What is a compensatory heat loss calculation, and when do you need one?
Many extensions meet the energy requirements of the building regulations the simple way: every new element meets the maximum U-values in the guidance, and the windows, doors and rooflights add up to no more than 25% of the extension's floor area, plus the area of any existing openings the extension builds over or removes.
A compensatory heat loss calculation is what you use when the design doesn't fit that pattern. It shows that the extension as designed loses no more heat than a compliant version of itself would. Where one part falls short, usually because there is more glazing, better performance elsewhere makes up for it.
When you need one
The two common reasons are:
- More glazing than the 25% allowance. A glazed gable to a garden, a run of sliding doors or a generous rooflight can easily take an extension past the limit.
- An element that can't meet its U-value. Scotland's own example is an attic conversion where headroom under the coombe ceilings leaves no room for enough insulation (Technical Handbook, Annex 6.A.1). The Welsh guidance gives the example of a poorly performing wall being compensated by a better one (paragraph 13.3).
How the calculation works
The method is the same in principle across the UK:
- Set up a notional (or benchmark) extension. It has the same size and shape as the one you are proposing, the maximum U-values from the guidance, and openings equal to the 25% allowance, plus any openings built over or lost.
- Work out its heat loss. For each element, multiply its area by its U-value to get a rate of heat loss in W/K, then add them up.
- Do the same for the extension as designed, using the areas and U-values you actually propose. These can be better or worse than the guidance, element by element.
- Compare. The proposed extension must not lose more heat than the notional one.
England, Wales and Northern Ireland express step 4 as an area-weighted U-value that must be no greater than the benchmark's. Because both extensions are the same size and shape, that comes to the same thing as comparing heat loss.
A worked example
This is Scotland's own example from Annex 6.A.2 of the Technical Handbook (Domestic). A single-storey, flat-roofed rear extension has a floor area of 18.0 m² and is built over 1.5 m² of existing openings. The design has 5.0 m² of windows and doors and 2.0 m² of rooflights, which is more than the allowance of 25% of 18.0 m² (4.5 m²) plus the 1.5 m² built over.
Notional extension (U-values from Table 6.2):
| Element | Area (m²) | U-value (W/m²K) | Heat loss (W/K) |
|---|---|---|---|
| Floor | 18.0 | 0.15 | 2.70 |
| Roof | 18.0 | 0.12 | 2.16 |
| External walls | 26.8 | 0.17 | 4.55 |
| Openings (25% + built over) | 4.5 + 1.5 | 1.4 | 8.40 |
| Total | 17.81 |
Proposed extension:
| Element | Area (m²) | U-value (W/m²K) | Heat loss (W/K) |
|---|---|---|---|
| Floor | 18.0 | 0.10 | 1.80 |
| Roof | 16.0 | 0.10 | 1.60 |
| External walls | 26.3 | 0.15 | 3.94 |
| Windows and doors | 5.0 | 1.4 | 7.00 |
| Rooflights | 2.0 | 1.7 | 3.40 |
| Total | 17.74 |
The proposed extension loses 17.74 W/K against the notional 17.81 W/K, so it complies. Better insulation to the floor, roof and walls, and better-performing rooflights, pay for the extra glazing.
Differences worth knowing
Scotland: the compensatory approach
- The comparison is made on the rate of heat loss (W/K). The notional extension's openings are 25% of the extension's floor area plus the area of existing openings built over (clause 6.2.8).
- Only the new work counts. The approach lets you vary the U-values of "the elements involved in the work" (clause 6.2.8), so improvements to the existing house can't be used to make up a shortfall in the extension.
- One compensatory calculation can cover separate areas of work under the same building warrant, provided the same assessment criteria apply to each (Annex 6.A.0).
- For conversions, the method can only be used if the recommended U-values are met in full, not where they are met only as far as is reasonably practicable (Annex 6.A.0).
- No localised area should be worse than 0.7 W/m²K for walls and floors or 0.35 W/m²K for roofs, and glazing poorer than 3.3 W/m²K should not be used (clause 6.2.1).
England: the area-weighted U-value approach
- The standard approach limits windows, roof windows, rooflights and doors to 25% of the extension's floor area, plus the area of windows and doors that no longer exist or are no longer exposed because of the extension (paragraph 10.7).
- The area-weighted U-value approach compares the area-weighted U-value of the whole extension, windows and doors included, with that of an extension of the same size and shape that complies with paragraph 10.7 (paragraph 10.9).
Wales: the U-value trade-off approach
- Windows and doors are limited to 25% of the extension's internal floor area, plus openings lost to the extension (paragraph 10.11). The guidance also advises not going below 20%, for daylight (paragraph 10.12).
- The benchmark is the same size and shape as the proposal. Unlike in Scotland, if you propose compensating insulation improvements to the existing dwelling, the calculation can include them (paragraph 13.6).
- The existing dwelling can't serve as the benchmark (paragraph 13.4), and no wall, floor or roof may be worse than the limiting values in column (b) of Table 11.1 (paragraph 13.9).
Northern Ireland: the Calculated Trade-off Approach
- The 25% allowance adds the area of windows, roof windows, rooflights or doors that no longer exist or are no longer exposed (paragraph 3.11).
- Larger areas of glazing "may be acceptable, especially where this is required to make the extension consistent with the external appearance or character of the building being extended", with compensating measures (paragraph 3.12).
The whole-dwelling alternative
Every nation also allows a route based on SAP (the Standard Assessment Procedure) that assesses the whole dwelling, extension included. In Scotland, it is the whole dwelling approach, where the dwelling's delivered energy rate must not exceed its target: the DDER (Dwelling Delivered Energy Rate) must not be more than the TDER (Target Delivered Energy Rate) (clause 6.2.8). In England, it is paragraph 10.10. Wales has the equivalent primary energy / carbon emissions target approach (Section 13). Northern Ireland has the Equivalent Carbon Target Approach (paragraphs 3.15 and 3.16). These routes are most practical where SAP data for the existing house is already available.
Sources
This guide refers to the editions in force in September 2026:
- Scotland: Building Standards Technical Handbook, Domestic (April 2026), clauses 6.2.1 and 6.2.8, and Annex 6.A.
- England: Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments), paragraphs 10.7, 10.9 and 10.10. A 2026 edition applies from 24 March 2027.
- Wales: Approved Document L, Volume 1: Dwellings (2022 edition), paragraphs 10.11 and 10.12, and Section 13. A 2026 edition applies from 4 March 2027.
- Northern Ireland: Technical Booklet F1 (2022), paragraphs 3.8 to 3.16.
The worked example is reproduced from Annex 6.A.2 of the Scottish Building Standards Technical Handbook (Domestic), © Crown copyright 2026. Contains public sector information licensed under the Open Government Licence v3.0.