Maximising Natural Light in a House Extension

The typical rear extension in a London Victorian terrace faces a fundamental lighting challenge:

• **Why rear extensions can be dark**:
• •The rear of most Victorian terraces faces north or east — meaning the extension receives limited or no direct sunlight through its rear glass wall
• •The depth of the extension away from the rear of the house means daylight from the rear elevation cannot reach the front of the extension effectively
• •The original rear wall of the house (which becomes the 'join' between old and new) is a solid masonry wall with no glazing — blocking light from travelling between the original house and the new extension
• •Tall fences, neighbouring extensions, and planting reduce light further

**The most effective natural light strategies**:

*1. Roof glazing (rooflights and roof lanterns)*: For single-storey rear extensions with a flat or low-pitch roof, overhead glazing is by far the most effective way to bring light in — daylight from above is on average 3× stronger than from the same area of vertical glazing on a wall.

• Options (see our dedicated rooflight guide for full detail):
• •Frameless rooflights (Velux CVP, Vision AGG, Korniche): flush with the roof plane; 300+ mm from the ridge rule applies
• •Pitched rooflights (Velux GGL/GGU): sit within the roof plane; the standard solution for pitched roofs and the cheapest reliable option
• •Roof lanterns: a box lantern sitting proud of the flat roof plane; architecturally prominent; more expensive than flat rooflights but very effective at creating a visual centrepiece
• •Walking-on / structural glass: a fully walkable glass panel flush with an adjoining upper floor terrace; highest specification and cost

Rule of thumb: for a 5m × 4m (20m²) single-storey extension on a flat roof, aim for at least 15% roof glazing area (3m² of rooflight) for good natural light. 20–25% (4–5m²) will feel generously lit.

• *2. Fully glazed rear wall or sliding/bifold doors*:
• •A full-width glazed rear wall — typically floor-to-ceiling bifold or slide-and-fold doors across the rear wall — maximises the light from the garden elevation
• •Most effective on south-facing rears where direct sunlight enters; less effective on north-facing where it provides reflected sky light only
• •Thermal performance consideration: large areas of glass have higher U-values than insulated walls — Part L energy assessment must account for this (see below)

• *3. Internal structural openings*:
• •A wide structural opening in the rear wall of the original house (the connecting wall between old and new) allows borrowed light from the extension to reach the existing interior
• •The wider the opening, the more light travels through — and vice versa: a narrow doorway maintains two separate spaces but restricts the light flow
• •Structural steel required for any opening (see our guide on RSJ and steelwork)

• *4. Sun tunnels and light tubes*:
• •Tubular daylighting devices (Velux Sun Tunnel, Solatube): a reflective tube running through the roof and ceiling, capturing daylight at roof level and diffusing it below
• •Effective where a rooflight cannot be installed (e.g., in a first floor bathroom with a tiled roof above, or in a central room with no roof access)
• •Capture area is limited — a 250mm Sun Tunnel delivers approximately equivalent light to a 250W bulb in direct sunlight conditions
• •Cost: £500–£1,200 installed

• *5. Glazed gable end*:
• •If the extension extends close to the side boundary and the gable faces east or west, a glazed gable can deliver significant light at specific times of day (morning from the east; afternoon from the west)
• •Planning implications: if glazed gable faces a neighbour's property or boundary, planning may restrict this; party wall/boundary considerations apply

Part L of the Building Regulations (Conservation of Fuel and Power) regulates the energy performance of extensions — including the proportion of the extension that can be glazed.

• **The Part L1B glazing area rule for extensions**:
• Under Part L1B (Existing Dwellings), the total glazing area in an extension (windows, doors, and rooflights combined) must not exceed:
• •Windows and doors: 25% of the floor area of the extension
• •Plus the area of any windows or doors lost (removed from the existing house as a result of the extension)
• •Plus rooflights: maximum 25% of the floor area (in addition to the window/door limit)

• **Practical example**:
• A 5m × 4m (20m²) single-storey rear extension:
• •Maximum windows/doors: 25% × 20m² = 5m² of vertical glazing
• •Plus rooflights: 25% × 20m² = 5m² of roof glazing
• •Plus any openings removed from the original rear wall (old back door, old rear windows)

• For a bifold door across the full 5m rear wall at 2.2m height = 11m² — this exceeds the basic 5m² vertical glazing limit. However, the compliance is assessed overall using a whole-extension energy calculation, not just individual glazing limits. Where you exceed the standard notional area, you must compensate:
• •By improving thermal performance elsewhere in the extension (better-insulated roof, better U-values on other elements)
• •By demonstrating via SAP calculation that the extension's fabric heat loss is not worse than a notionally compliant extension

In practice, most Building Control bodies allow larger glazed areas — particularly bifold or sliding doors across the full rear wall — if the roof and wall U-values are good and the overall extension energy balance is demonstrated. This is most effectively done through the architectural or energy consultant's submission at Building Regulations stage.

• **Double vs. triple glazing**:
• •Standard double glazing (argon-filled, low-e): U-value approximately 1.4–1.6 W/m²K
• •High-performance double glazing (krypton-filled, warm-edge spacer, low-e): U-value approximately 1.0–1.2 W/m²K
• •Triple glazing: U-value 0.6–0.8 W/m²K — approximately twice the performance of standard double glazing

For extensions with large glazing areas, upgrading to high-performance or triple glazing reduces the energy penalty of the glazing, making it easier to comply with Part L and reducing the heating cost of the space.

Beyond the specific elements above, a few design principles make a consistent difference:

**White ceilings and light colours**: Light bounces. A white or cream ceiling reflects daylight much more effectively than a dark or mid-tone ceiling. For an extension where light is a priority, white ceilings and light wall colours (or large expanses of light-reflecting materials like white-painted plaster) multiply the effect of whatever daylight enters.

**Minimal internal structure at roof level**: The more structural elements at ceiling level — downstand beams, structural walls — the more they cast shadows and interrupt the feeling of light. A clean, flat ceiling with rooflight glazing fully integrated creates a more luminous result than a ceiling with visible structural elements below.

• **Glazing orientation and solar gain**:
• Orientation affects which type of light you get, not whether you get any:
• •South-facing: direct sun for most of the day; maximum solar gain; potentially overheating risk in summer — roof glazing should include solar control specification (SGU solar control glass or external blinds)
• •North-facing: no direct sun; consistent cool diffused light; minimal overheating; good for kitchens and studios where consistent light without glare is preferable
• •East-facing: morning sun; bright and warm in the morning, shaded by afternoon
• •West-facing: afternoon and evening sun; particularly pleasant for dining and living rooms

**Internal borrowed light**: Where the extension connects to a dark internal hallway or room, a glazed internal door or internal window (borrowed light panel) allows light from the bright extension to spill into the darker adjacent space. This is architecturally simple, inexpensive, and highly effective.