Why dual-aspect light is preferred
A single-aspect rear extension (glazing on rear wall only) gives strong light at the rear glazing line but light dissipates within 4–5m. The host-side of the new room remains relatively dim. Dual-aspect lighting introduces a second glazed wall — typically the side — so light enters from two perpendicular directions; the result is more even illumination across the room volume, fewer shadows, brighter overall feel. Architectural lighting research (CIBSE Daylighting Code 2024 update) shows dual-aspect rooms achieve 35–55% higher Daylight Autonomy (the proportion of working hours with adequate natural light) than equivalent single-aspect rooms — the gain is largest in north-facing extensions where direct sun is absent. Estate agent feedback: dual-aspect rooms are explicitly valued in property descriptions (Knight Frank, Foxtons, Savills); single-aspect rooms are not. Buyers respond strongly to bright, dual-aspect kitchens — adds 1.5–3% to sale value.
Dual-aspect strategies for London extensions
Four strategies. Strategy A: side-return extension with side-wall glazing — when extending into a side return, include a fixed glass panel or sliding glass on the new side wall (where setback to neighbour boundary allows). 1.5–2.4m wide × 2.2–2.4m tall side glazing £2,400–£4,800. The new side wall faces east or west — provides morning or afternoon dual-aspect light. Most common on Victorian and Edwardian terrace extensions. Strategy B: glazed side slots — narrow slot windows (300–600mm wide × 2.0–2.4m tall) in the side wall; each slot provides a vertical column of borrowed light without compromising privacy. 2–3 slots per side wall £1,800–£3,500. Most effective when the neighbour's side fence runs close to the new wall. Strategy C: frameless mitred glass corner — the rear and side walls meet at a 90° frameless glass corner (no mullion); both walls are glazed for the corner 1.2–1.8m run; £4,500–£8,500 premium over standard rear glazing only. Strategy D: roof lantern positioned to one side — brings light down on the side opposite the rear glazing; £3,500–£6,500.
Planning and structural implications
Side glazing in London extensions has planning sensitivities. Side wall windows facing a neighbour boundary: planning officers concerned about overlooking — the test is whether the window provides direct sightline into a neighbour's habitable rooms or garden within 21m. Mitigation: (1) Obscure-glazed side glazing (Pilkington Texture 1, Sandblast frit) — eliminates direct sightlines; full daylight admitted; £40–£90/m² premium. (2) High-level side glazing (sill above 1.7m from internal floor) — natural sightlines from inside are upward only; planning officers generally accept. (3) Side glazing positioned to face the neighbour's blank gable end or boundary fence — no sightline issue. Permitted Development: Class A.1(f) GPDO requires upper-floor side windows in extensions to be obscure-glazed and non-opening below 1.7m — applies to first-floor side glazing in two-storey extensions; ground-floor side glazing has no such PD restriction. Structural: additional steel header beam required above side glazing in masonry side walls — £600–£1,800 over standard masonry wall.
Designing dual-aspect light for north-facing extensions
North-facing rear extensions are the strongest candidates for dual-aspect design — without dual-aspect, they feel grey and cool all day. North-facing strategy: maximise rear glazing (north light is cool and diffuse — generous glazing area is needed to compensate for absence of direct sun); add east or west side glazing (provides direct morning or afternoon sun). Specific recommendations: 3.6m sliding glass wall on rear (north); 2.0m × 2.4m sliding panel or fixed glazing on east side (morning sun) or west side (afternoon and evening); 1.5m × 1.0m roof lantern offset to the south-facing edge of the roof. Combined glazing area: 12–18m² on a 25m² extension = 48–72% glazing ratio (very high, requires premium thermal glazing and overheating mitigation). Floor finish: pale-toned oak or porcelain — reflects light. Wall finish: warm off-white (avoid cool greys which exaggerate cool north light); accent woods (walnut joinery, oak shelving) add warmth. North-facing dual-aspect extensions can achieve internal daylight quality similar to south-facing single-aspect extensions — the design effort pays back daily.
