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Velux Loft Conversion Cost London 2026

What a rooflight loft conversion costs in London, the headroom it needs, the permitted development rules and when a dormer is the better buy.

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A Velux (rooflight) loft conversion in London typically costs £25,000–£45,000 including VAT at 20%, and takes 6–10 weeks on site. It is the cheapest type of loft conversion because the shape of the roof does not change: the floor is strengthened, roof windows are cut into the existing slopes and the space is insulated, boarded and given a new staircase. It usually counts as permitted development, but it only works where the loft already has enough height under the ridge.

Velux loft conversion costs at a glance

These are our published London figures for 2026, including VAT at 20%. Every loft type is compared in our loft conversion cost guide.

TypeWhat it involvesLondon costTime on site
Velux (rooflight)Roof windows in the existing slopes; the roof shape does not change£25,000–£45,0006–10 weeks
Rear dormerA box structure built out from the rear slope for full headroom£45,000–£75,00010–14 weeks
Hip-to-gable with rear dormerThe hipped end rebuilt as a gable wall, with a rear dormer£55,000–£80,00010–16 weeks
MansardThe rear slope rebuilt almost vertically with a flat roof above£60,000–£100,000+14–20 weeks

What a Velux loft conversion involves

The existing roof structure is kept. New floor joists, sized by a structural engineer, are fixed alongside the old ceiling joists so the loft can take the weight of a room; roof windows are cut in between the rafters; and the slopes and walls are insulated, boarded and plastered. A new staircase is built down to the floor below, and the escape route is fire-protected under building regulations.

Because the roof shape stays the same, there is no new structure on the outside of the house, little or no scaffolding to the rear and a shorter programme than any other loft type. That is why it is the lowest-cost route into the loft.

What moves the price within the range

  • Size of the loft and how much floor needs strengthening.
  • Number and type of roof windows: standard centre-pivot windows at the lower end; larger, top-hung or conservation-style windows push it up.
  • An en-suite shower room: the plumbing, waste route and tiling are the biggest single addition.
  • The staircase: where it lands on the floor below, and whether a bedroom or landing has to be re-planned to fit it.
  • Finishes: flooring, built-in eaves storage and joinery. See our joinery and carpentry page for fitted storage.

Professional and statutory fees sit on top: structural calculations, building regulations and, where the work touches a shared wall, party wall notices. The fees table in our loft conversion cost guide lists each one.

Is there enough headroom?

Headroom decides whether a rooflight conversion is worth doing. The new floor sits above the old ceiling joists and the insulation sits inside the rafters, so the finished room is lower than the bare loft. In a typical Victorian terrace with a ridge height of around 2.3–2.5 metres, roof windows alone may leave a central standing area only 1.5–1.8 metres wide. That suits a study, a child's room or an occasional guest room rather than a main bedroom.

The staircase needs headroom too: building regulations allow a reduced clear height over a loft conversion stair, but it still has to land where there is room to stand. A survey of your loft settles both points before any design work. If the height is not there, a dormer loft conversion (£45,000–£75,000) adds full ceiling height across a much larger area.

Planning permission and permitted development

Roof windows are normally permitted development under Class C of Part 1 of the General Permitted Development Order 2015, so no planning application is needed, provided that:

  • they project no more than 150mm beyond the plane of the roof slope;
  • no part is higher than the highest part of the existing roof;
  • any window on a side-facing slope is obscure-glazed and non-opening, unless the opening part is more than 1.7 metres above the floor of the room.

Houses keep these rights in most conservation areas, but flats and maisonettes have no permitted development rights, and some Article 4 directions remove the right to add roof windows. A lawful development certificate (£274) confirms the position and is worth having when you sell. Our guide to permitted development in London covers the detail, and our London planning approvals study shows that councils granted 90.6% of loft lawful development certificates between September 2025 and August 2026.

Building regulations and party walls

Every loft conversion needs building regulations approval, including a rooflight conversion: the floor structure, the staircase, fire protection to the escape route, insulation and ventilation are all inspected, and a completion certificate is issued at the end. On a terraced or semi-detached house, work that bears on or cuts into the party wall usually needs party wall notices; see our party wall agreement cost guide.

How we price a Velux loft conversion

Our quotations are fully itemised and based on a complete specification, on a JCT Minor Works contract with a fixed price and a defined programme. We are CHAS Elite, Constructionline Gold and Federation of Master Builders members with £10M public liability insurance, and every job carries our 10-year workmanship warranty. More on how we build lofts is on our page on loft conversions in London.

Frequently Asked Questions

How much does a Velux loft conversion cost in London?

A Velux (rooflight) loft conversion in London typically costs £25,000–£45,000, including VAT at 20%. The lower end is a single room with two standard roof windows, an insulated floor and walls, a painted staircase and no en-suite. A shower room, larger or conservation-style roof windows and better flooring and joinery move it towards the upper end.

How much headroom do I need for a Velux loft conversion?

There must be enough height under the ridge for a usable room once the floor is strengthened and the roof insulated, which raises the floor and lowers the ceiling. In a typical Victorian terrace with a ridge height of around 2.3–2.5 metres, roof windows alone may leave a central standing area only 1.5–1.8 metres wide. A survey of your loft settles it; where the height is not there, a dormer is usually the answer.

Does a Velux loft conversion need planning permission?

Usually not. Roof windows are normally permitted development if they project no more than 150mm from the roof slope, are no higher than the highest part of the roof, and any on a side-facing slope are obscure-glazed and fixed shut unless the opening part is more than 1.7 metres above the floor. Flats have no permitted development rights, and some Article 4 directions remove the right to add roof windows, so check your address.

How long does a Velux loft conversion take?

Typically 6–10 weeks on site, the shortest of any loft conversion, because the roof structure is kept. Most families stay in the house throughout, as most of the work is above the existing rooms until the staircase goes in.

Velux or dormer: which is better value?

A rooflight conversion costs £25,000–£45,000 against £45,000–£75,000 for a rear dormer, but a dormer adds full ceiling height across a much larger area. Where the loft already has good height, a rooflight conversion is the better value; where it does not, the extra cost of a dormer usually buys a proper bedroom and en-suite instead of a study or occasional room.

Do I need building regulations approval for a Velux loft conversion?

Yes. Every loft conversion needs building regulations approval, whatever the planning position: the strengthened floor, a safe staircase, fire protection to the escape route, insulation and ventilation are all checked, and you receive a completion certificate at the end.

Prices shown are indicative ranges based on London market data for 2026. Actual costs depend on the existing structure, specification and programme. All figures include VAT at 20% unless otherwise stated. Your own price is fixed in a written, itemised quote.

Want a real number instead of a range?

Get a fixed-price quote for your loft

Tell us about your house and we send an itemised, fixed-price quote on a JCT contract. Every job carries our 10-year workmanship warranty.

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