Quick answer: new walls, roofs and floors should meet the Table 6.2 averages — 0.17, 0.12 and 0.15 W/m²K — with windows, doors and roof windows at 1.4 and rooflights at 2.1, and nothing worse than 0.70 (walls, floors), 0.35 (roofs) or 3.3 (glazing). Windows, doors and rooflights are limited to 25% of the floor area plus any openings built over (clause 6.2.8). Over that, the extension complies if its heat loss is no greater than a notional extension's (the compensatory approach, Annex 6.A); failing that, a SAP calculation for the whole dwelling.
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The U-values: Table 6.2
Clause 6.2.8 says the new fabric of an extension should be designed to the maximum U-values in clause 6.2.1 — Table 6.2. They are area-weighted averages for all elements of the same type, so one wall can be a little worse than 0.17 if another is better, as long as the average holds. Separately, no part of an element may be worse than the individual limits, which are there to limit the risk of condensation.
| Element | Table 6.2 average (W/m²K) | No part worse than | England, Table 4.2 |
|---|---|---|---|
| External walls | 0.17 | 0.70 | 0.18 |
| Floors | 0.15 | 0.70 | 0.18 |
| Roofs | 0.12 | 0.35 | 0.15 |
| Windows, doors and roof windows | 1.4 | 3.3 | 1.4 |
| Rooflights (on the horizontal plane) | 2.1 | 3.3 | 2.2 |
Roof windows sit in the plane of the roof and are assessed in the vertical position; rooflights, on a kerb, are quoted on the horizontal plane. Use the manufacturer's whole-unit U-value, never the centre-pane figure.
The 25% allowance
"To limit heat loss through openings", clause 6.2.8 limits the area of windows, doors and rooflights in an extension to 25% of its floor area plus the area of any existing openings built over — a back door that now opens into the extension, a window bricked up to build it. Those built-over openings are worth finding: they add to the allowance, and they add glazing to the notional extension too.
Over 25%: the compensatory approach
The compensatory approach lets the U-values and the glazing vary, provided the extension's overall heat loss — the sum of U × A, in W/K — is no greater than that of a notional extension of the same size and shape, built to the Table 6.2 values. Three details of the notional extension decide the answer:
- its windows and doors are at 1.4 and cover 25% of the floor area plus the openings built over;
- it has no rooflights — its roof is the whole roof area at 0.12;
- only the 25% comes off its walls; the built-over openings are added as glazing without taking any wall away.
Every element of the real extension still has to respect the individual limits — 0.70, 0.35 and 3.3 — however well the rest performs. And since the February 2023 changes the comparison is the extension on its own: upgrading the existing house no longer counts towards it (a Ministerial View, V2023/3, turned down an extension that relied on a loft upgrade).
Worked example: the handbook's Annex 6.A extension
An 18 m² extension with 5 m² of windows and doors at 1.4, two rooflights (2 m²) at 1.7, and 1.5 m² of existing openings built over. The allowance is 25% × 18 + 1.5 = 6.0 m² against 7.0 m² of openings, so it is over and goes to the compensatory approach. The gross walls are 31.3 m², the roof 18 m².
| Element | Proposed: area × U = W/K | Notional: area × U = W/K |
|---|---|---|
| External walls | 26.30 × 0.15 = 3.95 | 26.80 × 0.17 = 4.56 |
| Roof | 16.00 × 0.10 = 1.60 | 18.00 × 0.12 = 2.16 |
| Floor | 18.00 × 0.10 = 1.80 | 18.00 × 0.15 = 2.70 |
| Windows and doors | 5.00 × 1.40 = 7.00 | 6.00 × 1.40 = 8.40 |
| Rooflights | 2.00 × 1.70 = 3.40 | — |
| Heat loss | 17.75 W/K | 17.82 W/K |
The notional walls are 31.3 − 4.5 = 26.8 m² (only the 25% comes off), its roof is the whole 18 m², and its windows and doors are 4.5 + 1.5 = 6.0 m². The proposed extension loses 17.75 W/K against 17.82 — it complies, despite being over the 25%. (The handbook prints 17.74 and 17.81 because it truncates each figure; GlazeMonkey rounds. The conclusion is the same.) Swap the 1.7 rooflights for 2.1 units and the heat loss rises to 18.54 W/K: over the notional, and a fail.
Beyond that: a SAP calculation
If the compensatory approach cannot get there, the remaining route is a SAP calculation for the whole dwelling with its extension, shown to meet its target (DDER not more than TDER). That is a job for an energy assessor — ask us for a quote.
Conservatories and porches
- Exempt (schedule 1, type 18). A single-storey conservatory or porch attached to the house and ancillary to it, whose glazing meets the safety standard (regulation 9 and schedule 5 paragraph 4.8) — unless it is over 8 m², contains a flue, a fixed combustion appliance installation or a sanitary facility, or is within 1 m of a boundary.
- Not exempt, thermally divided and under 50 m² (clause 6.2.11). The glazing is unlimited in area but should have an area-weighted average U-value of 1.8 or better, with no unit worse than 3.3; walls, roof and floor to Table 6.2; the dividing walls, doors and windows no worse than the house's own exposed ones; and any heating on its own controls.
- 50 m² or more: subject to Standard 6.1 — shown with a SAP calculation.
- Open to the house, or a porch that is not exempt: an extension like any other, with the 25% limit.
A conservatory, for these purposes, has at least three-quarters of its roof and half of its external walls translucent. Less than that and it is an extension whatever it is called.
How GlazeMonkey does it
- Choose Scotland under "Where is the dwelling?" in step 1 (the button above does it for you).
- Enter the floor area, the windows, doors and rooflights, any openings built over, and the gross wall, roof and floor areas with their U-values.
- The result shows the Table 6.2 checks, the allowance, and the heat loss against the notional extension, live. If it fails, "How to make it comply" lists the changes that pass, each re-checked.
- The PDF — a Section 6 compliance report — shows every figure, the notional extension line by line, and cites the clauses it used.
Frequently asked questions
What U-values does an extension need in Scotland?
Section 6 of the Technical Handbook points an extension at the Table 6.2 values, as area-weighted averages for all elements of the same type: walls 0.17, floors 0.15, roofs 0.12, windows, doors and roof windows 1.4 and rooflights 2.1 W/m²K. No single part of a wall or floor should be worse than 0.70, no part of a roof worse than 0.35, and no glazing worse than 3.3.
Is there a 25% glazing limit in Scotland?
Yes. Clause 6.2.8 limits the windows, doors and rooflights of an extension to 25% of its floor area, plus the area of any existing openings the extension builds over. Going over it is not a fail: the compensatory approach can still show compliance.
What is the compensatory approach?
It lets the U-values and the glazing of an extension vary, provided its overall heat loss is no greater than a notional extension of the same size and shape built to the Table 6.2 values with windows and doors equal to 25% of the floor area plus the openings built over. The comparison is the sum of U × A in W/K; Annex 6.A of the handbook works an example.
Can I upgrade the existing house to make up for extra glazing?
Not within the compensatory approach: since the February 2023 changes it compares the extension alone, and a Ministerial View (V2023/3) refused an extension that relied on a loft upgrade. The remaining route is a SAP calculation for the whole dwelling with its extension, against its target.
Does a conservatory need a building warrant in Scotland?
Not if it is exempt under schedule 1, type 18: a single-storey conservatory or porch attached to the house, no more than 8 m², with no flue, fixed combustion appliance or sanitary facility, not within 1 m of a boundary, and glazed to the safety standard. Otherwise it needs a warrant; kept thermally divided from the house and under 50 m², it is assessed under clause 6.2.11.
How is Scotland different from England?
The values are tighter (walls 0.17 against 0.18, roofs 0.12 against 0.15, floors 0.15 against 0.18) but they are averages across each element type rather than caps on every element, with separate worst-case limits. And when the glazing is over 25%, Scotland compares heat loss in W/K against a notional extension with 1.4 windows and doors and no rooflights, where England compares area-weighted U-values.
Sources
- Scottish Government, Building standards technical handbook: domestic — April 2026 — Section 6: Table 6.2, clauses 6.2.1, 6.2.8 and 6.2.11, Annex 6.A; Section 0: schedule 1, type 18.
- Scottish Government, Ministerial View V2023/3 — building insulation envelope.
Also: the rules in Northern Ireland · England's area-weighted method · soakaways for a Scottish building warrant.