Windows are one of the most consequential decisions in any loft conversion. Get them right and the space is flooded with natural light, properly ventilated, and code-compliant for fire escape. Get them wrong and you end up with a gloomy room that fails Building Regulations sign-off — or an expensive retrofit after the fact.
This guide covers every window type used in UK loft conversions, with 2026 costs, sizes, and the regulatory requirements you cannot ignore.
Window Types Used in Loft Conversions
There are four main window types, each suited to different conversion approaches and planning contexts.
Roof Windows (Skylights / Rooflights)
A roof window sits flush within the existing roof plane, mounted between or alongside the rafters with no structural box or dormer surround. This is the defining feature of a Velux-style conversion (Velux is a brand name that has become generic; FAKRO and Keylite are the main competitors).
Roof windows are the cheapest way to admit light to a loft and the least visually disruptive from outside. They are ideal when:
- Ridge height is sufficient (≥ 2.2 m) but a dormer is not wanted or permitted
- The loft sits in a conservation area where dormers are refused
- Budget is the primary constraint
Standard sizes range from 550 × 780 mm (VS B02) up to 1,140 × 1,180 mm (VS SK06). The most popular size for loft conversions is 780 × 980 mm — large enough to satisfy fire-egress requirements when installed correctly.
Supply-and-fit cost per unit: £600–£1,200 for a standard top-hung or centre-pivot unit, including flashing kit, structural lintel above the opening, and plastered reveal. Solar-powered electrically operated units add £200–£400.
Dormer Windows
A dormer window is set within a dormer box that projects from the main roof slope, creating a vertical face in which the window is mounted. It gives significantly more headroom and light than a roof window because the window can be much larger and is positioned at wall height rather than overhead.
In a dormer conversion, the windows across the dormer’s front face are standard timber or uPVC casement or sash windows, sized and specified like any wall window. The dormer structure is what costs money, not the window itself (standard window units: £400–£900 supply only; £800–£1,600 fitted).
When people ask about dormer window cost, they usually mean the whole dormer including structural works. A single-window rear dormer adding one opening in a new dormer box costs £8,000–£15,000 including all structural, roofing, and plastering works. A full-width rear dormer (which might have two or three windows) costs £28,000–£50,000 as a complete conversion.
Juliet Balconies and Roof Windows with Balcony
A Juliet balcony (sometimes called a French balcony) is a fixed glass balustrade mounted immediately in front of a full-height window or door. There is no projecting platform — you cannot step outside. The window/door opens inward and the balustrade prevents falling. It admits more light than a standard window and provides ventilation without adding any roof projection.
The roof window with balcony (Velux Cabrio and similar products) is a combined unit where the lower half of the roof window folds down to create a small protruding balcony — typically 650–750 mm deep, with a folding balustrade. It is permitted development in most cases (as it folds flat when closed) and requires no structural dormer box.
Search volume for “roof window with balcony” is notably high because these units offer a middle ground: more light and ventilation than a fixed roof window, less structural work and cost than a full dormer, and visual impact that appeals to many homeowners.
| Window Type | Supply & Fit (per unit) | Planning Required? | Structural Works Needed? |
|---|---|---|---|
| Standard roof window (fixed) | £600–£1,200 | No (PD) | Minor (lintel only) |
| Solar/electric roof window | £850–£1,600 | No (PD) | Minor |
| Roof window with balcony (Cabrio-type) | £2,500–£4,500 | Usually no (PD) | Minor |
| Juliet balcony (wall-mounted in dormer) | £1,800–£3,500 inc. balustrade | No (within PD dormer) | Dormer required |
| Dormer window (within existing dormer) | £800–£1,600 | Within PD dormer | Dormer required |
Conservation Rooflights
For conservation areas or listed buildings where standard roof windows are too prominent, conservation rooflights are available. These sit closer to the roof plane (typically 60–80 mm above the tile surface versus 100–150 mm for standard units), have slimmer frames, and are designed to be less visible from street level. Brands include Keylite Conservation and the Velux Conservation range. Cost is approximately 30–50% higher than equivalent standard units.
Building Regulations Requirements
Fire Egress (Part B)
Every habitable room in a loft conversion must have a means of escape in case of fire. For loft conversions, this typically means:
- At least one window in each room (or on each level served by the stair) that provides a clear opening of at least 0.33 m², with a minimum clear dimension of 450 mm in any direction
- The sill height must not exceed 1,100 mm above floor level so an occupant can climb out
- The window must be openable from the inside without a key
Most 780 × 980 mm roof windows provide sufficient egress when a centre-pivot unit is fully opened — the clear opening is approximately 0.45 m², satisfying the 0.33 m² minimum. Check the manufacturer’s data sheet for the actual clear opening area; it varies by model and hinge type.
For dormer windows, any standard casement window of 650 mm × 750 mm opening size will satisfy egress requirements.
[!warning] Loft conversions on three or more storeys Where the loft conversion creates a third storey (ground floor + first floor + loft), Part B requires a protected stairway — a fire-rated enclosure from the loft level down to a final exit. This significantly increases the cost and complexity of the project. If in doubt, engage a fire engineer or Building Control surveyor at the design stage.
Thermal Performance (Part L)
Windows and roof lights in new loft conversions must achieve a minimum energy performance:
- Windows and doors: maximum U-value of 1.4 W/m²K (whole window, not just the glazing)
- Roof windows: maximum U-value of 1.4 W/m²K
Most modern double-glazed roof windows achieve 1.0–1.2 W/m²K; triple-glazed units reach 0.7–0.9 W/m²K and are worth specifying in north-facing lofts or where heat loss is a concern.
Ventilation (Part F)
Habitable loft rooms require background ventilation (trickle ventilators built into window frames, typically 4,000–8,000 mm² equivalent area) and rapid ventilation (the openable area of windows should equal at least 1/20th of the floor area). Both are easily satisfied by standard roof windows of the sizes described above.
Bathrooms require extract ventilation: an extractor fan rated at 15 l/s or a heat-recovery ventilation (HRV) unit. Velux and FAKRO produce combination roof window/ventilation units for bathrooms that extract through the roof window frame — useful where installing separate ductwork through the roof structure is difficult.
Sizing and Positioning Advice
Position roof windows to avoid obstruction by the ridge board or purlins. Standard rafter spacing in Victorian houses is 400–450 mm, allowing roof windows up to about 550 mm wide between rafters. Wider windows (780 mm+) require cutting a rafter and installing a structural trimmer — a straightforward carpentry task that adds £300–£600 but should be specified and approved by your structural engineer.
For the best daylight quality, south-facing rooflights maximise winter solar gain. North-facing rooflights give consistent, even light good for studios or offices. East and west-facing lights give strong morning or afternoon sun — pleasant but can cause glare and overheating in summer without external blinds (add £150–£350 per window for integral or external venetian blinds).
External solar-reflective blinds dramatically reduce summer overheating in south and west-facing roof windows and are worth including in the budget from the outset. Integral blinds (mounted between the panes) are lower maintenance than external systems.
The combination of a well-specified roof window with a balcony unit on the rear slope, complemented by fixed rooflights on the front slope, is an increasingly popular solution that maximises light and ventilation while keeping structural costs low — and stays within permitted development in the vast majority of cases.
