GlazeMonkey
Part L · Extensions & Renovations
This is the only structural decision in the tool, and everything below follows from it.
Tick both if the job does both. They are two independent tests against two different tables, and a great many real jobs trip over the second one while concentrating on the first.
The verdict is the worst of every active test, so one failing element fails the job.
The export unlocks once there is a real verdict — it will not sell you a report on half a model.
Free to use on screen. See an example report
New floor area. Its openings are limited to 25 % of the extension's floor area, and every new element must meet the Table 4.2 limits.
Windows, doors, roof windows and rooflights together may not exceed 25% of the extension’s floor area, plus the area of any existing openings the extension covers over (para 10.10).
Don’t forget the covered-over openings. A back door that now opens into the extension, a window bricked up to build it — their area is added to your allowance. Compare the existing and proposed elevations side by side and you usually find one or two.
Over the allowance is not a fail. You then either show the extension’s area-weighted U-value is no worse than a notional extension built to Table 4.2 with 25% openings — the tool computes that live — or commission a whole-dwelling SAP calculation.
U-values come from the manufacturer for openings (whole-unit, never centre-pane) and from a proper calculation for the opaque elements. U-Monkey does those free; the limits here are wall 0.18, floor 0.18, roof 0.15.
| Description | Type | W (m) | H (m) | Qty | Area | U (W/m²K) |
|---|
| Description | Area each (m²) | Qty | Area |
|---|
| Element | Proposed A (m²) | Proposed U | U·A (W/K) | Notional A (m²) | Notional U | U·A (W/K) |
|---|
Swipe the table sideways to see the notional-extension columns.
Approved Document L Volume 1 (2021) limits the windows, doors and rooflights of an extension to 25 % of its floor area plus any openings it covers over (para 10.10), and every new element must meet the Table 4.2 U-values. Go over the allowance and you have two routes: show that the area-weighted 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 25 % openings, or commission a whole-dwelling SAP calculation. GlazeMonkey runs the first two checks and the area-weighted comparison live, tests a conservatory or porch against the five exemption conditions, and checks renovated, replaced and retained elements against the Table 4.3 thresholds and the 15-year payback rule. The notional extension keeps the proposed openings in proportion up to the allowance and returns the excess to wall (0.18) or roof (0.15) at their limiting values. Method and figures: ADL1 2021 incorporating 2023 amendments, Sections 4, 10 and 11 and Appendix C. Not a substitute for a Building Control body's judgement.
A fully-branded, Building Control–ready PDF: the opening-area check, Table 4.2 element checks, the area-weighted U-value comparison against the notional extension, any conservatory exemption statement and existing-element upgrades.
£35 + VAT (20%)
Secure payment via Stripe · £35 + VAT (20%) = £42.00 total. Included: your Building Control–ready PDF with a unique Report ID (verifiable at energycount.co.uk/verify), re-download free for 24 hours and free re-export for genuine corrections within 1 hour (up to 5). A VAT invoice is emailed automatically — add your company name and VAT number at checkout. By paying you agree to our Conditions of Sale — the report is supplied immediately, so the 14-day cancellation right is waived on delivery.
This tool answers one question: does the work comply with Approved Document L for an existing dwelling? It runs two separate tests, and you only need the one that matches your job.
Adding floor area? That is an extension — step 2. Its windows, doors and rooflights are limited to 25% of the extension's floor area, and every new element has to meet the Table 4.2 U-values.
Working on what is already there? Re-roofing, re-rendering, dry-lining, cavity fill, new windows — that is step 3, and the test is Table 4.3 plus the 15-year payback rule.
Tick both if the job does both. The checkboxes at the top of step 1 decide which sections you see.
Every box has a small i beside it. Click it and it says what the number is and where to get it.
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 (para 10.10). Go over that and you are not automatically failed — you get two routes:
Covered-over openings are the bit people miss. A back door that now opens into the extension, a window bricked up to build it — their area is added to your allowance, not counted against it.
| New element | Limiting U-value |
|---|---|
| Wall | 0.18 W/m²K |
| Floor | 0.18 W/m²K |
| Roof | 0.15 W/m²K |
| Windows, doors, in-plane roof windows | 1.4 W/m²K (or WER band B / DSER band B–C) |
| Rooflights on an upstand | 2.2 W/m²K |
Use the manufacturer's whole-unit U-value, not the centre-pane figure — centre-pane always looks better and is not what the regulation asks for. For the opaque elements, work the U-value out properly in U-Monkey and bring the number back here.
A conservatory or porch is 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.
Fail any one and it is simply an extension, assessed like any other. Extending the house's heating into it is the single most common way the exemption is lost; a plug-in portable heater does not count.
An element only has to be upgraded once the work is big enough to trigger it: more than 50% of that element's surface, or a major renovation touching more than 25% of the whole envelope. Below that, you simply must not make it worse (para 4.11).
| Existing element | Threshold | Upgrade to |
|---|---|---|
| Cavity wall | 0.70 | 0.55 |
| Other wall | 0.70 | 0.30 |
| Floor | 0.70 | 0.25 |
| Roof | 0.35 | 0.16 |
If the existing element is already better than its threshold, no upgrade is required. If it is 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 fuel price and degree-days. Even then an upgraded element should generally not end up worse than 0.70.
It implements Approved Document L Volume 1 (2021, 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 — 01202 623236.