Extension Glazing Calculator
Planning an extension with bi-folds, a roof lantern or lots of glass? Part L limits glazing to 25% of floor area (plus any windows you're covering over), and anything above that counts as excessive glazing. Use this calculator to check if your design fits within that allowance. If your glazing is over the limit and it doesn't pass this reference method check, the same design could still pass with an area-weighted U-value check or a full SAP calculation.
Based on Approved Document L, paragraph 10.7(d). This tool is a reference method check only and doesn't replace a formal SAP calculation where one is required.
Glazing Area Check
PendingU-Value Check
PendingConfirm all elements meet Part L limits above.
Overall Result
Based on these figures, your extension should comply with Part L.
This calculator uses the simplified 25% rule. The good news is there's another way.
Let us run an area-weighted check — £95
If your floor or walls perform well thermally, they can make up for having more glazing. Lots of extensions over 25% still pass this way.
Upload Your Plans →If area-weighting doesn't pass, we'll credit the £95 toward a full SAP calculation.
When This Calculator Won't Be Enough
This works for most simple rear and side extensions. There are some limitations:
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Your glazing is excessive (over 25%)
Excessive glazing can often still pass using the area-weighted method or a full SAP calculation. We'll work out the best route for you.
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You're doing other work to the house
Replacing windows, adding insulation, or altering the heating system as part of the project? Part L has separate requirements for these that this calculator doesn't cover.
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It's a conservatory, porch or pool building
These have their own rules under Part L. Some are exempt, others need thermal separation from the main house. We can advise on what applies to yours.
Not sure? Send us your drawings and we'll tell you what you need. No obligation.
Get in touch →How Much Glazing Can My Extension Have?
The calculator does this for you, but the maths behind a house extension glazing calculation is simple enough to sanity-check by hand. Your extension is allowed glazing of up to 25% of its floor area (the 25% glazing rule), plus the area of any existing windows or doors the extension covers over (you are not losing those to the outside any more, so you get the area back as credit):
Glazing allowance = (extension floor area × 0.25) + area of existing openings covered over
Then add up the area of every proposed opening – windows, external doors, roof windows, rooflights and lanterns. If the total is within your allowance, the extension passes the reference method with no full SAP needed. This 25% allowance is set out in paragraph 10.7(d) of Approved Document L, Volume 1. Three worked examples:
Example 1 – a simple rear extension
A 4m × 5m extension (20m² floor area) that does not cover any existing windows or doors.
Allowance = 20 × 0.25 = 5m². Keep your windows and doors within 5m² of glazing and you pass.
Example 2 – built over an existing patio door
The same 20m² extension, but it is built across an existing patio door of roughly 2.1m × 1.9m (about 4m²).
Allowance = (20 × 0.25) + 4 = 9m². That 4m² of credit is often what lets a glazed rear extension pass.
Example 3 – an over-glazed design
The same 9m² allowance, but the design has a 3m bi-fold (about 6.3m²), two 1.2m × 1.2m windows (2.88m²) and a 1m × 1m rooflight (1m²) – 10.18m² of glazing in total.
10.18m² against a 9m² allowance is over by 1.18m², so the reference method fails and you move to the area-weighted route below.
What Counts as Glazing?
Part L counts every opening in the extension’s external envelope, not just the windows. When you total up your glazing, include all of the following:
- Windows – every new window in the extension walls.
- External doors – including French doors, sliders and bi-folds. Bi-folds are usually the single biggest item, so measure the full door opening, not just the glass.
- Roof windows – pitched, roof-mounted windows such as Velux-style units.
- Rooflights and lanterns – flat-roof glazing and roof lanterns count in full, measured on the developed (sloping) area of the glass.
Area is only half the test. Each element also has to meet the Part L limiting U-values – in an extension, windows and doors must be no worse than 1.4 W/m²K and rooflights no worse than 2.2 W/m²K. The calculator checks both the 25% area and these U-value limits.
See the building regs U-values in full →Is My Extension Over-Glazed?
If your glazing comes out above the allowance, your extension is over-glazed – but that does not mean redesigning. Going over the 25% limit is sometimes called excessive glazing, and there are two routes to prove compliance a different way:
1. Area-weighted U-value calculation
Bigger, better-insulated elements can offset the extra heat lost through the glass. You compare your design against a compliant, 25%-glazed version of the same extension – if its total heat loss is no worse, it passes, with no full SAP needed. In Example 3 above, that 1.18m² of excess could be balanced by upgrading the roof from 0.15 to around 0.11 W/m²K. Most over-glazed extensions pass this way. It is the area-weighted alternative in Approved Document L, paragraph 10.9.
How the area-weighted method works →2. Full (whole-house) SAP
For heavily glazed or complex extensions where even area-weighting cannot demonstrate compliance, a whole-house SAP assesses the extended dwelling as a whole. It is the most thorough route and the fallback when the others do not pass – the SAP alternative in Approved Document L, paragraph 10.10.
About whole-house SAP →Whichever route applies, we will always point you to the cheapest one that passes – and if you pay for a check that then needs a full SAP, we credit it against the cost.
Extension Glazing FAQs
Do rooflights and roof lanterns count towards the 25%?
Yes. Every rooflight, roof window and lantern counts as glazing and goes into the 25% total, measured on the developed (sloping) area of the glass. They also have to meet the 2.2 W/m²K rooflight U-value limit.
Do bi-fold and sliding doors count as glazing?
Yes. All external doors count, and glazed doors such as bi-folds and sliders are usually the largest single item in an extension. Measure the full door opening, not just the glass.
What U-value do the windows and doors need to hit?
In an extension, new windows and doors must be no worse than 1.4 W/m²K, and rooflights no worse than 2.2 W/m²K. Meeting the 25% area limit alone is not enough – the elements have to perform too.
What happens if my extension is over 25%?
You are not stuck. Most over-glazed extensions still comply through an area-weighted U-value calculation, and a full whole-house SAP is the fallback where even that cannot demonstrate compliance. See the section above for how each route works.
Does the existing house get included?
No. The reference method only looks at the new extension – its floor area, its glazing, and any existing openings it covers over. The wider house is only assessed if the project goes down the full SAP route.
Do I still need this if I am replacing windows elsewhere in the house?
Possibly. Replacing windows, adding insulation or changing the heating as part of the same project brings other Part L requirements into play that this calculator does not cover. Send us your plans and we will confirm what applies.
Do conservatories and porches need a glazing calculation?
Usually not, if they are thermally separated from the house – external-quality doors between the two and their own heating. If they are open-plan to the house or share the main heating, they are treated as an extension and the same rules apply.