A roof window transforms a dark loft space into a bright, usable room. Whether you are converting a loft, replacing a dated skylight, or adding natural light to a top-floor extension, the roof window market, dominated but not limited to Velux, offers a wide range of options with different opening mechanisms, glazing specifications, and price points.

This guide covers the standard sizes available in the UK, what installation typically costs, when you need planning permission, and how to choose the right window for your roof.

Velux and Its Competitors

Velux is the dominant brand in the UK roof window market and their product codes have effectively become the shorthand for the entire category, much like “hoover” for vacuum cleaners. Other reputable manufacturers include FAKRO, RoofLITE, and DAKEA, all of which make broadly the same sizes and are often cheaper than equivalent Velux units for a similar specification.

All major brands are available through builders’ merchants and roofing suppliers. Velux roof windows carry a 10-year guarantee; competitors’ guarantee periods vary, so compare the terms.1

Standard Sizes

Velux uses an alphanumeric coding system where the letters denote width and the numbers denote height; competitors use their own codes but make broadly the same sizes.2 Most of these sizes are wider than the gap between rafters, so fitting one usually means trimming the roof.

Velux CodeWidth (cm)Height (cm)Typical Use
CK025578Small utility rooms, stairwells
CK045598Compact bedrooms, bathrooms
MK047898Standard bedroom, most popular size
MK0678118Bedrooms, sitting rooms
MK0878140Living spaces, good egress
PK049498Wide, lower-pitch installations
PK0894140Large bedrooms, loft sitting rooms
SK06114118Feature windows, south-facing orientations
SK08114140Maximum daylight, large spaces

The MK04 (78×98 cm) is the workhorse of UK loft conversions. At 780 mm it is wider than the roughly 550 mm clear gap between rafters at 600 mm centres, so it needs a rafter trimmed, which is routine carpentry. The Building Regulations set no minimum glazed area for daylight in a habitable room in England. What Approved Document F does ask for is purge ventilation, and where that is delivered by a hinged or pivot window opening 30 degrees or more the openable area should be at least 1/20th of the floor area of the room.3 In an extension, Approved Document L also caps the total area of windows, rooflights and doors at 25% of the floor area plus any openings removed, unless a calculation justifies more.4

Opening Types

Centre-pivot: The most common type in the UK. The sash pivots on a horizontal axis at the centre of the frame, allowing the top of the window to swing inwards for cleaning and ventilation. The sash rotates 180° so the outer pane can be cleaned from inside.5 Velux rates its centre-pivot GGL for roof pitches between 15° and 90°.6

Top-hung: The sash hinges at the top, opening outward like a traditional window. Gives a wider, unobstructed view and a lower internal projection than centre-pivot. Velux rates its top-hung GPL for roof pitches between 15° and 55°, a narrower range than the centre-pivot.7 More expensive than equivalent centre-pivot.

Fixed / non-opening (rooflight): No ventilation function, provides light only. Lower cost, simpler installation, and suitable for areas where ventilation comes from elsewhere. Not acceptable as the sole ventilation source for a habitable room.

Electrically operated: Available in any opening type. Electric or remote operation is the usual choice for windows that cannot be reached by hand, and is available as an upgrade for lower ones. Remote control or app-controlled operation. Adds £150–£350 to the supply cost.

Glazing Options

All current Velux and equivalent products use double glazing as standard.8 Upgrades are available:

  • Triple glazing: whole-window U-value around 0.8–1.1 W/m²K on Velux’s centre-pivot range, against 1.3 W/m²K for its standard double glazing.6 Recommended for north-facing windows, cold climates, or high-performance envelopes. Adds £80–£200 to unit cost.
  • Laminated inner pane: holds the glass together if it breaks so it cannot fall into the room, and every glazing option on Velux’s current centre-pivot window includes one; a thicker laminated pane adds security.65
  • Solar control glazing: Limits solar gain, useful for south-facing roof windows in rooms prone to overheating. Reduces light transmission slightly.
  • Privacy (obscured) glazing: For bathroom roof windows where overlooking is a concern, though most bathroom roof windows are too high to be overlooked anyway.

Approved Document L, Volume 1 (2021 edition) caps windows, including roof windows, at a U-value of 1.6 W/m²K in new dwellings (Table 4.1) and at 1.4 W/m²K (or Window Energy Rating Band B) when fitted new or as replacements in an existing dwelling, which covers a loft conversion (Table 4.2); out-of-plane rooflights are limited to 2.2 W/m²K in both cases, and roof windows are assessed in the vertical plane.4

Flashings

A flashing kit seals the junction between the window frame and the roof covering, it is not optional and must be bought to match the roof tile profile and pitch.9 Velux and competitors offer specific flashing kits for:

  • Plain tile (standard, low-profile)
  • Interlocking concrete tile (deep profile)
  • Natural slate
  • Single-ply membrane / flat roof upstand
  • Corrugated metal / fibre cement

The wrong flashing causes water ingress: always specify the flashing kit at the same time as the window unit. Flashing kits cost £90–£220 depending on size and tile type.10

Cost to Supply and Fit

Supply Costs (mid-2026)

WindowVelux (Supply Only)Competitor Equiv.
MK04 centre-pivot, double glazing£400–£520£280–£380
MK06 centre-pivot, double glazing£480–£600£320–£430
PK08 top-hung, double glazing£600–£780£400–£550
SK08 centre-pivot, triple glazing£750–£950£520–£700
MK04 electric, double glazing£650–£800£480–£620

Flashing kit: add £90–£220 per window.

As a check on the table, a Velux GGL MK04 3070 centre-pivot window was listed at £414 including VAT by a roofing merchant in September 2026.1112

Installation Costs

ScenarioCost Per Window
Like-for-like replacement (existing opening, matching tile)£300–£450
New installation (cut new opening, new flashing, make good)£450–£700
New installation requiring structural work (rafter trimming)£600–£1,000
Electric window with new cable from consumer unitadd £150–£250

A competent roofer and carpenter typically complete a straightforward replacement in half a day. New openings requiring rafter trimming (cutting and doubling up rafters to create a structural trimmer frame) add a day’s carpentry work.

Planning Permission

Roof windows are generally permitted development in England under the GPDO 2015 (as amended), subject to conditions:13

  • The window must not protrude more than 150 mm above the existing roof plane.
  • The window must not rise above the highest part of the original roof.13
  • A roof window in a roof slope forming a side elevation must be obscure-glazed and non-opening unless the opening part is more than 1.7 m above the floor of the room.13
  • Not permitted on a listed building without listed building consent.
  • Conservation areas: permitted development rights for roof windows are not removed simply by being in a conservation area. The 150 mm protrusion limit applies to every roof slope regardless of what the highway can see, and there is no separate rule for the front.13

In Scotland, Wales, and Northern Ireland, permitted development rules differ, check with your local planning authority before proceeding in these jurisdictions.

Building Regulations

Roof windows in habitable rooms trigger Building Regulations compliance for:

  • Thermal performance (Part L): U-value requirements as above.
  • Ventilation (Part F): The opening area must meet the purge ventilation requirement for the room. Where a new habitable room is added, Approved Document F also looks for background ventilation, and most roof windows have a ventilation flap that acts as an integrated trickle vent, letting air in with the window closed.53
  • Fire safety (Part B): If the window serves as a means of escape from an upper floor, it must have an unobstructed openable area of at least 0.33 m², with a minimum 450 mm clear width and height, and be no more than 1,100 mm above floor level.14 Escape windows only count for storeys up to 4.5 m above ground level.14 A loft conversion that creates a storey above 4.5 m instead needs the whole escape route addressed, with fire-resisting doors of at least E 20.14 Whether a particular roof window qualifies depends on its clear opening, so check the manufacturer’s escape data; top-hung windows are usually chosen because a centre-pivot sash obstructs part of the opening.
  • Structural: Cutting a new opening in a roof requires confirmation that the remaining rafters can carry the load without deflection. On most domestic roofs this is straightforward with proper trimming, but a structural engineer’s sign-off may be needed for large openings or unusual spans.

Fitting a new roof window is building work, so it needs a building control application.15 Replacing an existing roof window is building work too: an installer registered with a competent person scheme such as FENSA or CERTASS can self-certify it, and otherwise it has to be notified to building control.16

References (16)

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. 1technicalsupplier figureVELUX, guidance.RN-1AMCI0VELUX UK's roof window product page. It states the guarantee terms: 10 years for roof windows, 3 years for blinds and 5 years for electrical parts.
  2. 2technicalsupplier 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.
  3. 3technicalvery 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.
  4. 4technicalvery 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.
  5. 5technicalsupplier figureVELUX, guidance.RN-LF9A97VELUX's data sheet for its centre-pivot GGU roof window. It confirms the sash rotates 180 degrees so the outer glass can be cleaned from inside, and lists pitch range and U-value.
  6. 6technicalsupplier figureVELUX, guidance.RN-JLV1GRVELUX's data sheet for its manual centre-pivot GGL roof window. It gives the permitted roof pitch range of 15 to 90 degrees, the sizes and the U-value of each glazing option.
  7. 7technicalsupplier figureVELUX, guidance.RN-8AXP21VELUX's data sheet for its top-hung GPL roof window. It gives the permitted roof pitch range of 15 to 55 degrees, narrower than the 15 to 90 degrees of the centre-pivot GGL.
  8. 8technicalsupplier figureVELUX, guidance.RN-ZWKM6NVELUX's UK product page describes its Glazing 70 double glazing as the standard glazing, with triple glazing offered as an option, which settles that Velux roof windows come double glazed as standard.
  9. 9technicalsupplier figureVELUX, guidance.RN-VBYCOZVELUX's technical library page for its EDW flashing shows that flashings are specific to the roofing material, this one being for profiled tiles up to 120 mm, so the kit must be chosen to suit the roof covering.
  10. 10costsupplier figureLawsons.RN-FTI45WA price, true at 2026-09. A builders' merchant product page for the standard Velux tile flashing for an MK04 window, with its underfelt collar, priced at £121.38 including VAT, within the stated band.
  11. 11costsupplier figureRoofing Outlet.RN-M82LC7A price, true at 2026-09. A roofing merchant's product page for the standard Velux MK04 manual centre-pivot window with laminated double glazing, priced at £414 including VAT, supply only.
  12. 12costsupplier figureJJ Roofing Supplies.RN-KLAX7CA price, true at 2026-09. the retail price JJ Roofing Supplies publishes for Velux GGL Centre Pivot Pine Roof Window, which is the supply figure the cost book records for this item
  13. 13technicalvery 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.
  14. 14technicalvery 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.
  15. 15technicalvery strongThe National Archives, legislation.RN-ZCMRHVRegulation 3 of the Building Regulations 2010, the meaning of building work. Lists what counts: erecting or extending a building, providing or extending a controlled service or fitting, material alteration, work required by a material change of use, and inserting insulation into a cavity wall. If a job is not on this list it is not building work, so this is the first place to check whether approval is needed at all.
  16. 16technicalvery strongThe National Archives (legislation.gov.uk), guidance.RN-BP3IWGSchedule 3 item 12 lets registered installers (APHC, BESCA, Certsure, HETAS, NAPIT) self-certify sanitary installations in a dwelling that involve no shared or underground drainage.

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

  2. VELUX roof window sizes, VELUX, accessed 30 September 2026. velux.co.uk

  3. Approved Document F, Ventilation, Volume 1: Dwellings, 2021 edition, Ministry of Housing, Communities and Local Government, accessed 13 September 2026. assets.publishing.service.gov.uk

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

  5. VELUX roof window GGU, product data sheet, VELUX, accessed 30 September 2026. contenthub.velux.com

  6. VELUX manual centre-pivot roof window GGL, product data sheet, VELUX, accessed 30 September 2026. contenthub.velux.com

  7. VELUX roof window GPL, product data sheet, VELUX, accessed 30 September 2026. contenthub.velux.com

  8. VELUX Conservation roof window, VELUX, accessed 30 September 2026. velux.co.uk

  9. EDW profiled tile flashing for single installation, VELUX, accessed 30 September 2026. velux.co.uk

  10. Velux EDW MK04 2000 Single Tile Flashing Including BDX 78x98, Lawsons, accessed 2 October 2026. Priced at 2026-09 and left there. lawsons.co.uk

  11. VELUX GGL MK04 3070 Pine Centre-Pivot Roof Window (78 x 98 cm), Roofing Outlet, accessed 2 October 2026. Priced at 2026-09 and left there. roofingoutlet.co.uk

  12. Velux GGL Centre Pivot Pine Roof Window, JJ Roofing Supplies, accessed 2 October 2026. Priced at 2026-09, so no calibration is needed. jjroofingsupplies.co.uk

  13. The Town and Country Planning (General Permitted Development) (England) Order 2015, Schedule 2, Part 1, Class C, legislation.gov.uk, accessed 14 September 2026. legislation.gov.uk

  14. Approved Document B (fire safety) volume 1: Dwellings, 2019 edition incorporating 2020, 2022, 2025 and 2026 amendments, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk

  15. The Building Regulations 2010, regulation 3 (meaning of building work), legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk

  16. The Building Regulations 2010, Schedule 3 (self-certification schemes and exemptions from requirement to give building notice or deposit full plans), legislation.gov.uk, accessed 30 September 2026. legislation.gov.uk