External Cladding for House Extensions and Renovations: Types, Costs, and Planning Rules

**Why timber cladding is popular for London extensions**:

Timber cladding became the dominant contemporary extension cladding choice in London through the 2010s and remains very popular. The reasons are: warm natural appearance that reads as a deliberate contemporary design choice rather than a brick or render imitation; adaptability to a wide range of profiles and installation patterns; lower cost than zinc or Corten; and a planning position that is generally more accepted than metallic claddings in many planning contexts.

**Timber cladding profiles and species**:

*Western Red Cedar*: The most commonly specified timber cladding for UK residential projects. Naturally durable (Class 3, no preservative treatment required for external use); attractive grain; weathers to a silver-grey if left untreated (many clients prefer the silver-grey natural finish). Available in a range of profiles: horizontal featherboard, square-edge, ship-lap, shiplap shadow-gap. *Cost*: £30–£55/m² (materials only); £55–£90/m² installed. *Lifespan*: 25–40 years if correctly installed with a ventilated cavity behind; may require treatment at 10–15 years if the original treatment (where applied) needs refreshing.

*Accoya modified wood*: Acetylated (chemically modified) softwood — dimensionally stable, Class 1 durability equivalent (can be used in contact with the ground), very low movement with moisture changes. Preferred where long-term stability and minimal maintenance are priorities. More expensive than cedar. *Cost*: £50–£90/m² (materials); £85–£130/m² installed. *Lifespan*: 50+ years claimed.

*Siberian Larch*: A dense, resinous softwood with good natural durability (Class 3). Available pre-treated or natural. The characteristic even grain gives a clean, contemporary appearance. Widely used as a lower-cost alternative to cedar. *Cost*: £20–£40/m² (materials); £45–£75/m² installed. *Lifespan*: 20–35 years depending on treatment regime and exposure.

*Charred timber (Shou Sugi Ban / yakisugi)*: Timber (typically cedar or Douglas fir) that has been surface-charred with a gas burner, then wire-brushed to remove ash. The charring carbonises the surface, creating a layer that repels moisture, UV, and insects. Very striking black appearance — distinctive but divisive. Increasingly popular on contemporary London extension projects. *Cost*: £60–£120/m² (materials); £95–£150/m² installed.

**Installation requirements for all timber cladding**:

• •**Ventilated cavity**: Timber cladding must be installed with a minimum 38mm ventilated air gap behind it. The cavity allows moisture to drain and air to circulate, preventing timber decay at the back face of the boards. Without a ventilated cavity, even naturally durable timber will decay within 5–10 years.
• •**Treated battens**: The battens (vertical or horizontal depending on cladding direction) that support the cladding boards must be pressure-treated to Class 4 (ground contact level) — the batten is in a more exposed and moisture-retaining position than the cladding itself.
• •**Stainless steel or hot-dip galvanised fixings**: Fixings that rust will stain the timber and corrode — use A4 stainless steel screws or ring-shank nails for all timber cladding.
• •**End-grain sealing**: End grain (top and bottom of cladding boards, cut ends) absorbs moisture more readily than the face grain — seal all cut ends with an end-grain stabiliser before installation.

**Zinc (VM Zinc, Rheinzink, Umicore)**:

*Characteristics*: Zinc is a fully recyclable, long-lasting, naturally weathering metal. Installed as either standing seam panels (the most common domestic application) or as interlocking flat panels. New zinc has a bright, slightly blue-grey colour; it weathers to a characteristic zinc patina (blue-grey to dark grey) within 12–24 months of London exposure.

*Performance*: Very long lifespan (60–100 years). Extremely low maintenance — the natural patina is self-sealing and protects the underlying zinc from further atmospheric corrosion. Suitable for both roofing and cladding applications.

*Planning position*: Zinc cladding is accepted in planning terms for contemporary extensions in non-Conservation Area settings; it is less commonly accepted in Conservation Areas where the planning policy context requires materials that respond to the character area. In some Conservation Areas, the planning officer may accept zinc cladding on a rear elevation not visible from the highway where the design rationale is well-argued.

*Cost*: £90–£180/m² installed (significant variation depending on panel system, profile, and installer).

*Suppliers*: VM Zinc, Rheinzink, VMZINC. Specify 0.7mm minimum thickness for wall cladding.

**Profiled metal (Kingspan, Colorcoat, Corus / Tata Steel)**:

*Characteristics*: Steel or aluminium sheet formed into corrugated or trapezoidal profiles. The most cost-effective metallic cladding option. Available pre-coated in a wide range of RAL colours. Very long lifespan if correctly coated and installed.

*Design note*: Profiled metal cladding was associated with agricultural and industrial buildings; in residential contexts it reads as deliberately industrial-chic or can look cheap if poorly specified. The profile selection (fine corrugation vs. heavy trapezoidal), colour (anthracite and dark grey work well for contemporary extensions), and installation detail (hidden fixings vs. visible fixings) all affect how the cladding reads.

*Cost*: £35–£75/m² installed (depending on profile and specification).

**Corten (weathering steel)**:

*Characteristics*: A steel alloy that forms a stable rust patina (orange-red to dark brown) on exposure to weather. The patina is protective — unlike standard steel, the rust layer on Corten forms a tight, impermeable surface that prevents deeper corrosion. Very distinctive aesthetic.

*Practical considerations for London*: Corten in the early stages of weathering (first 12–36 months of exposure) produces run-off that stains adjacent surfaces orange-brown. In a terrace house context this can stain a neighbour's wall, the patio, or the garden — significant neighbour relationship implications. After the patina stabilises, the staining run-off stops. This is a characteristic, not a defect, but the implications must be understood before specifying Corten adjacent to shared boundaries or light-coloured hard landscaping.

*Cost*: £70–£140/m² installed.

**Brick slip (thin-format brick veneer)**:

*Characteristics*: Thin-format clay brick tiles (typically 15–25mm thick) adhered to a structural substrate, giving the appearance of a full-depth brick skin at lower weight. Used where the structural capacity or budget does not allow full-depth facing brick but the planning requirement or design intention specifies a brick appearance.

*When brick slip is appropriate*: Where full-depth facing brick is required by planning (Conservation Area, matching the existing house) but the structural capacity of the extension does not allow a cavity masonry wall; on urban extensions where the brick appearance is desired for planning compliance but the contractor's programme benefits from a lighter system.

*Limitations*: Brick slips do not perform identically to full-depth masonry — particularly regarding resistance to impact, moisture management at reveals, and long-term durability. The adhesive bond must be correctly specified for the substrate (brick slips on polystyrene EWI board require different adhesive from brick slips on blockwork). Specify a quality brick slip system (NBSspecs, Eurobrick, Vande Moorele) rather than ad-hoc adhesive tiling of split bricks.

*Cost*: £70–£130/m² installed (including substrate preparation).

**Planning considerations by material**:

For any extension requiring planning permission or a Certificate of Lawful Development, the material specification is part of the application and can be controlled by planning condition. For PD rear extensions, the Class A condition requires materials to be 'of a similar appearance to the existing dwellinghouse' — which in practice means brick or render for most London houses, not metallic cladding.

*Material-by-material planning summary*:

| Material | PD (matching existing) | Planning permission (rear) | Conservation Area | |---|---|---|---| | Matching brick | ✓ | ✓ | ✓ (usually) | | Matching render | ✓ (if house is rendered) | ✓ | ✓ (usually) | | Timber (cedar/larch) | ✗ (not 'similar appearance') | ✓ (with good DAS) | Sometimes | | Charred timber | ✗ | ✓ (rear, good DAS) | Rarely | | Zinc standing seam | ✗ | ✓ (rear, good DAS) | Rarely | | Corten | ✗ | ✓ (rear, good DAS) | Very rarely | | Profiled metal | ✗ | ✓ (rear, contemporary design) | Very rarely | | Brick slip | ✓ (matching brick appearance) | ✓ | ✓ (usually) |

*DAS = Design and Access Statement arguing the 'honest contrast' approach.*

**Cost comparison for external wall cladding (all-in, supply and fix)**:

| Cladding type | Approximate installed cost /m² | |---|---| | Concrete blockwork + render (K-Rend) | £50–£90/m² | | Concrete blockwork + facing brick | £90–£150/m² | | Siberian larch cladding (on battens and membrane) | £55–£85/m² | | Western red cedar cladding | £65–£100/m² | | Charred timber (Shou Sugi Ban) | £100–£160/m² | | Zinc standing seam cladding | £110–£200/m² | | Profiled metal (Colorcoat Prisma) | £45–£85/m² | | Corten weathering steel | £90–£160/m² | | Brick slip system | £80–£140/m² |

*Note*: These costs are for the cladding system only — they exclude structural masonry or frame behind, insulation, windows, and doors.

**Maintenance summary by cladding type**:

| Material | Maintenance requirement | |---|---| | Brick (facing) | Minimal — repoint after 25–40 years | | Render (polymer-modified) | Biocide wash every 5–7 years; recoat every 15–20 years | | Cedar (untreated, silver finish) | Nil if left to weather naturally; annual wash recommended | | Cedar (treated) | Retreatment every 5–7 years | | Zinc | Nil — self-patinating | | Corten | Nil after patina stabilises (3+ years) | | Profiled metal (coated) | Inspect and touch up any damaged coating | | Brick slip | Check adhesion; replace any spalled slips |