A roof terrace above a flat-roofed extension — particularly at first floor level above a kitchen extension — is one of the most sought-after additions to a UK home. It transforms what would otherwise be a dead, maintenance-requiring flat roof into usable outdoor amenity space. It is also one of the more technically demanding and planning-sensitive projects, and one where cutting corners in waterproofing causes catastrophic and expensive failures.
Planning Permission
Unlike simply adding a flat roof extension, creating a roof terrace almost always requires planning permission, even where the extension itself was permitted development. The reason: a terrace creates overlooking opportunities for neighbouring properties. Local planning authorities take this seriously — permitted development rights explicitly exclude the creation of roof terraces by converting flat roofs. You must apply for full planning permission.
Planning considerations include: privacy screens or raised balustrades that prevent direct overlooking; the visual impact of any parapet or railing; and whether the terrace is on a rear elevation (much more likely to be permitted) or a side elevation (very unlikely in most residential settings).
Structural Requirements
A standard flat roof extension is designed for roof loading only — typically 0.75 kN/m² for maintenance access and snow load. A usable terrace with furniture and occupants requires a minimum of 1.5 kN/m² imposed load, and up to 3.0 kN/m² for higher-specification use. The structural engineer must confirm the slab, beams, and supporting walls can handle the increased load — in many cases, the roof build-up requires upgrading.
Balustrades must comply with Part K of Building Regulations: minimum 1.1m high (for terraces above ground level), structurally robust enough to withstand 0.74 kN/m imposed load horizontally, and with no opening large enough to permit a 100mm sphere to pass through.
Waterproofing for Occupied Terraces
A walkable roof terrace requires a waterproofing system rated for pedestrian traffic — typically a hot-melt rubberised system (Bauder, Sika Trocal), a single-ply system with a protection layer, or a liquid-applied system with a specified traffickable topcoat. Standard EPDM or felt is not appropriate for occupied terraces. The waterproofing system must be covered with a paving support system (pedestals and slabs) or decking, with adequate drainage beneath and at perimeter upstands.
Costs
| Element | Typical cost |
|---|---|
| Planning application | £250–£500 |
| Structural engineer's input | £600–£1,500 |
| Traffickable waterproofing (20m²) | £3,000–£8,000 |
| Balustrade (20m²) | £3,000–£8,000 |
| Paving on pedestals (20m²) | £2,000–£5,000 |