Every box has an i beside it saying where the number comes from. The same guidance is built into the tool under ? Help.
Renovating rather than extending? Open RenoMonkey → — same calculator, opens on the existing-elements section.
The 25% rule
An extension's windows, doors, roof windows and rooflights added together may not exceed 25% of the extension's floor area, plus the area of any existing openings the extension covers over (paragraph 10.10). Floor area is internal, all storeys added together.
Covered-over openings are the bit people miss. A back door that now opens into the extension, a window bricked up to build it — those are not new openings, they are existing ones that are no longer exposed, and their area is added to your allowance. Put the existing and proposed elevations side by side and you usually find one or two.
Over the allowance, you have two routes:
- The area-weighted route. Show the whole extension's average U-value is no worse than a notional extension of the same size and shape, built exactly to Table 4.2 with openings at 25%. The tool computes this live, always, as supporting evidence — whether you need it or not.
- A whole-dwelling SAP calculation. Outside this tool. That is assessor work, and we do it.
Table 4.2 — every new element
| New element | Limiting U-value | Where to get your figure |
|---|---|---|
| Wall | 0.18 W/m²K | Calculate the build-up — U-Monkey does it free |
| Floor | 0.18 W/m²K | A ground floor depends on its perimeter-to-area ratio too, so calculate rather than assume |
| Roof | 0.15 W/m²K | The tightest of the opaque elements |
| Windows, doors, in-plane roof windows | 1.4 W/m²K | Manufacturer's whole-unit U-value, or WER band B / DSER band B–C |
| Rooflights on an upstand | 2.2 W/m²K | Manufacturer's figure, quoted the way Table 4.2 assesses it |
Centre-pane U-values are not what the regulation asks for. They always look better than the whole unit, and using one is the fastest way to have a report sent back.
Conservatories and porches
A conservatory or porch is exempt only while all five of these hold:
- at ground level;
- not more than 30 m²;
- thermally separated from the house — the wall, windows and doors between are retained, or replaced to the same standard;
- glazing meeting Part K;
- no heating of any kind.
To be a conservatory at all it must also be substantially glazed: at least 75% of the roof and 50% of the external walls. Fail any one condition and it is simply an extension, assessed like any other. Extending the house's heating into it is the commonest way the exemption disappears — a plug-in portable heater does not count, but a radiator on the house system does.
Table 4.3 — work to existing elements
An element only has to be upgraded once the work is big enough to trigger it: more than 50% of that element's own surface, or a major renovation touching more than 25% of the whole envelope. Below that, you simply must not make it worse (paragraph 4.11).
| Existing element | Threshold | Upgrade to |
|---|---|---|
| Cavity wall | 0.70 | 0.55 W/m²K |
| Other wall | 0.70 | 0.30 W/m²K |
| Floor | 0.70 | 0.25 W/m²K |
| Roof | 0.35 | 0.16 W/m²K |
Already better than the threshold? Nothing to do. Worse? It must reach the improved value — unless that is not technically, functionally or economically feasible. Economic feasibility has a number attached: a simple payback longer than 15 years. Enter a cost and the tool runs that test on your own degree-days, fuel price and system efficiency. Even where the argument succeeds, an upgraded element should generally not end up worse than 0.70.
A replacement element has two tests to pass, not one: it must meet Table 4.2 and be no worse than the element it replaced.
Reading the result
- COMPLIES — every test in every active section passes.
- COMPLIES* — it passes, but an element relies on a recorded paragraph 4.13 feasibility justification. Keep the reasoning; Building Control will want to see it.
- DOES NOT COMPLY — the notes say which test failed. Where better glazing alone could close the gap, the tool tells you the average opening U-value you would need; where it cannot, it says so.
- INCOMPLETE or AWAITING INPUT — something essential is still blank.
The export unlocks only once there is a real verdict — it will not sell you a report on half a model.
What this tool is, and is not
It implements Approved Document L Volume 1 (2021 edition incorporating the 2023 amendments) for England — Sections 4, 10 and 11 and Appendix C — with the arithmetic shown line by line. The 2026 edition takes effect on 24 March 2027 (work with a building notice or full plans in before then keeps the 2021 edition if it has commenced by 24 March 2028); this tool follows the edition in force now.
It is not a substitute for a Building Control body's judgement, and where the area-weighted route is not enough, a whole-dwelling SAP calculation is the remaining option. We do those.
Frequently asked questions
What exactly is the 25% rule?
An extension's windows, doors, roof windows and rooflights added together may not exceed 25% of the extension's floor area, plus the area of any existing openings the extension covers over (Approved Document L Volume 1, 2021, paragraph 10.10). Going over is not an automatic failure — it just means you need one of the optional approaches instead.
What are covered-over openings, and why do they matter?
They are existing openings that no longer exist or are no longer exposed because of the extension: a back door that now opens into it, a window bricked up to build it. Their area is added to your 25% allowance rather than counted against it, so they are worth hunting for. Compare the existing and proposed elevations side by side and you usually find one or two.
What happens if I am over the 25% allowance?
Two routes. Either show the area-weighted average U-value of the whole extension is no worse than a notional extension of the same size and shape built to Table 4.2 with openings at 25% — the tool computes that live — or commission a whole-dwelling SAP calculation. The tool runs the first; the second is a job for an assessor.
When is a conservatory actually exempt?
Only while all five conditions hold: at ground level, not more than 30 m², thermally separated from the house, glazing meeting Part K, and no heating of any kind. To be a conservatory at all it must also be substantially glazed — at least 75% of the roof and 50% of the external walls. Extending the dwelling's heating into it is the commonest way the exemption is lost; a plug-in portable heater does not count.
When does an existing element have to be upgraded?
Not every time you touch it. The obligation is triggered when more than 50% of that element's surface is renovated, or by a major renovation covering more than 25% of the whole envelope. Below that you simply must not make it worse. Once triggered, compare the existing U-value against the Table 4.3 threshold — cavity wall 0.70, other wall 0.70, floor 0.70, roof 0.35 — and if it is worse, it must reach the improved value of 0.55, 0.30, 0.25 or 0.16 respectively.
What is the 15-year payback rule?
Approved Document L paragraph 4.13 lets an element be upgraded only to the best that is technically, functionally and economically feasible. Economic feasibility has a number attached: a simple payback longer than 15 years. Enter the cost of the upgrade and the tool runs that test on your own degree-days, fuel price and system efficiency. Even where the argument succeeds, an upgraded element should generally not end up worse than 0.70 W/m²K.
Where do I get the U-values from?
For windows, doors and rooflights, the manufacturer's whole-unit U-value — not the centre-pane figure, which always looks better and is not what the regulation asks for. For walls, roofs and floors, calculate the build-up properly; U-Monkey does that free, including the perimeter-to-area method a ground floor needs. For an existing element where the build-up is unknown, an age-band default from SAP or RdSAP conventions is acceptable, but say which you used.
Does this cover RenoMonkey too?
Yes. GlazeMonkey and RenoMonkey are the same calculator behind two front doors — GlazeMonkey opens on the extension section, RenoMonkey on the existing-elements section. Both sections are available in either, and everything on this page applies to both.
Where the method comes from
The document behind every figure in the tool.
- Approved Document L, Volume 1: Dwellings (2021, incorporating 2023 amendments) Section 10 for extensions and the 25% opening allowance, Table 4.2 for new elements, Section 11 and Table 4.3 for renovating thermal elements, paragraph 4.13 for the feasibility and 15-year payback tests, and paragraph 0.14 for conservatories and porches.