When the Government published Approved Document L 2026 on 24 March 2026 it added requirement L3: new dwellings must have on-site renewable electricity generation. Paragraph 5.73 sets the benchmark as an annual output at least equal to a reference solar PV array covering 40% of the house's ground floor area. This page explains the rule, works through examples and covers the alternatives when a roof cannot fit it.
Where the 40% rule comes from
The 40% figure is written into Approved Document L 2026 itself. Requirement L3, new in the 2026 edition, requires a system of on-site renewable electricity generation for new dwellings and buildings containing dwellings. Paragraph 5.73 then describes how much a new house should generate, measured against a reference array covering 40% of its ground floor area. The full rule and the transitional dates are in the Part L 2026 guide.
How the calculation actually works
The benchmark is an annual output, not a panel count. The house should achieve an output, in the approved calculation, at least equal to a reference PV array with a peak power of 0.22 kWp per m² over an area equal to 40% of the ground floor area, facing south-east to south-west, pitched at 45° and not overshaded. Equation 5.1 gives the reference peak power: area for PV panels multiplied by 0.22 kWp/m², which the Approved Document notes is 1 kWp for every 4.5 m². The ground floor area is measured to the internal face of the external perimeter wall and includes certain unheated spaces defined in the SAP 10 conventions. For a 42.5 m² ground floor: 17 m² × 0.22 = 3.74 kWp. The FHS calculator shows this reference figure for any plot.
When the roof cannot fit it
Paragraph 5.73(b) gives an alternative: an annual output at least equal to an array covering the reasonably practicable roof area at 0.22 kWp/m². Paragraph 5.71 allows renewable technologies within the curtilage of the building, so a garage roof or outbuilding can carry part of the array. For buildings containing flats, paragraph 5.74 applies the 40% reference to the building's ground floor area, to each flat's allocated share, or to the reasonably practicable roof area. Whichever route is used, the calculation must use the same version of the approved methodology as the rest of the compliance calculation.
Worked examples for three common house types
3-bed semi (42.5 m² ground floor): 17 m² reference area → 3.74 kWp.
4-bed detached (65 m² ground floor): 26 m² reference area → 5.72 kWp.
5-bed executive (95 m² ground floor): 38 m² reference area → 8.36 kWp.
These are reference arrays: south-east to south-west, 45° pitch, not overshaded. A real array on a different roof may need to be larger or smaller to produce the same annual output.
Common misconceptions about the rule
First, it is an output target: the 40% area defines a reference array, so a well-oriented roof may reach the output with a smaller array and an east-west roof will need a larger one. Second, the ground floor area is not simply the heated area of the lowest storey; the Approved Document measures it to the internal face of the external wall and includes certain unheated spaces. Third, for flats the reference can be set for the whole building or allocated per flat, with the roof-area route as the fallback. Finally, requirement L3 applies to new dwellings, not to existing homes.