Roof windows are one of the most effective ways to flood a loft room or attic conversion with natural light, but choosing the right size can feel baffling when every catalogue entry is just a code like “FK06” or “MK04”. This guide explains exactly how Velux size codes work, lists the common UK sizes, and helps you pick the right option for your rafter spacing and room layout.

How Velux Size Codes Work

Velux, the best-known roof window brand in the UK, uses a two-part alphanumeric code to describe each window’s width and height:

  • Letters indicate the width group (e.g. MK = 78 cm wide, FK = 66 cm wide, UK = 134 cm wide)1
  • Numbers indicate the height (e.g. 04 = 98 cm, 06 = 118 cm, 08 = 140 cm)1

The code is always Width Group + Height Number. So MK06 = 78 cm wide × 118 cm tall, and FK04 = 66 cm wide × 98 cm tall.1

Other manufacturers (Fakro, Keylite, RoofLITE) use their own codes, but most make windows in broadly the same sizes, which is useful when comparing prices.

Standard UK Roof Window Sizes

The table below lists the common UK sizes with the Velux code, nominal dimensions (width × height in cm), a typical use and a supply price; the rough opening is explained below it.

Velux CodeWidth (cm)Height (cm)Typical UseSupply Price (£)
CK025578Tight rafter bays, bathrooms220–320
CK045598Narrow rafter bays260–370
FK046698Single-rafter bay, shower rooms280–390
FK0666118Popular bedroom size300–430
MK047898Single-rafter bay, standard300–450
MK0678118Most popular UK size320–520
PK1094160Large bedrooms, open-plan420–650
SK06114118Double-rafter bay, living rooms480–720
SK10114160Maximum light, large lofts560–850
UK08134140Panoramic views, wide bays620–900

Prices are supply-only, ex-VAT, 2026. Installation adds £300–£600 per window depending on complexity.

What Does “Rough Opening” Mean?

The rough opening is the hole between rafters that takes the window frame. What matters is the clear gap between the rafters, not their centres, and that gap is usually narrower than people expect. Rafters at 600 mm centres, the normal spacing for modern trussed rafter roofs, leave a gap of only about 550–565 mm, depending on the rafter thickness, which is barely the width of a 55 cm CK window and well short of an FK (66 cm) or MK (78 cm). The manufacturer’s installation instructions give the exact opening each Velux window needs.

A wider window means cutting a rafter, and that is structural work. Get a structural engineer involved first, and in a trussed rafter roof do not cut or trim a truss except to details supplied by the truss manufacturer.2 Trimmers must be fitted above and below the opening and sized to Eurocode 5 (BS EN 1995-1-1).3

Centre-Pivot vs Top-Hung: Which Is Right for You?

FeatureCentre-Pivot (e.g. GGL/GGU series)Top-Hung (e.g. GPL/GPU series)
Opening styleRotates 180° for easy cleaningHinges at top, opens outward
HandleControl bar at the top of the sashHandle at the bottom of the sash
Unobstructed viewSash rotates into roomCleaner view when closed
Best forWindows set high, with furniture belowWindows set low, or where a clear view out matters
Typical price upliftBase model+£80–£150 over centre-pivot

Velux’s centre-pivot windows rotate on a central axis with a handlebar at the top, which suits a window with furniture below it, while its top-hung windows open upwards and outwards from the bottom.4 Centre-pivot is the default for most UK loft conversions because it allows the sash to flip for external cleaning from inside the room, important when a roofer is not regularly on site.

Planning & Building Regulations

Planning permission is generally not required for roof windows that:

  • Do not protrude more than 150 mm above the roof plane5
  • Do not exceed the highest part of the existing ridge5
  • On a roof slope forming a side elevation, are obscure-glazed and non-opening below 1.7 m above the floor5

Unlike dormers, roof windows are not excluded from permitted development in a conservation area, though an Article 4 direction can remove the right, and a listed building needs listed building consent.6

Under Part L of the Building Regulations (England, Approved Document L Volume 1, 2021 edition incorporating the 2023 amendments), a roof window is a controlled fitting. In an existing dwelling it must be no worse than 1.4 W/m²K, or Window Energy Rating Band B, under Table 4.2, and that is the same figure whether you are swapping a tired window for a new one or fitting new windows as part of a loft conversion, because a conversion is work to an existing dwelling rather than a new build.7 The 1.2 W/m²K figure quoted in a good deal of sales material is the roof window value in the notional dwelling used for new-build compliance, measured with the window in the vertical position: a sound product to buy, but not the threshold your conversion has to clear.7 Part L also separates a roof window, which sits in the plane of the roof, from a rooflight, which sits out of plane on a kerb or upstand and is allowed up to 2.2 W/m²K.7

For loft conversions, Part B (Fire Safety) matters just as much, and it is widely got backwards. A roof window is an escape route only on a storey no more than 4.5 m above ground level. Convert the loft of a typical two storey house and the new floor sits above that line, and at that point Approved Document B stops offering the window as an option: the house needs a protected stairway enclosed in fire resisting construction all the way to a final exit, or an alternative escape route from the top storey.8 Paragraph 2.21 is explicit that adding a storey above 4.5 m means addressing the full length of the escape route and fitting fire doorsets and partitions, upgrading the existing doors where they are not good enough.8 There is a second route in paragraph 2.23, which trades the stair enclosure for sprinklers in the open plan areas, a fire resisting separation between the ground storey and the ones above, and separated cooking facilities.8

So the big roof window is not what carries you out. Where an escape window is the strategy, on storeys up to 4.5 m, it needs an unobstructed openable area of at least 0.33 m², at least 450 mm both high and wide, and the bottom of that opening no more than 1100 mm above the floor. That last figure is the one that catches roof windows, because they are often set high in the slope for headroom and the sill ends up out of reach.8

How to Choose the Right Size

  1. Measure your rafter centres before ordering, the window must fit within an existing bay or between newly fitted trimmer rafters.
  2. Consider head height and glass area: a taller window (e.g. height code 10 = 160 cm) feels more generous if your ridge height allows, and Velux suggests a window area of around 15–20% of the room’s floor area on a pitched roof.1
  3. Check orientation: south- and west-facing roof windows may need solar-control glazing or an external blind to prevent summer overheating.
  4. Budget for the full package: a flashing kit matched to the roof covering adds about £90–£170 per window; blackout blinds typically cost £80–£220 depending on size.

Installer’s Rule of Thumb

For a standard loft bedroom, specify the largest window that fits your rafter bay without structural alteration. Daylight levels in a roof window are far harder to increase retrospectively than to reduce with a blind.

Cost Summary

ItemTypical Cost (per window)
Standard double-glazed roof window (FK06/MK06)£300–£5209
High-performance triple-glazed equivalent£480–£750
Flashing kit£90–£170
Blackout or solar blind£80–£220
Installation (carpenter + roofer)£300–£600
Total installed (mid-size window)£770–£1,510

For a straight replacement, Velux’s Replacement Supply & Fit service gives one inclusive quote, with the fitting done by a VELUX Certified Installer.10 Those installers are independent companies that set their own prices, so it is worth comparing a quote with a local roofer’s price.11 Always ask to see the installer’s public liability insurance and check they are familiar with the roof covering type (clay tile, concrete tile, slate) as each requires different flashing adaptors.

Common Mistakes to Avoid

  • Ordering the wrong flashing type: each window needs a flashing kit matched to your tile profile. A standard flashing will not seal against deep-profile interlocking tiles.
  • Ignoring solar gain: a large south-facing roof window without solar-control glass can overheat a loft room badly on sunny days.
  • Skimping on the trimmers: the doubled rafters and trimmers around the opening must be properly engineered. Undersized trimming lets the rafters deflect, and the window can rack and leak within a few years.
  • Forgetting extract ventilation: a bathroom or kitchen in the loft needs extract ventilation to the outside under Part F of the Building Regulations, and an opening roof window does not replace it.12
References (12)

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.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.
  2. 2technicalvery strongLABC Warranty, guidance.RN-A2P67ZLABC Warranty's technical manual chapter on roofs. It reproduces BS 5534 Table 3 minimum batten sizes, batten length, support and nailing rules, grading and marking, and the HR and LR underlay definitions.
  3. 3technicalstrongBSI (BSI Knowledge catalogue), a standard.RN-928IC7Dated 15 December 2004 with amendment A2 in 2014, this is the timber design Eurocode that UK joist and rafter span tables are calculated from. Its scope gives general design rules for timber structures with specific rules for buildings, arranged as Section 1 General, 2 Basis of design, 3 Material properties, 4 Durability, 5 Basis of structural analysis, 6 Ultimate limit states, 7 Serviceability limit states, 8 Connections with metal fasteners, 9 Components and assemblies and 10 Structural detailing and control. It is used with EN 1990 (basis of design) and EN 1991 (actions on structures), and it does not cover the design of structures subject to prolonged exposure at elevated temperatures.
  4. 4technicalsupplier figureVELUX, guidance.RN-3OZMMMThe roof window maker's product page. It recommends roof window area of 15 to 20 per cent of the floor area for a well-lit room.
  5. 5technicalvery 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.
  6. 6technicalvery 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.
  7. 7technicalvery 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.
  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. 9costsupplier 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
  10. 10technicalsupplier figureVELUX Company Ltd, guidance.RN-31P74JVELUX's page for its Replacement Supply & Fit service, which supplies a new roof window and has it fitted by a VELUX Certified Installer under one inclusive quote.
  11. 11technicalsupplier figureVELUX, guidance.RN-638OAEVELUX's UK find-an-installer page lists VELUX Certified Installers and explains that they are independent contractors, which settles that Velux publishes a list of certified installers on its website.
  12. 12technicalvery 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. VELUX Roof window sizes: Find the right fit for your home, VELUX, accessed 13 September 2026. velux.co.uk

  2. Technical Manual Version 11, Section 11: Roofs, LABC Warranty, accessed 30 September 2026. labcwarranty.co.uk

  3. BS EN 1995-1-1:2004+A2:2014 Eurocode 5. Design of timber structures. General. Common rules and rules for buildings, BSI, accessed 7 September 2026. knowledge.bsigroup.com

  4. Centre-pivot roof windows, VELUX, accessed 2 October 2026. velux.co.uk

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

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

  7. Approved Document L, Conservation of fuel and power, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments, Ministry of Housing, Communities and Local Government, accessed 10 September 2026. assets.publishing.service.gov.uk

  8. 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 11 September 2026. assets.publishing.service.gov.uk

  9. Velux GGL Centre Pivot Pine Roof Window, JJ Roofing Supplies, accessed 15 September 2026. Priced at 2026-09, so no calibration is needed. jjroofingsupplies.co.uk

  10. Replacement Supply & Fit service, VELUX Company Ltd, accessed 30 September 2026. velux.co.uk

  11. Find a VELUX Certified Installer, VELUX Company Ltd, accessed 30 September 2026. velux.co.uk

  12. Approved Document F, Ventilation, Volume 1: Dwellings, 2021 edition, Ministry of Housing, Communities and Local Government, accessed 14 September 2026. assets.publishing.service.gov.uk