Extension Roof Types in London: Flat, Pitched, Zinc, and Lantern — How to Choose

A flat roof (technically a 'low-pitch roof' — all roofs must have some fall for drainage) is the most common roof type for rear extensions in London. It is the cheapest to build, the simplest structurally, and the easiest to detail within the height restrictions typically imposed by permitted development rights.

**Modern flat roof construction** uses one of two main systems:

- **GRP (Glass Reinforced Plastic / fibreglass)**: The most widely used material in London residential extensions. GRP is moulded on-site into a seamless waterproof membrane. It is durable (25–30 year lifespan with good installation), low-maintenance, and available in a range of colours. Cost typically £80–£140/m2 installed. - **EPDM (Ethylene Propylene Diene Monomer)**: A single-ply rubber membrane bonded or mechanically fastened to the roof deck. Equally durable, slightly cheaper than GRP, and better-suited to larger or more complex roof areas. Cost typically £60–£110/m2 installed.

**The flat roof and planning**: Under permitted development, a single-storey rear extension cannot exceed 4m in height. A flat roof extension at eaves height of 3.0–3.5m (standard for most London terraced houses) gives a generous ceiling height of 2.8–3.2m inside — which is one of its main attractions.

**Natural light**: A standard flat roof extension has walls and no ceiling glazing. To bring natural light deep into the plan, roof lanterns or rooflights are added to the flat roof. This is the most popular current approach in London kitchen extensions — a flat roof with one or two rooflights or a full-width lantern.

A pitched roof extension matches the roof form of the host building — usually a gable end, hip, or lean-to pitch. In conservation areas, some planning authorities prefer or require a pitched roof that complements the existing house.

**Advantages**: More traditional appearance, better water drainage, longer lifespan (tiles or slates can last 50–100 years), and generally preferred by planning authorities where close material matching is required.

**Disadvantages**: More expensive than flat roof (additional structural framing, higher material cost), takes more height, and reduces internal ceiling height at the perimeter of the room (the eaves point is lower than the ridge).

**Typical pitched roof options**: - **Lean-to (mono-pitch)**: A single slope rising from the rear wall of the house, typically using plain concrete tiles or clay plain tiles to match the existing roof. Common in permitted development extensions that need to stay under 4m at the high point. - **Dual pitch (gable end)**: A full pitched roof with two slopes meeting at a ridge. Requires a wider extension to work proportionally. - **Hip roof**: A pitched roof that slopes on all four sides. Good appearance but more complex to build.

**Cost premium over flat roof**: typically £2,000–£5,000 additional for a standard single-storey rear extension (20–30m2 footprint) — primarily the extra timber framing, tiling, and valley or hip details.

Zinc, aluminium, and standing seam steel roofs have become increasingly popular for contemporary London extensions over the past decade. They offer a distinctive contemporary aesthetic at a cost below full-height glazed roofs but above standard flat or pitched construction.

**How they work**: Prefabricated or site-formed metal panels are fixed to a flat or low-pitched roof deck in a standing seam pattern (interlocking vertical ribs). The result is a clean, minimalist roof surface that weathers to a characteristic patina over time (zinc develops a grey-blue oxide layer; Cor-Ten steel develops a rust patina).

**Planning**: In conservation areas and for listed buildings, zinc and standing seam roofs may require planning approval if visible from the street. They are generally accepted on rear extensions where not visible from a public highway.

**Cost**: Zinc roofing installed typically costs £140–£250/m2 — significantly more than GRP or EPDM flat roofing, and comparable to high-quality natural slate pitched roofing. The premium is justified where the aesthetic is a design priority.

**Lifespan**: Natural zinc has an expected lifespan of 60–100 years with minimal maintenance. It is one of the most durable roofing materials available for residential use.

A roof lantern is a glazed upstand rising above the flat roof plane, bringing natural light vertically into the space below. It is the single most impactful natural-light feature in a rear kitchen extension and has become a standard element of London kitchen-diner extensions.

**Structural lanterns (aluminium or timber frame)**: Bespoke or semi-standard aluminium-framed glazed lanterns are the most common type in London residential extensions. They are available from specialist suppliers (Roof-Maker, Jones Roofing, Atlas and others) in standard sizes or bespoke dimensions. A 1.5m x 3m lantern installed typically costs £4,000–£9,000 supply and fit.

**Full-width glazed roofs**: For a more dramatic effect — particularly in contemporary extensions — a full-width or near-full-width glazed roof replaces the entire flat roof with structural glazing. This gives excellent natural light but at significant cost (£600–£1,200/m2 for structural glazing vs £80–£140/m2 for GRP flat roofing). Thermal performance requires careful specification — look for triple-glazed units with low-e coating and appropriate solar control in south-facing orientations.

**Rooflights**: Flush-fitting or slightly upstanding rooflights (Velux, Korniche, or structural polycarbonate) are a cost-effective alternative to a lantern for adding natural light to a flat-roof extension. A pair of 1000 x 1500mm rooflights installed costs approximately £2,000–£4,000 and significantly improves the quality of natural light.

The right roof type depends on four factors:

**1. Planning constraints**: In a conservation area or where close material matching is required, a pitched roof in matching tiles or slates may be required. Check your planning condition before specifying.

**2. Budget**: Flat GRP roof is the cheapest. Pitched adds £2,000–£5,000. Zinc adds £3,000–£6,000 over flat GRP. Structural glazing is the most expensive. Roof lanterns sit in the middle.

**3. Natural light requirements**: If the room runs deep (more than 4–5m from the rear wall), roof glazing — either a lantern or rooflights — is strongly recommended. Standard flat roof without roof glazing leaves the rear of the room dark.

**4. Aesthetic intent**: A contemporary rear extension typically suits a flat roof with lantern or structural glazing. A traditional or conservation-sensitive extension typically suits a pitched roof in matching tiles or slates.

Approximate all-in costs for the roof element of a 20m2 single-storey rear extension:

- GRP flat roof (no glazing): £2,000–£3,500 - GRP flat roof with two rooflights: £4,000–£6,500 - GRP flat roof with 1.5m x 3m lantern: £7,000–£12,000 - Lean-to pitched roof in concrete tiles: £3,500–£5,500 - Lean-to pitched roof in natural slate: £5,000–£8,000 - Zinc standing seam roof: £4,000–£7,000 - Full structural glazed roof: £15,000–£25,000

These are the roof element costs only — structural framing, insulation to Building Regulations standards, and internal finishes are separate elements within the overall extension cost.