Roof Lanterns for London Extensions: Types, Costs, Structural Requirements and What to Specify

**What is a roof lantern?**

A roof lantern (also called a pyramid rooflight or raised rooflight) is a glazed structure that sits on top of a flat roof, projecting upward above the roof line and incorporating multiple glazed faces. It allows natural light to enter from directly above and from multiple angles, providing excellent daylight distribution — particularly in deep-plan extensions where side glazing alone cannot reach the full internal depth.

• Roof lanterns are distinct from:
• •*Flat rooflights* (walk-on or non-walk-on flush-fitting rooflights installed within the flat roof plane — not projecting above the roof surface)
• •*Frameless structural glass roofs* (all-glass structural systems with minimal visible framing)
• •*Glazed ridge roofs* (pitched glazed roofs with a central ridge — common in orangeries and larger conservatories)

For a standard London single-storey rear extension of 15–25m², a roof lantern (projecting above the flat roof) or a flat rooflight (flush with the roof plane) is typically more appropriate than a glazed ridge roof.

**Roof lantern types by construction system**:

• *1. Aluminium thermally broken roof lanterns*:
• The standard specification for London residential extensions. Aluminium thermally broken systems:
• •Use a polyamide (nylon) thermal break within the aluminium frame to reduce heat transfer between the cold outer frame and the warm inner frame
• •Achieve a centre-pane U-value of ≤1.0 W/m²K (double-glazed) or ≤0.8 W/m²K (triple-glazed)
• •Whole-unit U-value typically 1.4–2.0 W/m²K (the frame itself has a higher U-value than the glass)
• •Available in standard sizes from leading manufacturers (CGI, Ultrasky, Roof-Maker, Atlas); also available as bespoke sizes for non-standard openings
• •Powder-coated in a wide range of RAL colours — anthracite grey (RAL 7016) is the dominant specification choice in London residential extensions 2023–2025
• •Low maintenance; durable; aesthetically versatile

• *2. Steel roof lanterns*:
• Steel lanterns allow finer frames and larger spans than aluminium, while maintaining thermal performance through thermal breaks. Steel lanterns have a more industrial aesthetic — popular in higher-end London residential projects and in Heritage or conservation-area contexts where the thinner sightlines feel more appropriate.
• •More expensive than aluminium (typically 40–80% more for a comparable size)
• •Longer lead times (8–14 weeks compared to 4–8 weeks for aluminium)
• •Available in custom powder coat finishes including specialist heritage colours

• *3. Timber roof lanterns*:
• Oak, accoya, or engineered timber lanterns — a premium option with a warmer aesthetic. Suitable where a natural material finish is preferred (higher-end refurbishments; Arts and Crafts style properties).
• •Higher cost; longer lead times; more maintenance than aluminium (periodic re-oiling or repainting)
• •Timber lanterns are more susceptible to movement than aluminium — more important that the structural opening is accurate

*4. Composite roof lanterns*: Timber inner face / aluminium outer skin — combining the warm aesthetic of timber internally with the low-maintenance aluminium external finish. Available from specialist manufacturers; typically between the cost of pure timber and steel.

**Flat rooflights vs. projecting roof lanterns — which is appropriate?**

• A flat rooflight (flush-mounted) is appropriate where:
• •The extension is used as a bedroom or bathroom above (flat rooflights provide a less dominant visual presence)
• •The extension roof pitch is very shallow (less than 5° — insufficient slope for a projecting lantern to drain effectively)
• •The local planning authority or conservation area requires a lower roof profile
• •The budget does not support a roof lantern

• A projecting roof lantern is appropriate where:
• •The extension is used as a kitchen/diner or living space (maximum daylight impact is desired)
• •The flat roof has sufficient structural capacity to support the lantern frame and its live and snow loads
• •There is no planning or conservation area restriction on the raised profile

For most London single-storey rear extension kitchen/diner extensions, a projecting roof lantern is the preferred specification — the daylight it introduces transforms the feel of the space compared to a flush rooflight of the same glass area.

**Standard sizes and the importance of planning the lantern size at design stage**:

Roof lanterns are available in standard modular sizes from leading manufacturers — typically in 150mm increments (e.g., 1500mm × 1500mm; 1500mm × 2500mm; 2000mm × 3000mm; 2500mm × 4000mm). Bespoke sizes are available but incur a cost premium of 20–40% and longer lead times.

The size of the roof lantern opening is a structural element of the flat roof — it must be designed into the structural engineer's roof layout from the start. A steel header beam around the lantern opening transfers the roof loads around the void. If the lantern size is changed after the roof frame is built, the structural opening must be modified — expensive and disruptive.

*Rule of thumb for lantern sizing*: the lantern glass area should be at least 10–15% of the floor area of the room it illuminates for good daylighting. For a 20m² extension kitchen/diner, this suggests a lantern glass area of at least 2–3m² — achievable with a 1500mm × 2000mm lantern (3m² glass area) or larger.

**Building Regulations Part L — the 25% glazing limit and SAP calculation**:

Building Regulations Part L (Conservation of Fuel and Power) requires that extensions comply with the energy efficiency requirements of Approved Document L (ADL). For a London residential extension, Part L requires:

• •Maximum 25% of the total floor area of the extension in fixed roof or overhead glazing (rooflights, roof lanterns, glazed roofs). Glazing above 25% of floor area requires a SAP calculation to demonstrate that the extension's total heat loss is no worse than it would be with 25% overhead glazing at the maximum U-value
• •Maximum whole-unit U-value for roof lanterns: **1.6 W/m²K** (centre-pane U-value for the glass: typically ≤1.0 W/m²K for double-glazed; ≤0.7 W/m²K for triple-glazed)
• •Maximum roof U-value for the flat roof area: **0.15 W/m²K** (warm deck PIR insulation)
• •Solar heat gain coefficient (SHGC / g-value) should be ≤0.35 to limit summer overheating

*The 25% rule in practice for a London extension*:

For a 20m² extension, the maximum overhead glazing under the standard Part L 25% limit is 5m² — which is achieved by a single 1500mm × 3000mm roof lantern (4.5m² glass area) plus a small remote rooflight (0.5m² glass area), or a single larger 2000mm × 2500mm lantern (5m² glass area).

Where the homeowner wants a larger roof lantern (or multiple rooflights), the SAP calculation route allows this — but the SAP calculation must demonstrate that the extension as a whole complies with Part L, which typically means increasing wall and roof insulation to compensate for the additional heat loss through the extra glazing.

**Glazing specification for roof lanterns — what to include**:

• *Glass type*:
• •Double-glazed unit (DGU): two panes of glass with an argon gas-filled cavity and warm-edge spacer. Standard specification for most London residential extensions. Centre-pane U-value: typically 1.0–1.2 W/m²K
• •Triple-glazed unit (TGU): three panes of glass with two gas-filled cavities. Higher thermal performance (centre-pane U-value: 0.6–0.8 W/m²K) but heavier (more structural support required), more expensive (+30–50%), and less light transmittance. Recommended for high-performance or Passivhaus standard extensions

*Low-e coating*: A low-emissivity (low-e) coating on the inner pane of the DGU reduces heat loss by reflecting radiant heat back into the room. Standard on all quality roof lanterns — must be specified correctly (coating position: position 3 in a DGU = inner face of inner pane, facing room)

*Solar control glass*: For roof lanterns, solar control glass (with a low g-value of ≤0.35) is strongly recommended to reduce summer overheating. London extensions with south-facing roof lanterns and inadequate solar shading can become uncomfortably hot in summer — particularly kitchen extensions where cooking appliances add additional heat gain. Solar control glass can significantly reduce this.

Common solar control glazing products for roof lanterns: Pilkington Activ Blue (self-cleaning + solar control); Guardian SunGuard; AGC Stopray Ultra. These products are available in standard DGU configurations and can be specified within most roof lantern systems.

*Self-cleaning glass*: For roof lanterns, self-cleaning glass is highly recommended — the glass is largely inaccessible for regular cleaning and accumulates bird deposits and atmospheric dirt rapidly. Self-cleaning glass uses a titanium dioxide photocatalytic coating activated by UV light to break down organic deposits, which are then washed away by rain. The most widely used product is Pilkington Activ, which is available as a stand-alone float glass (for DGUs with self-cleaning on the outer pane) or combined with solar control properties.

**Overheating risk and ventilation for roof lanterns**:

Overheating is a significant concern for London flat-roof extensions with roof lanterns — particularly in the summer months. Building Regulations Part O (Overheating) applies to new residential extensions and requires that the thermal comfort of the space is assessed and overheating risk managed.

The key strategies for managing overheating in a roof lantern extension:

*1. Solar control glazing*: specified above — reduces the solar heat gain through the lantern glass (g-value ≤0.35 recommended)

*2. Opening vents in the roof lantern*: Most quality roof lanterns are available with integral electric opening vents — motorised vents within the lantern frame that open automatically when internal temperature exceeds a set threshold. These provide both ventilation (air movement) and stack-effect cooling (hot air exits through the high-level vent; cooler air is drawn in at lower level). Electric opening vents typically add £300–£600 to the lantern cost depending on size.

*3. External shading*: External shading (a solar blind or canopy over the roof lantern) is more effective than internal blinds at reducing solar gain — external shading intercepts the solar radiation before it enters the glass. However, external shading for a roof lantern is complex and expensive. Internal blinds (pleated or roller, operated manually or motorised) provide partial benefit (reducing glare; providing some insulation in winter) but do not stop solar gain effectively.

*4. Cross-ventilation*: Ensuring that the extension has effective cross-ventilation (bi-fold doors to the garden + opening roof lantern vents) allows passive cooling through air movement on warm days.

**Structural requirements for a roof lantern opening**:

A roof lantern requires a structural opening in the flat roof — a rectangular void in the roof structure with steel or timber header beams spanning across the void to carry the roof loads around it. The structural requirements for the lantern opening are:

*1. Header beams around the opening*: For a roof lantern opening in a timber flat roof frame, the structural engineer will specify a steel or solid timber header (trimmer beam) on each side of the opening — transferring the joist loads around the void. For small lanterns (up to 1500mm wide), timber trimmers of appropriate size may be sufficient. For larger openings (2000mm+), steel UB or UC sections are typically required.

*2. Kerb height*: The roof lantern sits on a structural kerb — a vertical upstand built around the lantern opening. The minimum kerb height for a roof lantern in a UK climate is typically 150mm above the waterproof membrane level — to prevent rainwater from being driven into the lantern frame joint and to allow the membrane to be turned up at the kerb. Roof lantern manufacturers specify their required kerb height and construction method — follow the manufacturer's guidance precisely.

*3. Structural loading from the lantern*: The roof lantern imposes additional dead loads (its own weight), wind loads (suction and uplift), and snow loads on the structural kerb and the surrounding roof structure. For large lanterns (2500mm × 4000mm and above), the wind and snow loads can be significant — the structural engineer must confirm that the roof structure is adequate for the lantern loads. For standard residential applications with aluminium lanterns up to 2000mm × 3000mm, the structural loading is typically manageable within a conventional timber flat roof frame with appropriate trimmer sizing.

**Roof lantern costs in London (2025)**:

| Size | System | Supply cost | |---|---|---| | 1500mm × 1500mm | Aluminium DGU | £1,200–£2,000 | | 1500mm × 2500mm | Aluminium DGU | £1,800–£3,000 | | 2000mm × 3000mm | Aluminium DGU | £2,800–£4,500 | | 2500mm × 4000mm | Aluminium DGU | £4,500–£7,000 | | 2000mm × 3000mm | Aluminium TGU (triple) | £4,000–£6,500 | | 2000mm × 3000mm | Steel DGU | £5,000–£9,000 | | Bespoke size / custom | Any system | +20–40% premium |

Installation cost (London, over and above supply): £600–£1,500 depending on size and access complexity. Total installed cost for a 2000mm × 3000mm aluminium DGU lantern in a London extension: £3,400–£6,000.

• Additional costs:
• •Electric opening vents: £300–£600 per vent
• •Solar control glass (g-value ≤0.35 glazing): typically included in the product spec at this price range or available as an option
• •Self-cleaning outer pane: typically included in quality products or £150–£300 upgrade
• •Internal blind: £300–£700 motorised; £150–£350 manual

**Lead times for London roof lanterns**:

Standard aluminium roof lanterns from UK manufacturers (Ultrasky, CGI, Atlas, Roof-Maker): 4–8 weeks from order placement. Steel or timber lanterns: 8–16 weeks. Bespoke sizes: 8–14 weeks.

Roof lanterns must be ordered well in advance of the installation date — ideally at or before the structural frame is installed (so the opening dimensions can be confirmed before the kerb is built). Ordering a roof lantern after the roof frame and kerb are complete risks the lantern dimensions not matching the built opening.

*Lead time planning*: for a 16-week extension programme, the roof lantern should be ordered at Week 3–4 (when the structural frame dimensions are confirmed) to arrive by Week 10–12 (when the roof is weathertight and ready for lantern installation). A delayed lantern order holding up the weathertight roof is a common cause of programme overrun on London extensions.

**Planning implications for roof lanterns**:

• •For a rear extension under permitted development: a projecting roof lantern on the flat roof is PD as part of the extension (the extension's overall height is measured to the top of the lantern, not the flat roof level — the lantern must be within the 3m/4m PD height limit)
• •In conservation areas: roof lanterns on rear extensions (not visible from the street) are generally acceptable even in conservation areas; front-visible roof additions typically require planning permission
• •For larger extensions requiring planning permission: the roof lantern is specified in the planning application drawings. Some London Boroughs may request solar control glass specification or limit the size of roof lanterns as a planning condition