Timber Cladding for London Home Extensions: Species, Maintenance and Planning

Western Red Cedar (Thuja plicata): The classic timber cladding species for contemporary residential architecture. Naturally durable (Durability Class 2-3) — oils in the wood resist decay without treatment. Weathers to a silver-grey patina within 1-2 years if left untreated. Can be finished with a clear UV-stabilised oil to slow the greying and retain the warm red-brown colour, but this requires re-application every 1-3 years. Cedar is lightweight and stable dimensionally — it does not move significantly with moisture content changes, making it suitable for wider boards. Sourced from North America and Canada — availability can fluctuate; ensure FSC-certified supply. Siberian Larch (Larix sibirica): Dense, resinous softwood with good natural durability (Durability Class 3-4). Widely used in Scandinavian architecture. Colours range from pale yellow-brown when fresh to a silver-grey patina when weathered. On south-facing elevations, an annual or biennial application of a wood oil improves durability and slows surface checking (small surface cracks from drying). Widely available in the UK from Scandinavian and Baltic suppliers. Generally less expensive than cedar. Accoya (Acetylated Radiata Pine): A thermally and chemically modified softwood — radiata pine that has undergone an acetylation process to alter the cellular structure of the wood, dramatically improving durability (rated Durability Class 1 — equivalent to the most durable tropical hardwoods) and dimensional stability. Suitable for ground contact and fully exposed applications. Does not require preservative treatment. More expensive than cedar or larch but offers lower lifetime maintenance cost. Available pre-finished or left to weather. Suitable for both horizontal and vertical board profiles. Kebony (Modified Scots Pine or Radiata Pine): Another modified softwood, using a process that impregnates the wood cell structure with furfuryl alcohol to harden it (Durability Class 1). Similar performance to Accoya. Has a warmer brown colour than Accoya when new and weathers to a silver-grey. Manufactured in Norway using sustainably sourced FSC-certified timber.

Timber cladding can be installed in several profiles, each creating a different aesthetic: Featheredge (bevel) boarding: Boards that taper from thick at the bottom edge to thin at the top, overlapping each other. The classic traditional cladding profile used in barn and agricultural buildings. Creates a horizontal shadow line that emphasises the horizontal. Shiplap (rebated): Boards with a rebated profile that interlock — the lower board's top edge sits in a rebate on the underside of the upper board. Provides a slightly flatter appearance than featheredge with a tighter shadow line. Square-edged (open-jointed or closed): Square-edged boards installed vertically or horizontally with either open joints (a gap between boards allowing ventilation) or close-butted joints. Open-jointed systems rely entirely on the cladding board's natural durability rather than a lap joint to prevent water ingress. Board-on-board: Vertical boards installed in an alternating pattern, with narrower cover boards fixed over the joints between the main boards. Robust and ventilated at the joints. All timber cladding systems must be installed over a ventilated cavity — typically a 25-50mm clear air gap behind the cladding boards, maintained by vertical timber battens or cavity closers at the head and foot of the cladding. The ventilated cavity allows moisture that penetrates the cladding face to drain to the base and evaporate without reaching the structural wall behind. The cavity must be closed at the top and bottom with a vermin-proof mesh or equivalent to prevent birds and insects from nesting within it.

For most single-storey rear extensions in London, the choice of cladding material on the external walls does not require specific planning permission — the extension itself (if within permitted development limits) is approved as-is, and material choice is a matter for Building Regulations and the homeowner. However, planning considerations arise in the following situations: Conservation areas: Where an extension requires planning permission (either because it exceeds PD limits or because PD rights have been removed in that conservation area), the materials will be assessed for acceptability in the conservation area context. Timber cladding — particularly untreated weathering timber or charred timber — is sometimes resisted by conservation officers in traditional red-brick Victorian conservation areas on the grounds that it is incongruous with the local character. However, many conservation areas in inner London accept contemporary contrasting materials on rear extensions that are not visible from public views. Check with the conservation officer before finalising the material specification. Listed buildings: Any external work on a listed building, including cladding a rear extension, requires Listed Building Consent. The choice of materials is subject to assessment by the conservation officer. Article 2(3) land — other designated areas: Some AONB and National Park authorities may have specific guidance on the acceptability of timber cladding in the local context.

Building Regulations Part B (Fire Safety) contains requirements for the fire performance of external wall materials. The main restriction for residential buildings is that materials used on external walls above 11 metres above ground level must meet the reaction-to-fire classification A2-s1, d0 or better (limited combustibility). Timber, as a combustible material, does not meet this classification. For most London home extensions — single-storey or two-storey rear extensions — the 11-metre height threshold is not approached and Part B does not restrict the use of timber cladding. However, where the extension is part of a building that is, or will become after the extension, 11 metres or more in height, the Part B requirement for A2-s1, d0 classification or better applies to the upper storeys and the restriction on combustible cladding must be addressed. For standard London terraced houses, this is rarely a constraint on cladding choice for a rear extension. Charred timber (shou sugi ban): The charred layer on the surface of yakisugi timber provides some degree of resistance to fire spread along the surface — the char layer self-extinguishes rather than continuing to burn. However, the structural timber beneath the char layer remains combustible, and charred timber should not be assumed to have improved Part B compliance without specific fire performance data from the supplier. For standard 2-storey domestic extensions where the 11-metre threshold does not apply, charred timber cladding is used without specific fire performance concerns.