External cladding cost by material
External cladding cost in London (supply and fix, including breather membrane and battening, excluding scaffold): Timber — Siberian larch (featheredge or open-joint horizontal): £70–£110/m². Timber — Western red cedar (horizontal shiplap or board-on-board): £80–£130/m². Timber — Douglas fir (vertical board-on-board): £65–£100/m². Accoya modified timber: £90–£140/m². Fibre-cement — Hardie Panel (horizontal planks): £55–£80/m². Fibre-cement — Equitone Linea or Natura (large format panel): £80–£130/m². Pre-weathered zinc (corrugated or standing-seam): £120–£200/m². Powder-coated aluminium panel (Trespa, ALPOLIC): £100–£180/m². Typical project costs: single-storey rear extension side elevation (20m², timber larch): £1,500–£2,500. Double-storey side extension (40m², fibre-cement): £2,500–£4,500. Contemporary extension zinc rainscreen (30m²): £4,500–£7,000.
Timber cladding — species, finish and lifespan
Timber cladding is the most popular cladding material for London house extensions. Species comparison: Siberian larch: dense, resinous timber with natural durability (Class 3–4); weathers to silver-grey without treatment; with UV-stabilising oil treatment, the original pale honey colour can be maintained. Lifespan: 30–40 years unfinished, 40+ years with oil treatment. Western red cedar: lightweight, dimensionally stable, natural extractive oils provide durability (Class 3); characteristic reddish-brown colour weathers to grey. Lifespan: 25–35 years. Accoya: best-performing modified timber — Durability Class 1 above ground, dimensionally stable, 50-year guarantee against rot. Profiles: board-on-board, shiplap, open-joint vertical, featheredge. All timber cladding must be installed on a ventilated batten system with a 50mm clear cavity behind the cladding and a breather membrane on the structural substrate.
Planning and Building Regulations for external cladding
Planning considerations for external cladding in London: Non-conservation area, non-listed: cladding an extension is permitted development in most cases. Cladding the main house front elevation may require planning permission — check with the planning department. Conservation areas: cladding an extension visible from the highway usually requires planning permission and careful design — timber cladding on rear and side extensions is well-received by conservation officers. Front elevations in conservation areas: cladding the front elevation of a house almost always requires planning permission and may be refused. Building Regulations (Part B — fire safety): combustible cladding on buildings above 18m height is prohibited. For residential houses (well below 18m), all standard cladding materials are permissible. Cavities behind cladding must have cavity barriers at floor levels and around openings.
Cladding maintenance and fire safety
Timber cladding maintenance: unfinished larch or cedar — no maintenance required for silver-grey weathering; brush off algae growth on north-facing elevations every 3–5 years. Oil-treated timber: re-oil every 2–3 years on exposed elevations. Opaque stain-finished timber: restain every 5–8 years. Fibre-cement: virtually maintenance-free — factory paint finish guaranteed 15–25 years. Zinc (pre-weathered): no maintenance required — zinc develops a patina that provides permanent protection. Aluminium composite panels: wipe-clean; powder coat finish guaranteed 10–20 years. Fire safety of timber cladding: untreated softwood and hardwood cladding has a Euroclass D–s2,d0 reaction to fire rating — acceptable for residential buildings under 11m. For buildings between 11m and 18m, timber treated with fire-retardant (Thermoguard, Dricon) can achieve Class B — satisfying Part B requirements for this height band.
