Rooflights in London Renovations: Types, Planning, and Specification

Few architectural interventions deliver as much visual impact per pound as a well-positioned rooflight. In London's densely built terraces, where rear extensions face north or where central stairwells are trapped between party walls, a rooflight is often the only way to bring direct daylight into the building. Done correctly, it is transformative. Done incorrectly — with an underspecified product, inadequate upstand, or poor flashings — it leaks, condensates, and requires expensive remediation.

This guide covers rooflight types, planning requirements, and the specification details that determine whether a rooflight performs for 30 years or fails in the first winter.

Types of rooflight

Flat rooflights (fixed) sit on a low-pitch or flat roof, typically at a pitch of 0–15 degrees. They are the most common specification in rear extensions. The glazing unit is set into a kerb or upstand — either a proprietary aluminium kerb or site-built timber — and sealed with a continuous flashing system. The critical specification here is the upstand height: a minimum of 150mm above the finished roof surface is required to prevent water ingress. Many leaks trace back to inadequate upstands on flat rooflights.

Pitched rooflights (in-plane) sit flush with the roof slope, set between or over rafters. Velux and Fakro are the dominant manufacturers. These are appropriate in loft conversions and on pitched roofs. The flashing kit must match the tile or slate type exactly — manufacturers produce specific flashings for each roof covering. Generic flashings on premium natural slate are a common source of leaks.

Walk-on rooflights are specified at floor level — either as a glazed floor panel over a basement, or as a rooflight that functions as a terrace feature. These require significantly stronger glazing (typically triple-laminated safety glass) and a proprietary frame rated for pedestrian loading. They are considerably more expensive than overhead rooflights and require careful structural detailing.

Frameless/structural glazing rooflights use bolted point-fixings or structural silicone rather than visible aluminium frames. These deliver the cleanest aesthetic — a plane of glass with no visible border — but require specialist installation and regular silicone inspection. Not appropriate for DIY installation or less experienced contractors.

Lantern rooflights sit above a flat or low-pitch roof on a raised frame, creating a box or pyramid above the roof plane. They allow more vertical glazing area and feel more generous internally than a flat panel. They are structurally more complex and more expensive, and require careful thermal detailing to prevent cold bridging at the frame.

Planning considerations in London

Most rooflights fall under permitted development rights and do not require a planning application, subject to conditions:

• —The rooflight must not project more than 150mm above the roof plane (for in-plane pitched rooflights)
• —On a principal elevation facing a highway, rooflights are more restricted
• —In a conservation area, all rooflights visible from a public road require planning permission
• —On a listed building, all rooflights require listed building consent regardless of size or position

In practice, most rear extension flat rooflights in inner London do not require planning permission. Confirm with the local planning authority before proceeding if there is any doubt — the cost of a pre-application enquiry is small compared to an enforcement notice.

Performance specification

The two critical performance metrics for a rooflight are thermal transmittance (U-value) and condensation resistance.

U-value: Building Regulations Part L requires a maximum U-value of 2.0 W/m²K for rooflights in new dwellings. Premium products achieve 1.0–1.4 W/m²K with triple glazing. In a high-specification renovation targeting good thermal performance, specify triple glazed units with a U-value of 1.5 or better. A double-glazed unit meeting minimum regulation is technically compliant but will feel cold in winter and will show condensation at the frame in cold weather.

Solar control: south and west-facing rooflights in London receive significant solar gain in summer. Without solar control glazing, internal temperatures in a rear extension or kitchen under a rooflight can become uncomfortable. Specify solar control glass (typically a low-e coating with a solar factor of 0.3–0.5) for any rooflight with significant solar exposure. This is a glazing specification decision that cannot be changed after installation without replacing the unit.

Opening type: fixed rooflights are less expensive and simpler to detail. Opening rooflights (hinged, pivot, or electric) provide ventilation but introduce more complexity — seals, actuators, and drainage paths must all be maintained. In a rear extension kitchen, a single electrically-operated opening rooflight can meaningfully improve ventilation on warm days without opening doors. Specify opening where the room has limited wall ventilation.

The upstand: the detail that fails most often

A rooflight upstand is the vertical element that raises the rooflight frame above the roof surface, allowing the flashing to discharge water clear of the frame joint. The minimum upstand height is 150mm above the finished roof surface — this is a near-universal industry standard and is reflected in most manufacturers' installation guides.

In practice, this dimension is often compromised. On a flat roof with tapered insulation boards, the drainage falls are sometimes designed to create a low point at the rooflight location — either by careless design or because a contractor has not set out the falls correctly. When water pools against the upstand, even a well-flashed detail will eventually fail.

Ensure the structural opening and drainage falls are coordinated so the rooflight upstand sits at a high point of the drainage scheme, not a low one. Check this with the structural engineer and the roofing contractor before the concrete pour or insulation installation.

Key manufacturers

• —Glazing Vision — high specification flat and structural rooflights, used frequently in London residential work
• —IQ Glass — structural glazing, frameless systems, bespoke configurations
• —The Rooflight Company — heritage-appropriate rooflights, conservation area compliant designs
• —Velux — the default for pitched in-plane rooflights; reliable, well-supported, extensive flashing kit range
• —Fakro — strong competition to Velux for pitched rooflights, often better value at equivalent specification
• —Sunlux / Sky-Frame — premium frameless systems for high-specification contemporary interiors

For a standard rear extension flat rooflight in a quality renovation, budget £1,500–£4,500 for the product depending on size and specification, plus installation and structural opening costs.