Replacing a Velux in the Same Opening

Like-for-like means a new window of the same size, opening in the same way, in the same place in the slope, with the flashing renewed to suit. The hole in the roof stays as it is. The rafters, the timber trimmers around the opening and the plaster lining inside are left alone unless they turn out to be damaged.

It does not mean the same brand or the same age of design. Roof windows have changed their glazing, their seals and their flashing kits across the years, so a new unit is rarely a copy of the old. The thing that has to match is the opening, not the badge on the frame.

Is it cheaper to swap than to start again?

It is, because nothing structural is touched. A new opening means cutting a rafter, fitting a trimmer on each side, patching the covering and re-lining the room. A straight swap keeps all of that as it is. MyJobQuote's guide puts a straightforward swap at around £100 less than the mid-range figure for a standard fit (MyJobQuote, Sep 2026), and that gap reflects the work you do not need.

The saving only lasts while the opening stays intact. Have a roofer examine the timber round the frame before you order, because a window that has leaked for years can leave soft wood behind. If the trimmers are rotten the swap is no longer straight, and it is better to learn that on the survey than with the old unit already out and the room open to the weather.

For those wondering about the planning side, a like-sized unit in the same slope seldom changes the look of the house, though Class C and the listed building rules, covered below, still apply to some homes.

Every roof window has a size, and most makers print it as a short code on a label. The label is often on the side of the sash or the frame, and you can usually see it when the window is open. The code is built from the width and length of the unit, and the maker's chart turns it into millimetres.

What do you measure if the label has gone?

Measure the frame, not the glass. With the sash open, take the outer width of the frame and its outer height, from the top edge to the bottom, in millimetres. Then measure the opening in the lining inside the room. Write down both and take a clear photo of the frame and the surrounding flashing.

With those numbers a supplier can match the nearest listed size. Our page on Velux window sizes explains how the codes work. If your window comes from another maker, the chart is that maker's, and sizes from two makers can sit a few millimetres apart. Few millimetres decide whether a flashing kit closes the gap or leaves one.

A roofer measuring from the roof can also check the gauge of the covering round the frame, which affects how the new flashing laps the slates or tiles. Do not order a window from a photograph alone, however clear it looks. The code gives a good start, but the frame can have been altered by a previous repair, and a swap that fits on paper can still be a few millimetres out in the roof.

If a window was fitted a long time ago, the size may no longer be on the maker's current list. In that case ask the supplier for the nearest current size and whether it comes with a kit made to cover the difference.

A straight swap is a short job for two people on a dry day, though the length depends on the access and the roof. It follows a set order.

1. The blind, if there is one, and the inside trim are taken off, and the floor and furniture below are covered.
2. The sash is removed, then the old frame is released from the rafters and trimmers.
3. The slates or tiles round the opening are lifted, and the old flashing and soakers, the small L-shaped pieces under each course, are taken out.
4. The opening is checked for rot and for square, and the timber lining is made good.
5. The new frame is fixed, levelled and tied to the rafters, and the new flashing kit and soakers go in.
6. The covering is laid back against the new metal, the sash is hung and the inside is finished.

How long does the job take?

Longer than a quick call suggests, because the weather decides the order and the roof is open while the old unit is out. A well-planned swap keeps that gap short by having the new window and kit on site before anything is lifted. A fitting runs from the first lifted slate to the last sweep of the tidy-up, and the day should be explained to you beforehand.

Expect the work to be tidy. Dust sheets go down inside, the old unit and offcuts leave with us, and the covering we lift is stacked for reuse. If a slate or tile breaks as it comes off, we replace it with a matching one and tell you.

Several things can turn a like-for-like job into something bigger. None is a reason to put the work off, but each changes the plan and the price.

• The new size does not match the old, and the opening needs trimming.
• The timber trimmers or rafters round the frame are soft with old leaks.
• The covering was changed since the window went in, so the old kit no longer suits it.
• The lining inside has water damage or mould.
• The flashing sits on a roof that has tiles of a profile no longer made.

What if the size you want is no longer made?

Then you have a choice. A different maker's window of a close size can go into the opening if a kit exists that closes the gap. Otherwise the opening is altered, which means a rafter is cut, a trimmer fitted each side and the covering patched. That moves the job from a swap to alteration of the roof structure, which can bring Building Regulations into play, and your building control body will advise how the change is notified.

Do not trim a rafter to make a window fit. The timbers carry the roof, and a cut that is not supported by a trimmer weakens the slope. If the opening has to change, a roofer or builder who can show how the load is carried should do it.

Wider alterations are a separate subject. Adding a second window beside the first, or moving the window up the slope, is a new opening rather than a swap, and is covered in our guide to fitting a new roof window.

A replacement roof window counts as a new element, so it meets today's rules, not those in force when the old one was fitted. Table 4.2 of AD L Volume 1 (2021 edition, 2023 amendments) puts a window at a U-value of 1.4, or a Band B rating on the Window Energy Rating scale, and its note 8 includes roof windows. Rooflights take 2.2, which is a separate product. A newer AD L is due on 24 March 2027, with the tables renumbered 3.2 and 3.3, so the 2021 figures apply until then.

Does a swap need Building Regulations approval?

The new unit must reach the Part L figure whether or not any paperwork is needed. How a replacement window is notified is a question for your building control body, which in Taunton's area includes Somerset Building Control (0300 303 7790). Ask them how it applies to a roof window and keep the data sheet showing the figure with your house papers for the day you sell.

On planning, Class C covers roof alterations to a house. The unit may not project more than 0.15 metres from the original slope. A window set in a side-elevation slope carries a further condition: the glass must be obscure, and the sash fixed shut unless its opening part sits over 1.7 metres up from the floor of the room. A listed house keeps Class C rights but needs listed building consent for alterations that affect its character.

Ask Somerset Council's planning team, or Exmoor National Park Authority inside the Park, if the new window changes the look of the roof, or if a listing or a conservation area applies to your home. Our roof window Building Regulations guide covers the regulation side in more depth.

MyJobQuote's guide to Velux windows gives a standard mid-range window at £1,100, and a straightforward swap at around £100 less (MyJobQuote, Sep 2026). The guide is national and the figures are typical UK spend, not our prices, and they leave out the things that change a particular job.

What moves the price of a swap up or down?

• The size of the window and its glazing.
• How the sash opens: pivot, top-hung or motorised.
• The covering, which decides the flashing kit.
• How the roof is reached, including scaffold or a tower.
• Whether the timbers and lining need repair once the old unit is out.
• Whether the blind is kept or replaced.

Look at whether the total you are given takes in VAT and scaffold, since both change what you pay. On the highway in Somerset Council's area the licence goes to accredited firms alone, and it falls to the contractor doing the work to submit the application.

A window swap that goes smoothly is quick and keeps the cost down. One that uncovers rot is dearer, and the time to find out is before the old window comes out. That is why we start with a survey, look at the frame and the timber round it, and send you photos of what we find.

Send us the label code or the frame measurements and a photo of the window and the roof round it, and we will tell you whether a straight swap will do before we give you one written price.