SQUARE SECTION STEEL POST AND RAIL BALUSTRADE IN ANTHRACITE GREY POWDER COAT (RAL 7016) WITH HORIZONTAL OR VERTICAL INFILL BARS. COST: £200 TO £400 PER LINEAR METRE. COMPOSITE CABLE BALUSTRADE: STAINLESS CABLE INFILL ON POWDER-COATED STEEL POST SYSTEM — OPEN, CONTEMPORARY LOOK; £250 TO £500 PER LINEAR METRE.

ROOF TERRACE AND BALCONY COSTS IN LONDON VARY WIDELY BASED ON SIZE, STRUCTURAL UPGRADE REQUIRED, WATERPROOFING SPECIFICATION, AND SURFACE FINISH.

SMALL BALCONY ON DORMER (NO NEW STRUCTURAL WORKS — CANTILEVERED STEEL FROM EXISTING STRUCTURE, SARNAFIL WATERPROOFING, COMPOSITE DECKING, GLASS BALUSTRADE — 3M X 1M): £8,000 TO £18,000 INCLUSIVE OF PLANNING COSTS.

ROOF TERRACE ON EXISTING FLAT ROOF EXTENSION (STRUCTURAL UPGRADE, SARNAFIL WATERPROOFING, COMPOSITE DECKING ON PEDESTALS, GLASS BALUSTRADE — 20 M²): STRUCTURAL UPGRADE: £5,000 TO £10,000. WATERPROOFING (SARNAFIL G410 + PROTECTION LAYER): £2,000 TO £4,000. COMPOSITE DECKING ON PEDESTALS: £3,000 TO £6,000. GLASS BALUSTRADE (12M PERIMETER): £4,200 TO £9,000. PLANNING APPLICATION: £600 TO £1,500. TOTAL: £14,800 TO £30,500.

PREMIUM ROOF TERRACE (PORCELAIN PAVING ON PEDESTALS, FRAMELESS GLASS BALUSTRADE, BUILT-IN PLANTERS, LIGHTING): ADD £5,000 TO £15,000 TO THE ABOVE.

PLANNING SUCCESS RATES FOR LONDON BALCONIES AND ROOF TERRACES: HIGHLY VARIABLE BY BOROUGH AND SITE — BROADLY 40% TO 70% SUCCESS RATE FOR THOUGHTFULLY DESIGNED APPLICATIONS IN LONDON (HIGHER IN OUTER LONDON BOROUGHS WITH LARGER REAR GARDENS; LOWER IN INNER LONDON DENSE TERRACE CONTEXTS). ENGAGING A PLANNING CONSULTANT AND PREPARING A DETAILED DAYLIGHT AND PRIVACY ASSESSMENT SIGNIFICANTLY IMPROVES SUCCESS RATES.