What U-values does an extension need in England?
In England, new walls, roofs, floors, windows, doors and rooflights in an extension have to meet the limits in Table 4.2 of Approved Document L Volume 1 (2021 edition, in force from 15 June 2022). Those are 0.18 for walls, 0.15 for roofs and 0.18 for floors, in W/m2K.
What a U-value is
A U-value measures how much heat passes through a wall, roof, floor or window. It is given in W/m2K, which is watts of heat lost through each square metre for every degree of difference between inside and outside. A lower U-value is better, because less heat gets through.
The limits for new elements in an extension
These are the maximum U-values in Table 4.2 of Approved Document L Volume 1 (2021 edition incorporating 2023 amendments) for new fabric elements in existing dwellings, which includes extensions. Figures are in W/m2K:
- roof: 0.15
- wall: 0.18
- floor: 0.18
- windows: 1.4, or Window Energy Rating Band B minimum
- rooflights: 2.2
- doors with more than 60 percent of the internal face glazed: 1.4, or Doorset Energy Rating Band C minimum
- other doors: 1.4, or Doorset Energy Rating Band B minimum (external fire doorsets may be 1.8)
The table gives one figure for roofs. It does not set a separate figure for pitched or flat roofs, or for insulation at ceiling level or in the rafters. The figures are area-weighted averages, except for windows, doors and rooflights, which are checked individually.
Notes that affect real jobs
- A lesser wall standard may be appropriate if meeting 0.18 would cut the floor area of the room by more than 5 percent.
- The floor U-value of an extension can be worked out using the exposed perimeter and floor area of the whole enlarged dwelling.
- The total area of windows, doors and rooflights in the extension should not exceed 25 percent of the extension floor area, plus the area of any openings lost to the extension.
- Rooflight values are based on the developed surface area, which is often more than the roof opening.
The approved document also allows two other routes: showing that the area-weighted U-value of the whole extension is no worse than a compliant extension of the same size and shape, or using SAP to compare against a notional extension. Both need someone competent to run the numbers.
How layers add up
Each layer in a construction resists heat flow, and its resistance is its thickness divided by how well the material conducts heat. Add the resistances of all the layers, with a small allowance for the air films on each surface, and the U-value is 1 divided by the total.
A made-up example, to show the sum only: layers of 0.5, 3.0 and 0.2 m2K/W add up to 3.7, and 1 / 3.7 is a U-value of about 0.27. Real walls have studs, mortar joints and fixings that bridge the insulation, so a simple sum overstates performance. The approved document says U-values should be assessed using the methods and conventions in the BRE publication BR 443, for the whole element.
Who signs it off
Your building control body checks that the extension complies. Ask them at the start what calculations, product data and evidence they want. The U-values themselves should come from a calculation by someone competent, or from measured or rated product data. For windows and doors, the approved document says building control bodies may accept a Window Energy Rating or Doorset Energy Rating from a certification scheme.
Keep the calculation, the specification and the delivery notes with the job file so that what was built matches what was approved.
Which edition applies
The 2021 edition took effect in England on 15 June 2022. A 2026 edition has been published, taking effect on 24 March 2027 for building work that is not higher-risk, with transitional arrangements. The limits for new elements in existing dwellings are the same in both; in the 2026 edition they are in Table 3.2. Check which edition your job falls under with building control.