Single-Storey Rear Extension Costs in London: Full Breakdown from Foundations to Finished Room

The choice between a flat roof and a pitched roof for a single-storey rear extension affects both the cost and the visual appearance of the extension, and may be influenced by the LPA's design guidance.

Flat roof: A flat roof (technically a low-pitch roof, typically at 1:80 or 1:40 falls, finished with EPDM rubber membrane, mastic asphalt, or GRP fibreglass) is the more common and generally less expensive option for a single-storey rear extension in London. Advantages: allows for a higher internal ceiling height at the rear wall (since there is no pitched roof slope to compromise headroom); allows for the installation of a frameless flat rooflight or lantern directly over the plan; simpler to detail the junction with the existing rear wall; lower cost than an equivalent pitched roof. Disadvantages: some London boroughs' design guides express a preference for pitched roofs to match the host dwelling in conservation areas; flat roofs have a shorter effective lifespan than pitched roofs (typically 20–30 years for EPDM before major refurbishment is required, vs 30–50+ years for a tiled pitched roof). Typical cost of a flat roof (EPDM membrane, vapour-permeable insulation to achieve Part L compliance, OSB deck, softwood or steel roof structure): £4,000–£10,000 for a 15–25m² extension footprint, depending on specification and insulation standard.

Pitched roof: A pitched roof (typically at 30–45° to match the main house or at a shallower pitch if a mono-pitch or cat-slide profile is used) adds cost, complexity, and height to the extension but may be required by the LPA in specific locations. Advantages: longer lifespan; matches the main house appearance in some design contexts; can accommodate roof windows or small dormers. Disadvantages: higher cost (tile, timber roof structure, and increased wall height add approximately £3,000–£8,000 compared with a flat roof of the same footprint); reduces internal ceiling height at the lower end of the pitch (particularly on a mono-pitch slope draining towards the garden boundary); more complex junction with the existing rear wall; limits the ability to install a flat rooflight or lantern (a pitched roof requires a pitched rooflight or dormer instead). Typical additional cost of a pitched roof compared with flat roof: £4,000–£8,000 for a standard 15–25m² extension.

A single-storey rear extension typically involves modifications to the existing drainage and services, especially where the extension incorporates a new or relocated kitchen.

Surface water drainage: The flat or pitched roof of the extension must be drained. On a flat roof, the standard arrangement is a syphonic outlet (a concealed roof drain fitted into the flat roof deck, connected to a downpipe running vertically from the roof to a drain at ground level). For a larger flat roof, two outlets at opposite ends are often specified. The drain connection must comply with Part H of the Building Regulations (drainage). If the extension roof is over the line of an existing surface water drain, the drain may need to be diverted or bridged with an engineered foundation.

Foul water drainage: Where the extension involves relocating the kitchen sink or adding a new WC, the existing foul water drainage connection must be extended or modified. The foul drain from the existing kitchen typically runs under the existing floor slab to a gully in the rear yard, then connects to the main foul water sewer. Extending the kitchen into a new extension typically means extending the drain beneath the new slab to serve the new sink position and (if relevant) any new dishwasher or washing machine connection. A party wall surveyor may specify the drainage arrangement as part of the party wall award if the extension works are near the party wall drain. Thames Water build-over approval is required if the extension is constructed over or within 3m of a public sewer.

First-fix plumbing: Kitchen extension first-fix plumbing scope typically includes: capping off the existing gas and water supply at the existing kitchen position; extending gas supply and water supply pipes (hot and cold) to the new kitchen position in the extension; extending the waste connection from the new sink position to the existing foul drain run; providing under-floor heating manifold connections if UFH is specified. Budget £1,500–£4,000 for first-fix plumbing in a standard kitchen extension.

First-fix and second-fix electrics: Extension first-fix electrics includes: running new cables from the existing consumer unit (or from a new sub-board in the extension) to all new socket, switch, and lighting positions; installing any additional circuits required (typically a 20A ring main for the extension plus a dedicated circuit for any electric oven, hob, or cooker hood, plus a circuit for underfloor heating if specified). Budget £1,500–£4,000 for first-fix electrics in a 15–25m² extension.

After the structural shell is complete (foundations, walls, roof, glazing, drainage), the internal fit-out works commence. The cost of these works depends heavily on specification.

Plastering and internal partitions: The inner leaf of the extension cavity wall and the party walls typically receive a sand-and-cement render coat followed by a finish plaster skim. Any new internal partition walls (for example, a boot room partition or utility room) are built from timber stud and boarded with plasterboard, taped, jointed, and skimmed. Budget £2,000–£5,000 for internal plastering in a 15–25m² extension.

Flooring: The floor slab in the extension (typically a reinforced concrete ground-bearing slab, insulated to meet Part L with 100mm+ rigid insulation below the slab) receives a floor finish. For an open-plan kitchen-diner extension, a single floor finish running from the existing house through the extension creates visual continuity. Common options: porcelain tile (£3,000–£8,000 supply and fix for 15–25m² depending on tile spec and any underfloor heating screed); engineered oak (£4,000–£9,000 for 15–25m² depending on grade, thickness, and finish); polished concrete (£5,000–£12,000+ for a bespoke polished concrete floor). For budget builds: LVT (luxury vinyl tile) or similar resilient flooring (£1,500–£4,000 for 15–25m²).

Kitchen fit-out: Kitchen fit-out is typically the largest single cost item in a kitchen extension project. Budget ranges: entry-level fitted kitchen (B&Q Magnet or similar, standard appliances): £5,000–£12,000 supply and fit; mid-range kitchen (Howdens, Wren, or equivalent, integrated appliances): £10,000–£25,000 supply and fit; bespoke/high-specification kitchen (John Lewis of Hungerford, Tom Howley, DeVOL, or custom joinery): £20,000–£60,000+.

Total cost summary for a typical London single-storey rear kitchen extension (15–25m², flat roof, aluminium bi-folds, standard specification, excluding kitchen fit-out): Demolition and enabling: £2,000–£5,000 Foundations: £4,000–£8,000 External walls: £5,000–£10,000 Roof structure and membrane: £5,000–£10,000 Structural steel opening: £4,000–£8,000 Glazing (bi-folds + rooflight): £8,000–£15,000 Drainage and plumbing: £2,000–£5,000 Electrics: £2,000–£4,000 Plastering: £2,000–£4,500 Flooring (tile or engineered oak): £3,000–£8,000 Decoration: £1,500–£3,000 Prelims and site management: £3,000–£7,000 Total build cost (exc. kitchen, professional fees, planning): £41,500–£87,500 + VAT