Part L 2026 is the first version of the Building Regulations to expect solar on every new home in England. The question every self-builder, architect and developer asks next is how much. The answer comes from one paragraph of Approved Document L, and it is easier to apply than it looks. The rule itself, the dates and the transitional arrangements are covered in the Part L 2026 guide; this article is the working.
Step 1: find the ground floor area
Start with the ground floor area of the house. Approved Document L notes that it is measured to the internal face of the external perimeter wall, and that it includes certain unheated spaces within the dwelling as defined in the SAP 10 conventions. Your architect or SAP assessor will have this figure from the drawings.
Step 2: take 40% of it
Forty per cent of the ground floor area is the reference panel area. A house with a 42.5 m² ground floor has a reference area of 17 m².
Step 3: multiply by 0.22 kWp per m²
Equation 5.1 turns the area into a peak power: installed peak power equals the area for PV panels multiplied by a panel efficiency of 0.22 kWp/m² or higher. The Approved Document notes that this is the same as 1 kWp for every 4.5 m² of panel. For the 42.5 m² house, 17 m² × 0.22 gives a reference array of 3.74 kWp.
Step 4: check it against your actual roof
The reference array faces south-east to south-west, is pitched at 45° and is not overshaded. Your real array has to produce at least the same annual output in the approved calculation, which at launch is SAP 10.3. On a south-facing roof close to that pitch, an array of about the reference size will usually do it. An east-west roof, a shallow pitch or some shading all reduce output per kWp, so the array needs to be larger to match. If the roof simply cannot take it, paragraph 5.73(b) allows the alternative of an array covering the reasonably practicable roof area.
Worked examples
Two-storey three-bed, 42.5 m² ground floor: 17 m² reference area, 3.74 kWp.
Detached four-bed, 65 m² ground floor: 26 m² reference area, 5.72 kWp.
Bungalow, 90 m² ground floor: 36 m² reference area, 7.92 kWp.
The bungalow is the surprise: because its whole floor area is on the ground, it needs a much larger array than a two-storey house of the same floor area, although it also has a larger roof to put it on.
Flats and apartment blocks
For a building containing flats, paragraph 5.74 works from the building. Equation 5.2 takes the ground floor area as the total floor area divided by the number of storeys, and equation 5.3 gives each flat a share in proportion to its floor area. A six-storey block with 3,000 m² of total floor area has a ground floor area of 500 m², so a reference area of 200 m² and a reference array of 44 kWp for the building. A 60 m² flat in that block is allocated 10 m² of ground floor area, a 4 m² reference area and 0.88 kWp. On tall blocks the roof is usually the limit, which is where the reasonably practicable roof area route comes in.
Turning kWp into panels
Divide the target kWp by the rated power of the panel you intend to use, taken from its data sheet, and round up. Higher-power panels mean fewer panels for the same kWp, but the output target is what counts, so check the result in the compliance calculation rather than the panel count.
If the roof will not fit it
Three things help. Paragraph 5.71 allows renewable technologies within the curtilage of the building, so a garage roof or an outbuilding can carry part of the array. The reasonably practicable roof area route in paragraph 5.73(b) sets the target from the roof you actually have. And the design can change: a simpler roof form with one large south-facing slope carries far more PV than a roof broken up by dormers and valleys. The FHS PV calculator gives a quick first estimate, and the 40% PV rule explainer covers awkward roofs in more detail.
