Will a roof lantern overheat the room in summer?

Sometimes. South-facing lanterns can add 3–5°C peak summer indoor temperature. Solutions: specify a lantern with solar-control glazing (£450–£900 premium); add openable lantern vents for stack-effect cooling (£600–£1,400 over fixed); orient lantern to sit centrally rather than fully south. North-facing or central lanterns rarely overheat.

What is the minimum window-to-floor area ratio in Building Regs?

Approved Document F (ventilation) requires 1/20th openable window area to floor area in habitable rooms (5%). Approved Document L does not set a glazing ratio but penalises U-value performance — high glazing requires premium thermal performance (1.2 W/m²K or better) to compensate. SAP balances glazing area against U-values, solar gain factors and overheating risk.

Is a rooflight cheaper than a roof lantern per m² of glazing?

Yes — 1m × 1m fixed rooflight £1,800–£3,500; 1.5m × 2m roof lantern £3,500–£6,500 (£1,200–£2,200/m²). Lantern cheaper per m² because of larger area, but install (taller upstand, parapet detail) adds complexity. Two 1m × 1m rooflights vs one 2m × 1.5m lantern: similar cost; lantern usually wins on appearance and central focal positioning.

Do glass-box extensions work in London?

Yes but expensive. Frameless structural-glass extensions (Trombe, IQ Glass) cost £4,500–£8,500/m² of floor area — 2–3× a conventional masonry extension. Planning is straightforward if to the rear and conforming to PD volumes. Best for compact garden-room extensions (8–15m²) where the glass-box aesthetic is the design driver; less viable for full kitchen-diner extensions over 25m² where cost runs to £150k–£250k+.

Natural Light Rear Extension London 2026 | Builderr

How Do You Maximise Natural Light in a London Rear Extension?

Maximise natural light in a London rear extension with a 30–45% glazing-to-floor-area ratio: full-width sliding or bifold rear glazing, a 1.5m × 2m roof lantern, light-toned floors and ceilings, and reflective surfaces. North-facing extensions need 40%+ glazing ratio; south-facing need solar shading. Budget £8,500–£22,000 for the light-strategy components above base extension cost.

The orientation problem in London

London terraces orient predominantly north-south due to street grid history — rear extensions typically face north (kitchen at rear, sun in the front) or south. North-facing rear extensions: cool, even sky light all day but no direct sun; risk feeling cold and grey without aggressive glazing ratios and warm interior palettes. South-facing: strong direct sun midday-to-afternoon; risk overheating and glare; need solar shading and cooler interior palettes. East-facing: morning sun only — bright breakfast room. West-facing: afternoon and evening sun — best for kitchen-diner with evening entertaining. Orientation analysis should happen at concept stage; Builderr's design team plots a sun-path diagram for every rear extension brief at week 2 of design to inform glazing, shading and palette decisions.

Glazing ratio and types

Total glazed area divided by floor area: 25% is minimum (Building Regs Part L), 30–45% is the practical target, 55%+ moves toward glass-box territory and demands premium thermal glazing. Rear-wall glazing: 3.0–4.5m of full-height bifold (Schüco, Reynaers, Sunflex) or sliding (Schüco ASE, Sky-Frame, Vitrocsa) — bifold £2,800–£4,500 per linear metre; sliding £3,500–£6,500 per linear metre. Sliding is becoming the default in 2026 — cleaner sightlines, fewer mullions, modern aesthetic. Roof glazing: rooflight (1m × 1m fixed or opening) £1,800–£3,500. Roof lantern (1.5m × 2m) £3,500–£6,500 — biggest light gain, visual focal point. Side glazing: where setback allows, 600mm × 2.4m glazed slot in the side wall £900–£1,800 — adds 'borrowed light' from the side passage. Frameless glazed corner (rear and side walls meeting at a frameless mitred glass corner) £4,500–£8,500 premium.

Internal light strategy beyond glazing

Light gained at the rear glazing dissipates within 4–5m. Three strategies extend light deeper. Strategy A: rooflight or lantern over the original-kitchen zone (where the rear extension meets the existing host) — this is the darkest zone of the combined room; a 1.2m × 1.5m rooflight here transforms it. Strategy B: borrowed-light internal glazing — Crittall screen with clear glass between kitchen-diner and an adjacent room (utility, snug, study) lets light pass through; £2,500–£4,500 for a 2.4m × 2.4m Crittall. Strategy C: light-tinted finishes throughout — white or pale ceiling, off-white walls, pale floor (engineered oak in pale rift-sawn cut, or pale large-format porcelain) reflect light deep into the room. Dark floors (smoked oak, dark walnut) absorb light and worsen the gloomy-end-of-extension problem. Mirror placement: a 1.5m × 2.5m mirror on the wall opposite the rear glazing visually doubles the apparent depth and bounces light back; cheap, effective.

Solar shading for south-facing extensions

South-facing rear extensions overheat without shading: SAP overheating calculations frequently fail on south-facing schemes with 35%+ glazing. Three strategies. Mechanical: external roller blinds or brise-soleil — most effective; £2,500–£6,500 for external roller blinds on a 4m bifold; £4,500–£9,500 for brise-soleil at roof-overhang. Architectural: 600mm roof overhang above rear glazing — blocks summer sun (high angle) while admitting winter sun (low angle); built into the design from concept; cost-neutral over a flush parapet detail. Glazing: solar-control glazing (selective coatings such as Pilkington Suncool 70/40) — admits visible light but reflects infrared; cost premium £400–£800 on a 25m² extension. Internal blinds help but are less effective than external shading. Best practice on south-facing: combine modest roof overhang with solar-control glazing — usually sufficient without external blinds.