A roof terrace converts an existing or newly constructed flat roof into accessible outdoor amenity space. In London, this is most commonly proposed on top of a rear kitchen extension, a lower-ground addition, or a garage conversion. The concept is simple and appealing, but the planning and structural requirements are often more complex than homeowners expect.
Planning permission: A flat roof in itself does not usually require planning permission. But creating an accessible terrace on a flat roof almost always does. The planning concern is primarily overlooking — a terrace at first-floor or higher level creates a raised vantage point from which the occupants can see into neighbouring gardens and habitable rooms that were previously private. Most London boroughs take a cautious approach to roof terraces adjoining residential neighbours, and refusal rates for roof terrace applications are significantly higher than for extensions. Key planning factors are whether the terrace is visible from the street, whether it creates new overlooking into neighbouring gardens or windows, the height and design of any proposed balustrade or privacy screen, and the borough's specific design policies on roof extensions.
Structural requirements: An existing flat roof built for non-accessible use (maintenance access only) is typically designed to carry a load of around 0.6 kN per square metre. An accessible terrace must carry a live load of 1.5 kN per square metre minimum, plus the weight of the decking, balustrade, and any planters or furniture. In most cases, the existing floor structure must be assessed by a structural engineer and may need to be upgraded before terrace use is permitted. This is particularly important for extensions built with lightweight timber joists, which may need to be replaced with a heavier-duty floor structure or supplemented with additional structure.
Waterproofing: An accessible flat roof requires a higher-specification and more robust waterproofing system than a non-accessible roof. Single-ply membrane systems from brands such as Sika Sarnafil or Firestone are appropriate, installed over a suitable insulation layer with a decking system on top. The waterproofing must be designed to accommodate foot traffic and the point loads from decking supports and furniture.
Cost: A straightforward roof terrace on an existing extension, assuming the structure is adequate and planning is granted, costs eight thousand to twenty-five thousand pounds depending on size, balustrade specification, and decking material. Where structural upgrading is required, add three thousand to ten thousand pounds for the structural works. Planning professional fees for a contentious application add one thousand five hundred to three thousand pounds.
RCB advises clients on roof terrace feasibility during the pre-construction consultation and coordinates planning, structural, and waterproofing specialists on terrace projects across London. 07359 872594 | contact@rcbgroup.co.uk | www.rcbgroup.co.uk