How to fit a basement window
Updated: 23 October 2026
You have an opening in a shed or a thin wall, and the listing says the job is screws and foam. That sentence is how people order the printed size and then discover the wall was doing more than holding a frame. This is how to fit a basement window when the hole already exists. Products Compared editorial team used the seven amazon.co.uk listings and UK measuring and notification guidance. We did not fit a window in a basement. The products are on the main [Basement window](/hub/diy-supplies/basement-window) comparison. A qualifying purchase through tag pcuk26-21 can earn a commission at no extra cost to you.
You have an opening in a shed or a thin wall, and the listing says the job is screws and foam. That sentence is how people order the printed size and then discover the wall was doing more than holding a frame.
You have an opening in a shed or a thin wall, and the listing says the job is screws and foam. That sentence is how people order the printed size and then discover the wall was doing more than holding a frame. This is how to fit a basement window when the hole already exists. Products Compared editorial team used the seven amazon.co.uk listings and UK measuring and notification guidance. We did not fit a window in a basement. The products are on the main Basement window comparison. A qualifying purchase through tag pcuk26-21 can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Nominal size | Opening | Frame | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  Kathfly Kathfly 16 x 8 inch recessed basement window | 16 x 8 inch; hole under 14.9 x 7 inch | Sliding, recessed | Aluminium | 8.8 | about £39.10 | View offer |
| Alternative |  Sion's store Sion's store grey slider, 53 x 50 cm | 53 x 50 cm | Flush slider | Aluminium alloy | 0.0 | about £75.71 | View offer |
| Alternative |  ZUGIATP ZUGIATP 24 x 36 inch black pivot window | 24 x 36 inch | Pivot; also hopper and double hung | Aluminium alloy | 0.0 | about £100 | View offer |
On this page
Measure the opening, not the sticker
Put the tape on the structural opening, not on the old sash and not on the listing title. Trade guidance from Huws Gray is to take width at the top, middle, and bottom, and height at the left, middle, and right. Order from the smallest of each.
If the old frame is still in, you usually need the masonry opening once that frame is out. Do not cut a new hole to the carton size. The STARHMLUX listing says the same three-and-three method. That is for an opening you already have, not permission to widen brick. A hole that crosses the damp-proof course is no longer a screw-in vent. The buying tests beside the tape are on the basement window buying guide.
The 10 mm allowance is a trade figure
Huws Gray's measuring guide deducts 10 mm from the smallest width and 10 mm from the smallest height when you order a window into a straight reveal. That 10 mm is their fitting allowance, not a figure printed on every amazon.co.uk basement window. Do not subtract it from a stock unit and expect the listing's own hole note to agree.
The Kathfly page, and only that ASIN, says the hole should be smaller than 14.9 by 7 inches, because the unit is recessed. That is not a 10 mm deduction from 16 by 8. Do not copy it onto the Sion sliders, the vinyl rows, or the 24 by 36 inch title. If glazing will sit within 800 mm of the floor, Huws Gray also flags toughened glass. Some bullets say tempered. The £93.40 vinyl page does not settle whether the panes are glass.
What the listings say about fixing
The Sion install bullet says to set the frame in the opening, tighten the screws, fit the panel, and fill the gap with foam sealant. The ZUGIATP bullet lists mounting screws and sealing foam, and it says to secure the unit from the top and then apply the foam. That is the whole method on those pages. It does not mention a lintel or a below-ground seal. Foam in that sentence is not BS 8102.
A frame that only needs screws is going into an opening that already has something above it to carry the wall. Screws into a fresh cut do not replace that support, and a foam bead does not design a light well or a drain. The shed version of a simple existing hole is on basement window for a shed. A habitable basement is not that shed.
When you notify, and when you stop
GOV.UK says replacement windows are notifiable unless a competent-person installer self-certifies. FENSA tells homeowners that new openings, new builds, and extensions sit outside that replacement certificate. Fitting a replacement window in a dwelling yourself does not create a FENSA certificate. You notify building control, or you use an installer who can self-certify. A garden shed is not automatically that dwelling case, and a house cellar is not exempt because the product title says shed.
Stop fitting when the opening is new, when you cannot see what is carrying the wall, when the well floods, when the building is listed, or when the basement is habitable. An escape window, if the room needs one, has to meet the opening size before you foam anything. Nothing here is building-control approval for your house.
Common questions
Do I measure the old frame or the brick?
For a replacement you need the opening the new frame has to sit in, which is usually the brick or block once the old frame is out. Measuring only the old timber under-orders the unit if the timber is smaller than the hole, and over-orders it if you include architrave. If you cannot see the masonry, do not cut a new hole to the sticker.
Which listing says the hole must be smaller than the window?
The Kathfly 16 by 8 inch page says the opening should be smaller than 14.9 by 7 inches, because that unit is recessed. That note is for ASIN B0FM83BZR7 only. The other six listings do not print that hole. Copying it onto a flush-mount slider will leave a frame that does not cover the cut.
Can I just foam the gap?
The Sion and ZUGIATP bullets tell you to screw the frame and then fill the gap with foam. That is a listing sentence, not a tanking specification and not a lintel. Foam does not carry brick, and it does not design a drain in a light well. If the gap is the only thing keeping rain out of a below-ground room, the job is bigger than the kit.
Do I need a FENSA certificate if I fit it myself?
FENSA registration is for installers who self-certify replacement windows. Fitting it yourself does not make you that scheme. GOV.UK treats replacement windows as notifiable unless a competent person certifies them. New openings sit outside a FENSA replacement certificate. A shed is not the same notification as a dwelling. Check which building you are actually altering.
When should I stop and call someone?
Stop if the opening is new or wider than the old one, if you cannot identify the lintel, if the well holds water, if the building is listed, or if people sleep in the basement. Also stop if the glass line will be within 800 mm of the floor and the listing will not say the glass is toughened. Screws and foam are the kit. They are not those jobs.
Related pages
Sources
Public UK sources and the amazon.co.uk listings compared on this page. Updated 22.10.2026.
- Approved Document B Volume 1, paragraphs 2.10 and 2.16 (retrieved 22.10.2026)
- Approved Document L Volume 1, Table 4.2 (retrieved 22.10.2026)
- Approved Document K, critical glazing locations (retrieved 22.10.2026)
- Approved Document F Volume 1, paragraphs 3.14 to 3.15 (retrieved 22.10.2026)
- FENSA: jobs you can register (retrieved 22.10.2026)
- Planning Portal: basements and building regulations (retrieved 22.10.2026)
- BS 8102:2022, protection of below-ground structures against water (retrieved 22.10.2026)
- Hampshire building control: replacement windows and doors (retrieved 22.10.2026)
- diy.com RISOR 90 by 40 cm tilt window, not in this set (retrieved 22.10.2026)