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:

  • There is enough head height (a common rule of thumb is at least 2.2 m, measured from the top of the joists in the loft to the underside of the ridge) but a dormer is not wanted or permitted1
  • The house is in a conservation area, where a roof dormer is not permitted development and needs planning permission2
  • Budget is the primary constraint

Standard sizes range from 550 × 780 mm (CK02) up to 1,340 × 1,400 mm (UK08).34 A common size for loft conversions is 780 × 980 mm (MK04).3 Whether a particular model and size can serve as an escape window depends on how it opens, so check the manufacturer’s data (see Fire Egress below).

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. Approved Document K gives 1,100 mm as the guarding height for external balconies, including Juliet balconies.5 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, with a folding balustrade. It needs no structural dormer box, but whether it is permitted development is less clear-cut than for a standard roof window: permitted development for rooflights is lost if the alteration protrudes more than 0.15 m beyond the plane of the roof slope, and the open balcony projects well beyond that.6 Check with the council or get a lawful development certificate first.

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 TypeSupply & Fit (per unit)Planning Required?Structural Works Needed?
Standard roof window (fixed)£600–£1,200No (PD)Minor (lintel only)
Solar/electric roof window£800–£1,600No (PD)Minor
Roof window with balcony (Cabrio-type)£2,500–£4,500Check with councilMinor
Juliet balcony (wall-mounted in dormer)£1,800–£3,500 inc. balustradeNo (within PD dormer)Dormer required
Dormer window (within existing dormer)£800–£1,600Within PD dormerDormer required

Conservation Rooflights

For conservation areas or listed buildings where standard roof windows are too prominent, conservation rooflights are available. These sit lower in the roof, closer to flush with the covering than standard units, have slimmer frames, and are designed to be less visible from street level.7 Brands include Keylite Conservation and the Velux Conservation range. Expect them to cost more than equivalent standard units.

Building Regulations Requirements

Fire Egress (Part B)

An escape window is only the fire strategy for a floor no more than 4.5 m above ground, which a loft on a typical two-storey house is not.8 There, the loft rooms are served by a protected stairway instead (see the warning below), and the window rules that follow apply where an escape window is the route, such as a bungalow conversion:

  • An escape window in each habitable room (two rooms can share one if a door between them gives access to it without passing through the stair enclosure, and each room has its own access to the stair), with an unobstructed openable area of at least 0.33 m², at least 450 mm high and 450 mm wide8
  • The sill height must not exceed 1,100 mm above floor level so an occupant can climb out8
  • Locks (even with removable keys) and child-resistant opening stays may be fitted, but the window must be able to stay open without being held8

A centre-pivot roof window’s sash turns about the middle of the opening, so escape windows are normally top-hung or dedicated escape models: VELUX, for example, marks the top-hung GPU sizes that suit emergency escape in its product data.9 Check the manufacturer’s data sheet for the model and size you intend to use; suitability varies by model and hinge type.

For dormer windows, a casement with a clear openable area of 650 mm × 750 mm (about 0.49 m²) meets the area and dimension rules, provided the bottom of the opening is no more than 1,100 mm above the floor.8

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.8 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)10
  • Roof windows: maximum U-value of 1.4 W/m²K, assessed in the vertical position10
  • Rooflights (flat or low-pitch units such as lanterns): maximum U-value of 2.2 W/m²K, assessed in the horizontal position10

VELUX’s centre-pivot GGU, for example, is rated at 1.3 W/m²K with double glazing and 0.83–1.1 W/m²K with triple glazing.4 Triple glazing is 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, 8,000 mm² equivalent area for a habitable room, 4,000 mm² for a bathroom) and purge ventilation (an openable window area of at least 1/20 of the floor area if the window opens 30° or more, or 1/10 if it opens only 15–30°).11 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.11 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. Rafters at 400–450 mm centres, common in older houses, leave a clear gap of only about 350–400 mm, narrower than even the smallest standard roof window, so most roof windows (and every 780 mm unit) mean 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; confirm the balcony unit’s planning position before ordering.

The rules in this note are those that apply in England. Scotland, Wales and Northern Ireland have their own building standards and permitted development rights.

References (11)

Figures in this note were checked against the sources below on 2 October 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

  1. 1technicalfairFederation of Master Builders, guidance.RN-MK8GBNThe builders' trade body's homeowner guide to loft conversions; it gives 2.2 m as the working head-height figure and says how to measure it.
  2. 2technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-KHYKNAClass B permits roof enlargements provided the roof space does not increase by more than 40 cubic metres for a terrace house or 50 cubic metres in any other case (B.1(d)), no part exceeds the height of the highest part of the existing roof (B.1(b)) and the work does not extend beyond the plane of any roof slope forming the principal elevation that fronts a highway (B.1(c)). The right is withdrawn on article 2(3) land, including conservation areas, and for houses created under Classes G, M, MA, N, P, PA or Q (B.1(a), (f) to (h)). Conditions in B.2 require materials of similar appearance, an enlargement set back not less than 0.2 metres from the eaves measured along the roof slope, and side-elevation windows that are obscure-glazed and non-opening unless the opening part is more than 1.7 metres above the floor.
  3. 3technicalsupplier figureVELUX.RN-HF2FVEThe manufacturer's own size code table, the only reliable source for what a Velux code means. Pitched roof window codes and dimensions include FK06 at 66cm by 118cm, MK04 at 78cm by 98cm, MK06 at 78cm by 118cm, MK08 at 78cm by 140cm, PK06 at 94cm by 118cm, PK08 at 94cm by 140cm, SK06 at 114cm by 118cm, SK08 at 114cm by 140cm, UK04 at 134cm by 98cm and UK08 at 134cm by 140cm. Flat roof window sizes run from 60cm by 60cm up to 150cm by 150cm and 200cm by 100cm. The first letter pair gives the width band and the digits the height, which is the pattern a size-code note should explain.
  4. 4technicalsupplier figureVELUX, a data sheet.RN-WXE894VELUX product information for the GGU centre-pivot polyurethane roof window, edition 3.0, July 2020. Gives the size codes and their actual dimensions, the U-values by glazing unit, the ventilation flap and filter, and the roof pitch range the window is made for. The manufacturer's own data, and the source behind any statement about what a given VELUX size code physically measures.
  5. 5technicalvery strongMinistry of Housing, Communities and Local Government.RN-XH6E39The standard that decides whether a loft ladder is legal and when a raised deck needs a balustrade. Published 3 January 2013, it covers protection from falling, collision and impact including stairs, ladders, ramps, guarding and vehicle barriers. Paragraph 1.31 states that retractable ladders must not be used as a means of escape. Paragraph 1.32 permits a fixed ladder, with fixed handrails on both sides, only for access in a loft conversion containing one habitable room, and only where there is not enough space without altering the existing space for a compliant stair, which is the rule that catches most loft ladder installations. Paragraph 1.29 restricts alternating tread stairs to loft conversions. For a private stair the maximum pitch is 42 degrees (Table 1.1 note 1), the normal relationship between rise and going is twice the rise plus the going between 550mm and 700mm, and for dwellings external tapered steps forming part of the building need a going of at least 280mm. Paragraph 3.1 requires guarding wherever it is reasonably necessary for safety at the edge of a floor, gallery, balcony, roof, light well, basement or similar sunken area. Paragraph 3.2 requires guarding to be at least the height in Diagram 3.1, allows any wall, parapet or balustrade to serve as guarding, and requires it to resist the loads in BS EN 1991-1-1 with its UK National Annex and PD 6688-1-1, with BS 6180 for barrier and infill panel design. Handrails should be positioned 900mm to 1000mm from the pitch line or floor.
  6. 6technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-5QRXP7Class C permits any other alteration to the roof of a dwellinghouse, which is the right rooflight and Velux loft conversions rely on. Paragraph C.1(b) makes development not permitted if the alteration would protrude more than 0.15 metres beyond the plane of the roof slope, measured perpendicular to the original roof plane, and C.1(c) if the highest part of the alteration would be higher than the highest part of the original roof. C.1(d) excludes chimneys, flues, soil and vent pipes and solar equipment from the class, and condition C.2 requires any window in a roof slope forming a side elevation to be obscure-glazed and non-opening below 1.7 metres above the floor of the room.
  7. 7technicalsupplier figureVELUX, guidance.RN-INIP52VELUX's UK conservation roof window page describes slim black profiles designed with conservation officers and a finish that sits flush to the slates, which settles the lower, slimmer profile of conservation rooflights.
  8. 8technicalvery strongMinistry of Housing, Communities and Local Government.RN-J0UZDBApproved Document B Volume 1, fire safety in dwellings, 2019 edition with the 2020, 2022 and 2025 amendments and the forthcoming 2026 and 2029 changes collated in. Covers means of warning and escape, internal fire spread through linings and structure, external fire spread and access for the fire service. This is the document behind escape window sizes, protected stairways, the 4.5 metre rule and the fire doors a loft conversion needs.
  9. 9technicalsupplier figureVELUX, guidance.RN-ORUQSDThe manufacturer's data sheet for its top-hung GPU roof window; its size chart flags which sizes suit emergency escape, which its centre-pivot GGU sheet does not.
  10. 10technicalvery strongMinistry of Housing, Communities and Local Government.RN-L0FCJXThe in-force U-value tables a DIY insulation or door note must quote, with the exact numbers. Table 4.2, limiting U-values for new fabric elements in existing dwellings, sets roof 0.15, wall 0.18, floor 0.18, swimming pool basin 0.25, window 1.4 or Window Energy Rating Band B minimum, rooflight 2.2, doors with more than 60 per cent of the internal face glazed 1.4 or Doorset Energy Rating Band C minimum, and other doors 1.4 or Doorset Energy Rating Band B minimum. Table 4.3, for existing elements, gives a roof threshold of 0.35 and an improved value of 0.16, which is the figure a loft insulation note needs: paragraph 4.12 says a retained element whose U-value is higher than the column (a) threshold, for example through a loft or garage conversion, should be upgraded to the column (b) value. Paragraph 4.13 allows a lesser upgrade only where the improved value is not technically or functionally feasible or would not achieve a simple payback of 15 years or less. Paragraph 4.8 requires a replacement element to be no worse than the one it replaces as well as meeting Table 4.2. Paragraph 4.10 gives the heritage exception: where character must be maintained, a centre pane U-value of no more than 1.2 or low-emissivity secondary glazing. Note 9 records that the timber window concession of 1.6 or Band C ended on 14 June 2023. The 2021 edition came into force on 15 June 2022; a 2026 edition exists at https://assets.publishing.service.gov.uk/media/69c122a6cfa346b9d4704a55/ADL1_2026.pdf.
  11. 11technicalvery strongMinistry of Housing, Communities and Local Government.RN-QD1SFFTable 1.1 sets minimum intermittent extract rates: kitchen with a cooker hood extracting outside 30 l/s, kitchen without one 60 l/s, utility room 30 l/s, bathroom 15 l/s, sanitary accommodation 6 l/s. Extract terminals and fans other than cooker hoods should be as high as practicable and no more than 400mm below the ceiling; a cooker hood should sit between 650mm and 750mm above the hob if the manufacturer gives no figure. Table 1.7 requires background ventilators of at least 8000mm2 equivalent area in habitable rooms and kitchens for a dwelling, 10,000mm2 for a single-storey dwelling. Background ventilators should be at least 1700mm above floor level and at least 500mm from a fan in the same room. On replacement windows, paragraph 3.14 says new windows must carry background ventilators no smaller than the originals, and paragraph 3.15 gives the fallback minimums where the old size is unknown: 8000mm2 habitable rooms, 8000mm2 kitchen, 4000mm2 bathroom. Paragraph 3.32 confirms that replacing an extractor fan or cooker hood on existing cabling is not notifiable.

  1. Loft conversions: The ultimate guide, Federation of Master Builders, accessed 30 September 2026. fmb.org.uk

  2. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class B, The National Archives (legislation.gov.uk), accessed 30 September 2026. legislation.gov.uk

  3. VELUX Roof window sizes - Find the right fit for your home, VELUX, accessed 7 September 2026. velux.co.uk

  4. Product information: VELUX Centre-pivot polyurethane roof window GGU, Edition 3.0, 01.07.2020, VELUX, accessed 7 September 2026. resources.velux.co.uk

  5. Approved Document K: Protection from falling, collision and impact, 2013 edition, HM Government (GOV.UK), accessed 30 September 2026. assets.publishing.service.gov.uk

  6. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class C, The National Archives (legislation.gov.uk), accessed 30 September 2026. legislation.gov.uk

  7. VELUX Conservation roof windows, VELUX, accessed 30 September 2026. velux.co.uk

  8. Approved Document B (fire safety) volume 1: Dwellings, 2019 edition incorporating 2020, 2022, 2025 and 2026 amendments, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  9. Product information: VELUX Top-hung polyurethane roof window GPU, Edition 3.0, 01.07.2020, VELUX, accessed 30 September 2026. resources.velux.co.uk

  10. Approved Document L: Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk

  11. Approved Document F: Ventilation, Volume 1: Dwellings, 2021 edition, HM Government (GOV.UK), accessed 7 September 2026. assets.publishing.service.gov.uk