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Basement escape window size

Updated: 23 October 2026

You are treating a small amazon.co.uk window as the way out of a habitable basement because the title says egress. The basement escape window size that matters is the unobstructed opening, not the carton and not the word in the title. Products Compared editorial team checked seven listings, from £39.10 to £100, against Approved Document B for dwellings in England. None of the pages prints a clear opening, so we did not measure one. The rows are on the main [Basement window](/hub/diy-supplies/basement-window) comparison. Wales, Scotland, and Northern Ireland use their own handbooks, so these figures are England's, not a UK-wide stamp. A qualifying purchase through tag pcuk26-21 can earn a commission at no extra cost to you.

You are treating a small amazon.co.uk window as the way out of a habitable basement because the title says egress.

Our top picks

Kathfly Kathfly 16 x 8 inch recessed basement window

Best overall

Kathfly Kathfly 16 x 8 inch recessed basement window
8.8/10about £39.10
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Sion's store Sion's store grey slider, 53 x 50 cm

Alternative

Sion's store Sion's store grey slider, 53 x 50 cm
0.0/10about £75.71
View offer
ZUGIATP ZUGIATP 24 x 36 inch black pivot window

Alternative

ZUGIATP ZUGIATP 24 x 36 inch black pivot window
0.0/10about £100
View offer
On this page
  1. 1.The two numbers that both have to pass
  2. 2.Cill height and a way out
  3. 3.What that does to these listings
  4. 4.The other legal route

The two numbers that both have to pass

Approved Document B Volume 1, paragraph 2.10, asks for an unobstructed openable area of at least 0.33 m², and at least 450 mm high and 450 mm wide. Both tests have to pass. A square of 450 mm by 450 mm is only 0.2025 m², not 0.33 m². North Norfolk illustrates 450 mm by 734 mm as a rectangle that can clear the area. The frame from the box is not that opening. The sash and the screen sit inside the outer frame. Paragraph 2.10 also says the window should stay open without being held. The buying order around that test is on the basement window buying guide.

Cill height and a way out

Paragraph 2.10 says the bottom of the opening should be no more than 1100 mm above the floor. That is a maximum, not a minimum. CNC's guide also puts the bottom between 800 mm and 1100 mm. The 800 mm floor is their guidance, not the sentence in paragraph 2.10. A cill at 700 mm is not failed by that paragraph for being under 800 mm. A cill at 1200 mm is over the 1100 mm maximum. The opening has to lead to a place free from fire. A window well with no steps, no ladder, and no route away from the building is not that place. The fitting point, where a new hole becomes structural, is on how to fit a basement window.
Kathfly Kathfly 16 x 8 inch recessed basement window

Testsieger

Kathfly Kathfly 16 x 8 inch recessed basement window
8.8/10about £39.10
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What that does to these listings

A 16 by 8 inch frame is about 406 mm by 203 mm, so both sides are under 450 mm. The Kathfly, frosted Croblissful, and clear CroBlissful units are all that nominal size and fail before the sash opens. The STARHMLUX row is 58 by 38 cm and the white Sion slider is 55 by 40 cm. Each has a side under 450 mm. The grey Sion slider is 53 by 50 cm, so both nominal sides clear 450 mm, and it is still not a pass: no clear opening is printed, and egress is only in the title. The ZUGIATP title says 24 by 36 inch. The package line is 60 by 40 by 10 cm, and the page calls the action a pivot, a casement, a hopper, and double hung. Hampshire building control has noted that some pivots do not give a suitable clear area. No ASIN here prints the gap in paragraph 2.10.

Common questions

What size is the escape opening, not the frame?
The escape opening is the unobstructed gap, at least 0.33 m² and at least 450 mm by 450 mm, under Approved Document B paragraph 2.10 for dwellings in England. The bottom of that opening should be no more than 1100 mm above the floor, and the sash should stay open without being held. The outer frame on the carton is larger than that gap. None of these amazon.co.uk listings prints the clear size.
Why is 450 mm by 450 mm not enough?
Because 450 mm times 450 mm is 0.2025 m², and the area test is 0.33 m². Both have to pass. North Norfolk illustrates 450 mm by 734 mm as a rectangle that can meet the area when one side is at the minimum. CNC's guide says a 450 mm width needs about 750 mm of height. A square at the side minimum fails the area even though it meets both sides.
Does a 16 by 8 inch window pass?
No. Sixteen inches is about 406 mm and 8 inches is about 203 mm, so the frame fails both 450 mm sides before it opens. The Kathfly, frosted Croblissful, and clear CroBlissful rows are that nominal size. There is no clear-opening figure on those pages that could overturn the frame.
What if the basement already has a protected stair?
Paragraph 2.16 allows that protected stair as the alternative to the escape window for a habitable basement. The small window can then stay a light, if the stair is really the strategy your design uses. It does not become an escape window by default. Ask building control to confirm the layout. A title that says egress is still not evidence.
Is a window well with no way out still an escape?
No. The opening has to reach a place free from fire. A well you cannot climb out of is not that place, however large the sash is. This page does not describe how to excavate or ladder a well. If the only route stops at a concrete box, the frame size is not the missing piece. Speak to building control about the plan, not to the product title.

Related pages

Basement window for a shed

Basement window

Best basement window: which listing fits a named job

Basement window buying guide before you pick a row

How to fit a basement window into an existing opening

Sources

Public UK sources and the amazon.co.uk listings compared on this page. Updated 22.10.2026.

  1. Approved Document B Volume 1, paragraphs 2.10 and 2.16 (retrieved 22.10.2026)
  2. Approved Document L Volume 1, Table 4.2 (retrieved 22.10.2026)
  3. Approved Document K, critical glazing locations (retrieved 22.10.2026)
  4. Approved Document F Volume 1, paragraphs 3.14 to 3.15 (retrieved 22.10.2026)
  5. FENSA: jobs you can register (retrieved 22.10.2026)
  6. Planning Portal: basements and building regulations (retrieved 22.10.2026)
  7. BS 8102:2022, protection of below-ground structures against water (retrieved 22.10.2026)
  8. Hampshire building control: replacement windows and doors (retrieved 22.10.2026)
  9. diy.com RISOR 90 by 40 cm tilt window, not in this set (retrieved 22.10.2026)