Two limits for every element
Anyone insulating a wall, re-roofing or cladding a basement ceiling in Germany in 2026 has to keep two different U-values in mind. The first is required by law and applies to almost every major change to the building envelope. The second is stricter and decides whether you receive a subsidy for the measure. This post compares both values and explains when each limit applies.
The legal duty: Annex 7
The requirements for existing buildings are set out in Annex 7 of the Building Modernisation Act (Gebäudemodernisierungsgesetz). That is the name of the former Building Energy Act (GEG) since the end of July 2026. The maximum U-values did not change with the new name; the corresponding section is now § 36.
The duty applies when you renew, replace or install an external element for the first time. Small changes affecting no more than 10 percent of the area of the relevant element group are exempt. So if you re-render one side of the house that makes up a quarter of the wall area, you must meet the maximum value there. If you patch a small area, you do not.
The subsidy: BEG technical minimum requirements
The federal funding programme for efficient buildings (BEG) subsidises insulation as a single measure with a base rate of 15 percent. With an individual renovation roadmap (iSFP) another 5 percentage points are added. Under the rules in force since summer 2026, however, this bonus is only granted for single-unit buildings from eligible costs of €30,000. Every subsidy requires that the element meets the technical minimum requirements after renovation and that an accredited energy efficiency expert is involved.
The values compared
| Element (residential) | Law, Annex 7 | BEG subsidy |
|---|---|---|
| External wall | 0.24 | 0.20 |
| Pitched roof, dormer | 0.24 | 0.14 (dormer 0.20) |
| Top floor ceiling | 0.24 | 0.14 |
| Flat roof with waterproofing | 0.20 | 0.14 |
| Basement ceiling, ceiling to unheated rooms | 0.30 | 0.25 |
| Wall against ground | 0.30 | 0.25 |
| Floor against ground | 0.30 (floor build-up only: 0.50) | 0.25 |
| Ceiling above outside air | 0.24 | 0.20 |
| Windows, French windows | 1.3 | 0.95 |
| Roof windows | 1.4 | 1.0 |
| External doors | 1.8 | 1.3 |
All figures in W/(m²K). The legal values come from Annex 7 as amended on 23 July 2026. The subsidy values come from the technical minimum requirements of the BEG EM guideline of 21 December 2023. For the new guideline in force since 17 August 2026, the technical annex was not available at our editorial deadline (24 September 2026). Please check the values with your energy consultant before applying.
Special cases you should know
- Limited insulation thickness: if insulation cannot be made arbitrarily thick for technical reasons, for example because of a roof overhang, the legal requirement is met when the thickest possible insulation with λ 0.035 is installed. For blown-in insulation or renewable materials λ 0.045 is sufficient.
- Cavity wall insulation: for two-leaf masonry the subsidy does not demand a U-value but an insulation material with λ of at most 0.035.
- Listed buildings and buildings worth preserving: the subsidy then requires 0.45 for external walls and 0.65 for internal insulation of exposed timber framing.
- Roofs of listed buildings: here the subsidy requires the thickest possible insulation with λ 0.040.
What this means in practice
For external walls, the gap between legal duty and subsidy is often only two to four centimetres of insulation. For roofs it is larger: getting from 0.24 to 0.14 means roughly ten centimetres more at λ 0.035. Since labour for scaffolding, render or roofing has to be paid anyway, stepping up to the subsidy threshold almost always pays off, especially as the measure lasts for decades.
With the U-value calculator you can check your planned build-up against both values. It also shows how many centimetres of insulation with λ 0.035 are missing to each limit. How the U-value is calculated is explained in the pillar article Calculating the U-value.