Roof windows wear out, leak or simply no longer suit the room. When one needs replacing, the first question is whether the new window will go in the same opening. If it will, the job is mostly about the window, the flashing and the weatherproofing around it. If it will not, because you want a larger window, a different position or an extra one, rafters have to be cut and the structure reworked.
That split drives everything else: how long the work takes, who needs to be told, what Part L asks of the new window and whether planning comes into it. Owners sometimes assume that a window swap is always simple, and sometimes assume the reverse. The truth depends on the opening.
This guide explains the two routes, what trimming and structure involve, the thermal figure for a roof window, how planning and building control apply, what to check on the old window before ordering a new one and what the national guides publish on cost, for roof windows in Farnborough and the neighbouring towns.
Like for like: the same opening
Like for like means the new window matches the old in size and type and sits in the opening the old one left. The rafters, the header and the trimmers around the opening already exist and stay in place.
What the job involves
The covering around the window is lifted, the old frame and flashing are taken out, and the new frame is fixed to the existing structure. A new flashing kit is fitted so that water runs around and away from the window, and the covering is replaced around it. Inside, the lining and finish are made good.
Why the flashing is renewed
A flashing kit is made for a particular window and roof covering. Reusing an old flashing with a new window risks leaks, because the dimensions and the fixing details differ. Our guide to roof window flashing kits explains the kits by covering.
Checking the old window
Look at the label or the frame for the size code and series. A roofer can measure the opening and the rafter spacing and match the window. Where the original has been discontinued, the nearest current size is chosen, and a small difference in size may mean adjusting the lining or the flashing.
Matching the interior
The inside of the opening is as important as the outside. The lining, the splay and the plaster finish around the old window may not match a new frame exactly, so a replacement of a different series can leave gaps that need filling and painting. We check the inside at the survey and say what making good is allowed for.
What it does not involve
No rafters are cut and no header is added. That is why a like-for-like swap is the smaller job, and why it is commonly done in one visit with the right access.
If you are replacing the window because it leaks, our guide to why a Velux window leaks helps you check whether the window or the flashing is at fault first.
A new or enlarged opening
If the new window is bigger, in a different place or an additional one, the roof structure has to change. This is where a window job becomes a structural one.
Trimming the opening
Cutting through a rafter removes part of what carries the roof. The load has to be taken around the opening by a new horizontal member, a header, fixed between full-length rafters, with trimmers alongside. The members are sized for the span and the load, and they are fixed with the right hangers and fixings.
Why a survey matters first
The roof's structure varies between houses. Older roofs may have purlins, collar ties or old repairs that affect where an opening can go. Modern trussed roofs, built from factory-made trusses, cannot simply be cut, and altering them needs an engineer's design. The survey looks at which you have before any window position is promised.
Other effects
A new opening affects insulation, the ceiling or lining inside and any wiring or pipework in the slope. Changing the number of windows also changes the daylight and heat balance of the room.
Approvals
Structural alteration is building work, and it may need a Building Regulations application. Planning can be relevant too, as the next sections explain. Where the window also has a safety job, the escape window rules add their own sizes.
Costs rise with the structural work, which is why a new opening is priced after the survey and not from a catalogue. A like-for-like swap carries far fewer unknowns, though a rotten batten or a failed lining found on opening up can change the plan, and if it does you are told before extra work is added.
The thermal figure for a replacement window
A replacement roof window has to meet the energy rules in force when the work is done. The relevant figures are in Approved Document L, volume 1.
The numbers
For new or replacement windows in an existing dwelling, In Table 4.2 of the 2021 edition (2023 amendments) the figure is 1.4 W/(m²·K), with a Window Energy Rating in Band B as the other way to comply, and a note confirms that roof windows belong in that row. Rooflights, on flat roofs, have a different figure, 2.2, and the two are often confused.
Not a thermal element
Roof windows are outside the thermal element definition. Regulation 2(3) of the Building Regulations 2010 defines a thermal element as a wall, floor or roof, excluding windows, doors, roof windows and rooflights. So replacing a window is treated under the window figure, and not under the renovation rules for roofs. The roof around the window may be a separate question.
Editions
The 2021 edition took effect on 15 June 2022. The next edition begins on 24 March 2027, with renumbered tables, and until that date the 2021 figures stand.
Ask for the evidence
The maker's data sheet or label for the new window gives its U-value or rating, and it belongs in your file. Our guide to roof window U-values and Part L gives the full picture, including how the figures are measured.
Planning and permitted development
For many houses, replacing or adding a roof window needs no planning application. The detail depends on the house and the street.
Class C
Schedule 2, Part 1, Class C of the General Permitted Development Order 2015 takes in roof alterations to a house. A roof window normally qualifies, so long as it stands no more than 0.15 metres proud of the original roof plane.
Side slopes
Condition C.2 applies to a window in a roof slope that faces the side of the house. It must always be obscure-glazed, and the 1.7 m measurement only decides whether it can open: opening parts must be higher than 1.7 metres above the floor if the window is to open at all.
Flats, listed buildings and conservation areas
- Flats, including a house split into flats, have no permitted development rights, so an application may be needed.
- A listed building retains its Class C rights, yet it also needs consent for anything affecting its special character.
- In a conservation area, ask the local planning authority about any Article 4 direction. In Rushmoor, those at Cargate and Wellesley are about chimneys and antennas, not roof windows.
Dormers are different
A dormer is a different kind of alteration, with its own limits, and it is not available as permitted development on land in a conservation area. Our roof windows and skylights page covers the range of jobs.
When in doubt, a short written enquiry to the council settles the position before you order anything.
Building control and the paperwork
Whether a window replacement needs Building Regulations approval depends on what is being done. The distinction maps neatly onto the two routes.
Like for like
A replacement window in the same opening is normally handled as a replacement of a controlled fitting, and it needs the right thermal performance. Many owners use a registered installer scheme or notify building control, and the evidence of the window's U-value should be kept. Ask your building control body how it wants a straight replacement handled.
New or enlarged opening
Cutting rafters is structural work, and the design of the trimming may need to be shown to building control. The window's thermal performance, the ventilation of the room and, in a loft conversion, fire safety are part of the assessment. A building control application is likely to be needed.
Who to ask
In Rushmoor, the Hart and Rushmoor Building Control Partnership answers such questions on 01252 398715. Registered building control approvers, the private alternative, have operated under the regime that began on 6 April 2024.
Planning Portal test
The Planning Portal's test about the building envelope relates to re-roofing. A roof window is part of the envelope, and an opening adds to the area affected, but the practical advice is to ask building control rather than calculate.
Records
Keep the data sheet, any completion certificate and the invoice. A buyer's solicitor may ask for them, and they are the evidence that the work met the standard.
Cost and what to ask before ordering
Cost depends on whether the opening is new, and the national guides give a rough picture for the window alone.
| Source | Job | Figure |
|---|---|---|
| MyJobQuote (Sep 2026) | Velux window, standard mid-range | £1,100 |
| MyJobQuote (Sep 2026) | Straightforward swap | around £100 less |
What the figures show
A straightforward swap is cheaper than a first-time installation, which fits the picture above: the structure is already in place. A new opening costs more because of the structural work, the extra lining and the need to make good. The window's size, the access and the roof covering move the price as well.
Questions to ask
- Is this a like-for-like swap or a new opening, and who decides?
- Which flashing kit goes with my roof covering?
- What is the stated U-value or energy rating?
- Will the lining and finish inside be made good?
- What does the job mean for building control and planning?
One written total
Our price comes after the survey as a single written figure, with VAT shown and access allowed for. A new opening brings the structural work into that same figure. Where a window is leaking, the survey checks whether the window or the flashing is at fault, so you do not pay to replace the wrong thing.
If the roof around the window is also worn, ask for the options to be priced together.
