What is the 25% glazing rule?
In England, Approved Document L limits the glazing in a new dwelling extension to 25 percent of the extension’s floor area. That total counts windows, doors, rooflights and roof windows. A design over the limit needs a calculation route to prove it still complies.
What counts towards the 25 percent?
- Windows and external doors
- Rooflights, including lanterns
- Roof windows
- Any openings in the extension wall that are new
Can the allowance go up?
Yes. The limit can be raised by the area of any windows and doors removed or covered by the extension. Over that, the installer needs a compensating calculation, using better U-values elsewhere or a whole-dwelling method.
Does it apply to conservatories?
Not to a conservatory that qualifies for the exemption, because it is thermally separated from the house. It does apply to extensions such as orangeries, and the rooflight U-value limit of 2.2 W/m²K also applies.
Order from the trade counter at Unit 1, Friar Walk, Worthing, with free delivery within 30 miles. See the rooflights range for specs.
Need a trade price? Call the trade counter on 01903 262 636 or visit us at Unit 1, Friar Walk, Worthing. We offer free delivery within 30 miles.
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