Part L Extension Calculator — 25% Glazing Rule & Area-Weighted U-Value

1 Project details

Project / address

Client

Reference

Date

What does the work include?

An extension — new floor area added to the dwelling (including a conservatory, orangery, garden room or porch) Work to existing elements — re-roofing, re-rendering, dry-lining, cavity fill, replacing a wall, roof, floor, window or door, or a garage / loft conversion

Applies to dwellings in England under Approved Document L Volume 1 (2021 edition incorporating 2023 amendments). The 2026 edition takes effect for work starting on or after 24 March 2027.

Result

Free to use on screen. See an example report

Report logo: Energycount change — consultants: brand the report as your own

2 The extension

Floor area of the extension (m², internal)All storeys of the extension added together

Type of extension

Storeys Only affects the default floor and roof areas

Windows, doors, roof windows and rooflights in the extension

Description	Type	W (m)	H (m)	Qty	Area	U (W/m²K)

Table 4.2 limits: windows, doors and 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. For an odd shape enter the area as the width with a height of 1.

Existing windows and doors the extension covers over added to the allowance

Description	Area each (m²)	Qty	Area

Only openings that no longer exist or are no longer exposed because of the extension count (para 10.10) — a back door that now opens into the extension, a window bricked up to build it.

Walls, roof and floor of the extension net areas, heat-loss elements only

External wall area, net of openings (m²)Exclude the wall shared with the house

Wall U-value (W/m²K)

Limit 0.18 · exact value in U-Monkey

Roof area, net of rooflights (m²)Measured on the slope for a pitched roof

Roof U-value (W/m²K)

Limit 0.15

Ground floor area (m²)The heat-loss floor — normally the footprint

Floor U-value (W/m²K)

Limit 0.18

Assessor options

Conservative notional — set every notional opening at 1.4 W/m²K rather than crediting rooflights at their 2.2 limit. Harder to pass; some building control bodies prefer it. Always include the area-weighted comparison in the report, even when the reference method passes.

Area-weighted U-value — proposed extension vs notional extension

Element	Proposed A (m²)	Proposed U	U·A (W/K)	Notional A (m²)	Notional U	U·A (W/K)

3 Work to existing elements — Table 4.3 thresholds, replacements and controlled fittings

Description	Element	Work	Area (m²)	Existing U	% renovated	Proposed U	Cost (£, opt.)	Result

Major renovation — the works renovate more than 25 % of the dwelling's total envelope area, so every renovated element is caught whatever its own percentage

Payback assumptions (para 4.13 economic feasibility — indicative only)

Heating degree-days (K·day/yr)UK typical 1,800–2,400 at base 15.5 °C

Fuel price (£/kWh delivered)Mains gas ≈ 0.06–0.07; electricity ≈ 0.25

Heating system efficiencyCondensing boiler 0.85–0.92; heat pump 2.5–3.5

Annual saving = ΔU × area × degree-days × 24 ÷ 1000 ÷ efficiency × price. Simple payback = cost ÷ saving. A payback over 15 years lets the element be upgraded to the best that is economically feasible instead of the Table 4.3 improved value (para 4.13). Building Control may ask for your own figures.

About this tool & the method

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.