Skylight Calculator

Under Part L 2021 (Approved Document L Volume 1), the limiting U-value for rooflights is 2.2 W/m²K, measured on the developed surface area (effectively the horizontal plane). This 2.2 limit applies both to new dwellings (Table 4.1) and to new or replacement rooflights in existing dwellings and extensions (Table 4.2). Roof windows are different: they are not a separate row but fall under the Window row (Table 4.1 note 5 and Table 4.2 note 8), measured vertically, so the limit is 1.6 W/m²K for a new dwelling (Table 4.1) and 1.4 W/m²K for a new or replacement roof window in an existing dwelling or extension (Table 4.2). Quality argon-filled double-glazed units typically achieve about 1.2-1.5 W/m²K and triple about 0.8-1.1 W/m²K — comfortably inside these limits.

Flat skylights under permitted development don’t usually need planning permission if they don’t protrude more than 150mm above the roof. Roof lanterns may need permission. Conservation areas and listed buildings have stricter rules.

Roof lanterns provide dramatic light from multiple faces. Fixed skylights give excellent overhead daylight. Tubular sun tunnels suit internal rooms. As a rule of thumb, a roof aperture admits at least twice and up to about three times the daylight of a vertical window of the same area (VELUX/BRE sky-component guidance) — the gain is a roughly constant ratio, not something that scales with size.

A standard 600x900mm fixed flat skylight costs £700-1200 supplied and fitted. Opening skylights add £200-400. Roof lanterns cost £1500-4000+. Tubular sun tunnels cost £400-700 installed.

South and west-facing skylights can cause summer overheating. Solutions include solar control glass (g-value 0.3 or lower), external blinds, and opening skylights for ventilation. Part O now addresses overheating risk.