| Element | U-value (W/m²K) |
|---|---|
| Walls | 0.18 |
| Roofs | 0.15 |
| Floors | 0.18 |
| Windows and roof windows | 1.40 |
| External doors | 1.40 |
| Rooflights | 2.20 |
These are the standard extension figures used for the reference method. They are also the fabric starting point for the notional extension used in a whole house SAP comparison. They are different from the notional U-values for a new home.
The figures describe the complete wall, roof, floor or opening, including the window frame. They are not the performance of the insulation or glass alone. Some windows and doors can also comply using an energy rating.
Meet these U-values and you might not need SAP
If your extension meets the reference method requirements, you do not need an Area Weighted U-value check or whole house SAP calculation. The U-values are one part of that: your openings must also stay within the allowance, and the heating and other building services must meet the relevant standards.
Any existing construction becoming part of the heated building envelope needs checking too. Meeting the energy requirements does not replace the other Building Regulations checks for your extension.
Check the opening allowance
Permitted opening area = 25% of the extension floor area + existing windows and doors covered or removed by the extension.
Count windows, doors and rooflights, including their frames. Our free extension glazing calculator does the sums for you. For an example, read how the 25% glazing rule works.
How do you know your construction meets the U-values?
A thickness of insulation is not a U-value. The materials, their arrangement and the construction details all matter. For windows and doors, ask your supplier for the whole unit figure.
We can calculate the U-values for your proposed wall, roof and floor for building control. Book your U-value calculations.
What if you want more glass?
Exceeding the opening allowance does not mean abandoning the design. An Area Weighted U-value check can show whether better insulation or glazing offsets the extra heat loss. Have your measurements and calculated U-values? Try the free online Area Weighted U-value calculator. Our extension calculations guide explains the available approaches.
Source: Approved Document L, Table 4.2 and paragraphs 10.7–10.10. This reference is for extensions to existing homes in England subject to the 2021 standards.
