A like-for-like VELUX window replacement is a straight swap. The old unit comes out, a new one of the same size code goes into the structural opening you already have, and the rafters are never touched. Because the opening is trimmed and the reveal is already formed, most single replacements are a one-day job with no structural work and no engineer. The price sits between £1,150 and £1,650 for a unit up to MK06, fixed after survey, and every replacement carries our 10-year workmanship guarantee. This page covers exactly what the straight swap involves and where it stops being straight.
Most of the roof-window work in Ipswich is now replacement rather than new installation. The town filled with roof windows between 2000 and 2010, in loft conversions done during the extension boom, and those sealed units are reaching the end of their service life together. When people ring us they are usually describing a like-for-like swap without knowing that is the term. Sometimes they are describing a new opening. The two jobs cost very different amounts, so the first thing we do is work out which one you actually have.
What a straight swap actually is
The defining feature of a like-for-like replacement is that the hole in the roof does not change. The gap between the trimmed rafters, the trimmers above and below, and the ceiling reveal inside all stay exactly as built. You are changing the window, not the opening. That is why it is quick, why it is predictable, and why it rarely troubles Building Control beyond the thermal requirement.
This works because roof-window size codes have barely moved in twenty five years. A window fitted in 2004 can almost always be matched to a current model of the same code without disturbing a single rafter. Where a straight swap gets slightly less straight is the frame depth, which has changed between generations. A deeper or shallower frame changes how the internal lining and the plasterboard reveal meet the window. That is a finishing question, solved with a modern lining kit, not a structural one. We cover the code system in full on our VELUX size codes page.
The service, stage by stage
Here is what the straight swap involves on the ground. The full sequence for every job, new openings included, is on our VELUX installation process page, but the replacement-specific version is shorter and worth setting out on its own.
1. Survey and size confirmation
We start with the data plate on the top edge of the sash, or we measure the structural opening directly if the plate has gone, which is common on pre-2000 units where the label has cooked on a south-facing slope. We never order from a tape reading of the visible frame, because the visible frame is not the opening and the difference runs about 40mm a side. If you want to check your own code before we come out, the find your VELUX size code page walks you through it. At the same visit we check the covering type and profile depth, the pitch, the rafter type and the internal reveal, then issue one written price.
2. Removal of the old unit
On the day, the working area of slate or tile is stripped back and stacked for reuse, then the old window is lifted out. This is the point where the opening, the trimmers and the roofing underlay are exposed and inspected. Any rot in the trimmers surfaces here. We photograph it and show you before we touch it, and if there is remedial work it is priced before we proceed rather than added quietly. On a sound opening the old frame is simply unfixed from its brackets and taken out.
3. New flashing kit matched to the covering
The old flashing does not go back on. A new kit, matched to your specific roof covering, is fitted every time. Slate and low-profile coverings take a slate kit, profiled tiles and pantiles take a deeper pleated kit, and conservation settings sometimes take a recessed combination kit. Fitting the wrong family is the single most common built-in leak we find on other people’s work, which is why the flashing detail gets its own flashing kit explainer.
4. Insulation collar and underfelt collar
Two components that almost never appear on a competing quote, and both do real work. The insulation collar is a pre-formed jacket that wraps the frame and links the roof insulation to the window, closing the cold bridge around the perimeter. Leave it off and you get a black mould line at the head of the window in January, which people report to us as a leak when it is condensation on a cold reveal. The underfelt collar connects the roofing underlay to the frame so water passing the covering drains over the head of the frame and back out. The bodge is stapling felt roughly over the frame, which fails in the first driven easterly. Both collars are in every price we issue.
5. Roof-covering reinstatement
The stacked slates or tiles are cut and re-hung around the flashing skirt with the head lap maintained and no nail hole left exposed. On natural slate we cut with a slate cutter, not an angle grinder, and we source matching reclaimed slate rather than dropping a modern fibre-cement slate into a Victorian roof where it reads as a patch from the street. The roof is watertight the same day, never left open overnight.
6. Internal lining repair
Inside, the reveal lining is made good. Where the frame depth has changed, the lining is adjusted to suit. The standard detail is a vertical reveal at the head and a horizontal reveal at the sill, which throws daylight further into the room and lets warm air wash up the glass to cut condensation. Plasterboard and skim are included where that is in scope and stated on the quote either way.
The 10-year workmanship guarantee. Every replacement is backed for ten years on the work we do: the frame fixing, the flashing detail, both collars and the weathering. That is a guarantee on our labour and the way the unit is installed, separate from the manufacturer’s own product warranty on the window itself. We are experienced VELUX installers, not part of any manufacturer approval scheme, and the guarantee is ours to stand behind directly.
Like-for-like or a new opening?
This is the fork that decides the price. A straight swap reuses the opening. A new opening cuts fresh structure into the roof, which means removing part of a rafter and transferring its load into doubled trimmers. On a traditional cut roof one rafter can normally be trimmed for a window up to about 940mm wide without an engineer. A trussed roof, which is most housing built after about 1970 around Kesgrave and Martlesham, cannot be cut without a designed solution. The table below sets the two jobs side by side. The full new-opening figures are on the new VELUX fitting cost page.
| Factor | Like-for-like replacement | New opening |
|---|---|---|
| Structural work | None. Existing opening reused, rafters untouched | Rafter cut, doubled trimmers formed above and below |
| Engineer needed | No | Only for trussed roofs, openings over about 1.2m, or irregular rafters |
| Days on site | 1 for a single unit | 2 per window in a cut roof |
| Building Control | Thermal element notice only (Part L) | Notifiable as a structural alteration and a thermal change |
| Cost band, up to MK06 | £1,150 to £1,650 | £1,900 to £2,700 |
| Cost band, PK10 or larger | £1,450 to £2,100 | £2,300 to £3,300 |
Bands are supply and fit including flashing matched to covering, both collars, fixings and covering reinstatement, assuming sound trimmers and reasonable access. Full detail on the installation cost page.
If your existing window already sits where you want it and you simply want a modern, warmer, watertight unit in its place, you want a like-for-like replacement and this is the cheaper, faster job of the two. If you want light in a different part of the room, or a bigger opening, or a first window where there was none, that is a new opening and it is a different conversation.
How long the roof is open
The question behind most enquiries is not really about price, it is about how long there is a hole in the roof. On a like-for-like replacement, hours. We strip the working area, take the old unit out, set and square the new frame, fit both collars and the matched flashing kit, then re-hang the covering, and the roof is watertight by the end of that stage. It is never left open overnight. If weather makes it genuinely impossible we sheet, secure and come back rather than pressing on into a downpour. The VELUX replacement time page breaks the timings down job by job, including two or three units on the same slope.
What moves the price on a replacement
- Top hung instead of centre pivot: add roughly 15 to 30 per cent on the unit. Worth it where a desk or bed sits under the window and you want a clear view out.
- Triple glazing: add around 20 to 30 per cent on the unit. Compelling on a north slope in a room you heat hard, less so on a small landing.
- White painted or polyurethane finish: a modest add, and the right choice in any bathroom or humid room.
- Coastal specification: 316L stainless fixings and upgraded flashing, included as standard within a mile of open sea, not an upsell.
- Matching reclaimed slate reinstatement: adds for materials and slower working on a period roof.
- Access: full scaffold rather than a tower is quoted separately and is the biggest single variable on terraced houses.
- Rotten trimmers found on strip-back: priced before we proceed, never assumed.
Common questions about VELUX window replacement
Is a like-for-like replacement really quicker than fitting the same window new?
Yes, and by a clear margin. A straight swap reuses the trimmed opening and the reveal, so there is no cutting, no trimmers to form and no engineer. A single like-for-like unit is a one-day job. Cutting the same size window into a fresh part of the roof is two days because the structure has to be formed first. The unit and the flashing are identical, the difference is entirely the structural work.
Do I need Building Control for a straight swap?
A replacement is a controlled fitting under Part L, so the new unit has to meet the thermal standard for a replacement rooflight, currently a whole-unit U-value of 2.2 W/m²K or better in the vertical plane. Every unit we fit beats that comfortably. Because we are not under a competent-person scheme, that goes to your local authority Building Control as a building notice, which we handle. There is no structural notification on a like-for-like because nothing structural changes.
Can you match a window fitted in the early 2000s?
Almost always. Size codes have been broadly stable for decades, so a 2004 unit usually matches a current model of the same code with no change to the opening. The one thing that shifts between generations is frame depth, which affects how the internal lining meets the window. That is handled with a lining kit at the finishing stage. Send us a photo of the data plate and we can confirm the match before anyone comes out.
What if the old flashing looks fine, can you leave it?
No. A new flashing kit matched to your covering goes on every replacement. Old flashing has weathered for fifteen to twenty years alongside the failed window, the laps have relaxed, and reusing it undoes the point of the job. The kit is part of the supply-and-fit price, not an extra, and it is the component that actually keeps the water out.
Get a fixed price for your replacement
Send a photograph of the data plate if you can reach it, one of the window from inside, and one of the roof from the garden showing the covering. That is usually enough for us to say whether a like-for-like swap is possible and roughly what it costs before anyone visits. Then we survey properly, confirm the opening, confirm the flashing kit for your covering, and give you one written price with the U-value on it. That price is fixed, and the replacement carries a 10-year workmanship guarantee behind installers with 25 years fitting roof windows.
Request a fixed quote for your VELUX window replacement, or read more about roof-window installation on our VELUX window installation hub.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.