Windows, Rooflights, and Glazing for London Extensions: Choices, Standards, and Design

A typical London rear extension has several glazing elements, each serving a different function:

**Bifold or slide-fold doors:** The large opening to the garden — typically occupying most or all of the rear wall width. Bifold doors fold back in panels; slide-fold (sliding-folding) doors use a similar mechanism with slightly different tracking. The choice between bifold and slide-fold is largely aesthetic and spatial — bifolds require a clear space to the side as panels fold out, while slide-folds can fold to one side or both.

• *Key decisions:*
• •**Frame material:** Aluminium (thinner sections, lower maintenance, typically standard in extensions), timber (warmer appearance, higher maintenance), UPVC (lower cost, larger section sizes).
• •**Glazing specification:** Double glazed (standard, U-value 1.2–1.6 W/m²K for the unit) or triple glazed (U-value 0.5–0.8 W/m²K for the unit, better thermal performance but heavier and higher cost).
• •**Opening width:** Most residential bifold/slide-fold systems go up to 6m in a single run without structural support. Wider openings need intermediate posts.

**Fixed glazed panels:** Fixed glass panels alongside doors or between structural columns. Lower cost than openable units, slightly better thermal performance than an equivalent openable panel. Commonly used to maximise the glass-to-wall ratio without the cost of a fully openable run.

• **Rooflights:**
• Rooflights (skylights) on a flat or pitched roof extension bring natural light deep into the space from above — which is fundamentally different from and often more effective than side glazing at illuminating an interior. Types include:
• •**Flat rooflights (kerb-mounted or frameless):** For flat roof extensions. Frameless (flush) rooflights are more expensive but significantly more attractive. Most flat rooflights open for ventilation.
• •**Lantern lights:** Raised glazed structures on a flat roof. More complex and expensive than a flat rooflight but provide a more generous light source and a distinctive architectural element.
• •**Conservation rooflights:** Low-profile rooflights designed to sit nearly flush with a pitched roof — required in conservation areas where traditional-looking pitched roof extensions are expected.

**Glazed roof (orangery or garden room glazing):** A fully or partially glazed roof — either a structural glass system or a lantern on a solid-framed flat roof. Maximises light but introduces thermal performance challenges: solar gain in summer, heat loss in winter. High-specification triple-glazed roof systems address this but at high cost.

**Clerestory windows:** High-level horizontal windows set in the wall above head height. In a rear extension, clerestory windows set in the wall between the extension and the main house can bring light into the original room as well as the extension — a useful strategy where the original kitchen is dark.

**Planning considerations:**

For permitted development extensions, glazing is not typically a planning constraint — the extension itself may or may not require planning permission, but the glazing specification within a permitted development scheme is largely unrestricted.

• Where planning permission is required:
• •**Conservation areas:** Glazing must typically be sympathetic to the character of the area. Aluminium frameless glass will usually not be acceptable on a conservation area street-facing elevation — though rear extensions (not visible from the street) are generally treated more permissively.
• •**Overlooking and privacy:** Extensions with large rear glazing can be refused or conditioned on overlooking grounds — particularly where the extension is at first floor level and overlooks neighbouring properties. Obscure glazing or limiting window height may be required.

**Building Regulations Part L — Thermal performance:** Part L of the Building Regulations sets thermal performance standards for new extensions. Key constraints on glazing:

• •**Maximum glazing area:** Part L limits the total area of windows, doors, and rooflights in an extension to 25% of the floor area of the extension, plus the area of any walls and roof openings that are being removed as part of the extension (typically the original rear wall).
• •**Minimum glazing U-values:** Individual window and rooflight units must meet minimum performance standards: typically U-value ≤1.6 W/m²K for windows and 1.8 W/m²K for rooflights (whole-unit values).
• •**Compensation:** If the glazing area exceeds the 25% threshold, the excess glazing can be compensated by improving the thermal performance of the opaque elements (walls, floor, roof) of the extension — a 'whole building' calculation.

**FENSA:** Replacement windows (but not extension windows — windows in newly built structures) require notification under the FENSA scheme, which self-certifies that the replacement windows meet Building Regulations Part L. A FENSA certificate is provided on completion — important for future sale.

**Frame material comparison:**

| Material | Section size | Maintenance | Cost | Longevity | |---|---|---|---|---| | Aluminium | Slim — 50–80mm | Very low | Mid–high | 40+ years | | Timber | Medium — 60–100mm | High (painting, oiling) | Mid–high | 40+ years if maintained | | UPVC | Larger — 80–120mm | Very low | Low–mid | 25–35 years | | Steel | Very slim — 30–50mm | Medium (painting) | High | 50+ years |

**Aluminium** is the most common choice for London rear extensions — slim sections allow maximum glazing area, low maintenance, and modern appearance. Powder-coated to almost any RAL colour.

**Timber** has a warmer appearance and is often preferred in conservation area contexts. Requires regular maintenance (repainting or oiling every 5–10 years).

**Steel** (Crittal-style or modern equivalents) is high cost and used on premium extensions where the very slim sightlines and industrial aesthetic are specifically desired.

**UPVC** is significantly lower cost and suitable for projects where budget is a priority — section sizes are larger, which slightly reduces the glass area achievable.

• **Rooflight specification:**
• For flat roof rooflights, key specification choices are:
• •**Opening vs. fixed:** Opening rooflights add ventilation (often required for habitable rooms) but add cost
• •**Frameless vs. framed:** Frameless (flush) rooflights are more expensive but more attractive and lower profile
• •**Double vs. triple glazed:** Triple glazed is significantly more thermally effective but heavier and more expensive; double is standard for most residential flat roof rooflights