Choosing External Materials for a London House Extension

**Why brick is the default material for London extensions**:

London's residential housing stock is predominantly Victorian and Edwardian brick construction. Most householder planning policies in London (and the Permitted Development conditions) require new extensions to be of 'similar appearance' to the existing dwelling — in practice, this typically means facing brick to match the existing, or brick compatible with the Conservation Area character.

Brick is also structurally versatile, durable, and requires minimal maintenance — making it the practical as well as the planning-safe choice for most London extensions.

**Matching existing brick**:

For a rear or side extension on a Victorian or Edwardian house, the specification goal is typically to match the existing brick as closely as possible in colour, texture, and mortar joint colour. Common existing brick types in London:

• •*London stock (yellow stock)*: The most common original brick in South and East London (LB Hackney, LB Southwark, LB Lambeth, LB Tower Hamlets, LB Islington). A pale yellow/buff-grey colour, traditionally made from the local 'brickearth' (London Clay with chalk). Modern equivalents: Michelmersh stock bricks, Northcot stock bricks, or genuine hand-made stocks from Bulmer or Smeed Dean
• •*Red facing brick*: Common in North and West London (LB Barnet, LB Hammersmith, Ealing) and in late Victorian and Edwardian properties. Numerous modern equivalents from Ibstock, Forterra, and Wienerberger
• •*Yellow/grey Fletton*: A machine-made brick common in mid-20th century extensions and outbuildings — less architecturally valued; can be matched from any major brick manufacturer

*Getting the match right*: Take a sample of the existing brick to a brick merchant or manufacturer's brick library (Ibstock, Forterra, and Wienerberger all offer matching services). Request samples and compare them at the property against the existing brick in both wet and dry conditions — brick colour varies significantly with moisture. Consider the mortar joint colour separately — a well-matched brick with the wrong mortar joint colour will still look wrong.

*A note on PD and brick matching*: The Permitted Development condition (Class A, Part 1) states that the 'materials used in any exterior work shall be of similar appearance to those used in the construction of the exterior of the existing dwelling-house.' An extension built in a significantly different brick colour is in breach of PD conditions — technically not PD and potentially enforceable as an unauthorised development. Always match brick to the existing where the extension is intended to be PD.

**Contrasting brick as a deliberate design choice**:

Not all extensions need to match. For a clearly contemporary architectural intervention — a roof extension, a heavily glazed extension, or an extension that is intended to read as 'new' — a contrasting or complementary brick (darker, lighter, or a different texture) can be a deliberate and successful design choice. This requires planning permission (the PD condition requires 'similar appearance') and the planning officer's approval. In practice, planning officers in many London boroughs are willing to support a carefully considered contrasting material palette for an extension that is clearly differentiated from the original house — particularly for flat-roof contemporary extensions where trying to match original Victorian brickwork creates an artificial result.

**Render**:

Render (a cement-based, lime-based, or polymer-based external coat applied to the blockwork substrate) provides a smooth or textured external finish. Common render types:

• •*Monocouche render (through-coloured polymer render)*: A single-coat system applied at 15–25mm, pre-coloured (no painting required), with good crack resistance. Popular finish for contemporary extensions. Available in a range from off-white through grey to dark charcoal. Main suppliers: Weber, Parex. Cost: £35–£65/m² applied.
• •*Sand/cement render + masonry paint*: Traditional two-coat render, sand/cement applied in two coats and finished with masonry paint. Lower material cost; requires repainting every 5–10 years. Cost: £20–£45/m² applied.
• •*Lime render*: A hydraulic lime render used for breathable applications (older solid-wall buildings) or for conservation applications requiring a historical feel. More expensive than cement render; requires experienced applicators. Not commonly used for new extensions unless specified by the planning authority.

*Render and PD*: Render is an acceptable material under PD for modern detached or semi-detached houses that are themselves rendered (the extension matches the existing). For a brick Victorian house, a rendered extension is unlikely to comply with the PD 'similar appearance' condition — planning permission would be required.

**Timber cladding**:

Timber cladding (horizontal or vertical boards fixed over a ventilated cavity on the extension walls) provides a warm, natural aesthetic that has become popular for garden studios, home office extensions, and contemporary rear extensions.

• *Common timber cladding species for London*:
• •*Western Red Cedar*: Naturally durable (Class 3 durability), weathers to a silver-grey if left untreated; a classic choice for contemporary buildings. Cost: £30–£55/m² supply and installation
• •*Siberian Larch*: Harder and denser than Cedar, naturally durable (Class 3–4); tends toward orange-brown if treated, silver-grey if left. Cost: £25–£45/m²
• •*Kebony*: Modified softwood (Norwegian pine or maple treated with furfuryl alcohol), Class 1 durability, very stable; more expensive than natural hardwoods. Cost: £45–£70/m²
• •*Thermally modified timber*: Various species heat-treated to improve durability; Class 3–4 durability; better dimensional stability than untreated timber

*Timber cladding and PD*: Timber cladding is unlikely to be 'of similar appearance' to a Victorian brick house — planning permission is required for a cladded extension on a brick house. In a Conservation Area, timber cladding is unlikely to be approved if visible from the street.

**Zinc and other metal cladding**:

Zinc, Corten steel, and aluminium cladding panels have become popular for contemporary single-storey extensions and roof extensions on Victorian terraces — particularly as a 'honest new addition' strategy (the new extension is clearly differentiated from the historic fabric by its material). These materials require planning permission (not PD-compliant for brick-built original houses) and a planning officer who is sympathetic to contemporary design.

*Zinc*: VMZINC or Rheinzink pre-weathered zinc panels; naturally weathers to a dark grey; cost £80–£130/m² *Corten steel*: Weathering steel that oxidises to a characteristic orange-brown rust patina and then stabilises; strong visual character; cost £70–£120/m²

**Glazing and glass extensions**:

For extensions with large areas of glazing (bifold doors, sliding glazing, fixed glazing panels, structural glass walls), the glazing specification must meet:

• •*Part L U-value*: Maximum 1.6 W/m²K for windows (recommended 1.2 W/m²K); total glazed area should not exceed 25% of the total floor area of the new extension and the area of any windows lost in creating the extension (the 25% rule is the SAP calculation default for Part L compliance in an extension — if total glazed area exceeds 25%, an energy calculation is required to demonstrate overall compliance)
• •*Part K safety glazing*: All glazing below 800mm from floor level and all glazing in doors must be safety glass (toughened or laminated) complying with BS 6206 — specified by the glass thickness and type from the supplier
• •*Part Q security*: Windows and doors in new extensions must resist attack to BS PAS 24:2016 — most mainstream aluminium and timber window and door systems from reputable manufacturers (Origin, Schuco, Smart) comply as standard

**Flat roof covering materials**:

As covered in the flat roof drainage guide, the main options are GRP (fibreglass), EPDM (rubber), and TPO/PVC single-ply. These are covered by planning as follows:

• •In a Conservation Area, a flat roof on a rear extension is generally acceptable as it is not visible from the public realm
• •A flat green roof (sedum or extensive wildflower mix) is positively encouraged by many London planning authorities as a biodiversity and surface water benefit — and may not add significantly to cost if specified at the design stage

**Pitched roof tiles for extensions with a pitched roof**:

For double-storey extensions and certain single-storey extensions with pitched roofs, the roof tiles should match the existing dwelling. Common tile types in London:

• •*Plain clay tiles*: The original tile type for Victorian and Edwardian terraces; 265mm × 165mm format; typically hand-made or machine-made in a range of clay colours. Best match found at specialist salvage yards (for genuine matching) or from manufacturers (Keymer, Dreadnought, Tudor Roof Tile)
• •*Concrete interlocking tiles*: Common in 1960s–1990s extensions and re-roofing; available in a wide colour range; not always a good match for original plain tile properties
• •*Slate*: Natural or fibre-cement slate; common in certain London areas (North London in particular); must match the format and colour of the existing slate

**Glazed rooflights and roof lanterns**:

• •*Conservation Area rooflights*: In a Conservation Area, rooflights on a rear slope (not visible from the street) are generally PD-compliant if they project no more than 150mm above the plane of the roof and are not on a roof slope facing a highway
• •*Roof lanterns*: Increasingly popular for kitchen extensions — a raised glazed structure set in the flat roof providing natural light and a volume to the extension. Leading suppliers: Atlas, Origin, Apropos, Sky-Frame. Planning assessment: a roof lantern that projects above the ridge of the existing roof in a Conservation Area may require planning permission

**The 'similar appearance' principle in practice**:

• For the majority of London householder extensions, the most straightforward approach to materials and planning compliance is:
• •Match the facing brick of the existing house as closely as possible
• •Use the same roof tile or flat roof covering type as the existing rear slope (where applicable)
• •Specify windows and doors in a compatible format (ideally aluminium-framed, which is acceptable as a 'similar appearance' material in most London LPAs)
• •Reserve the contrasting material choices for elements that are clearly new (the glazed element of a bifold door run, a flat roof section, a garden office)

Only move to a fully contrasting material palette if the design is sufficiently resolved to support a planning application and the planning authority has given a positive pre-application steer.