Quick answer: for a building containing flats, Approved Document L takes the ground floor area as the gross internal area ÷ the number of storeys (equation 5.2). The benchmark is 0.22 kWp/m² over 40% of that, facing south-east to south-west at 45°, unshaded. You can meet it for the whole building (paragraph 5.74(a)) or flat by flat (5.74(b), equation 5.3). A 1,200 m² block over 4 storeys has a whole-building target of 21,204 kWh a year; each of its 60 m² flats, 1,060 kWh.
The rule for a building containing flats
Paragraph 5.74 gives three routes, and asks for one of them:
- (a) The whole building — an annual output at least equal to that of 0.22 kWp/m² panels over 40% of the building's ground floor area, AGRND = ATOTAL ÷ number of storeys (equation 5.2);
- (b) Each dwelling — the same benchmark over 40% of the flat's share of that area, AGRNDDWELL = (ADWELL ÷ ATOTAL) × AGRND (equation 5.3);
- (c) The reasonably practicable roof area — where neither can be met, the output of 0.22 kWp/m² panels covering the usable roof.
Both (a) and (b) use the same fixed benchmark as a house: orientated south-east to south-west, pitched at 45°, not overshaded, worked out in SAP 10.3 on UK-average weather. The notes to the equations define the terms: ATOTAL is the gross internal area of all spaces within the building, floor areas are measured to the internal face of the external walls, and storeys are counted as in section 2.08 of the SAP 10 conventions. Equation 5.3 comes out at the flat's floor area divided by the number of storeys.
Three blocks, worked through
| Block A | Block B | Block C | |
|---|---|---|---|
| Storeys | 3 | 4 | 5 |
| Flats | 9 × 60 m² | 16 × 60 m² | 25 × 64 m² |
| Gross internal area | 600 m² | 1,200 m² | 2,000 m² |
| Communal and plant share | 10% | 20% | 20% |
| AGRND, equation 5.2 | 200 m² | 300 m² | 400 m² |
| Benchmark array | 17.6 kWp | 26.4 kWp | 35.2 kWp |
| Whole building, 5.74(a) | 14,136 kWh | 21,204 kWh | 28,273 kWh |
| Each flat, 5.74(b) | 1,414 kWh | 1,060 kWh | 905 kWh |
| All flats under (b) | 12,723 kWh | 16,964 kWh | 22,618 kWh |
| Floor per flat, 5.77(b) | 240 kWh | 180 kWh | 144 kWh |
Targets in kWh a year. Read as due south, the benchmark is 6.4% higher; the calculator shows both readings.
Whole building or flat by flat?
Route (a) suits one system for the building. Paragraph 5.75 only asks that the electricity is available to the residents, and it says that in a building containing dwellings this can be to individual dwellings and to communal spaces used for their benefit — so a landlord's supply feeding the lifts, corridors and plant is a legitimate design. Route (b) suits arrays split between the flats' own supplies, where each flat's SAP carries its share.
The two routes don't add up to the same thing. Equation 5.3 shares AGRND out in proportion to each flat's floor area, and the flats' floor areas don't add up to the gross internal area — corridors, stairs, plant and any other space make up the rest. So the per-flat targets total less than the whole-building target: 80% of it for Block B, where 20% of the gross internal area is communal. Which route a scheme takes is a design decision to settle early, with the assessor and Building Control.
Taller blocks: the same target, smaller shares
AGRND is roughly the building's footprint, so for the building the target follows the footprint, not the height. Each flat's share shrinks as storeys are added — and so does its exceptional-circumstances floor:
| Storeys | Flat's target, 5.74(b) | Floor, 5.77(b) |
|---|---|---|
| 2 | 2,120 kWh | 360 kWh |
| 4 | 1,060 kWh | 180 kWh |
| 6 | 707 kWh | 120 kWh |
| 8 | 530 kWh | 90 kWh |
The roof doesn't grow with the height either, which is why the reasonably practicable roof area route matters more for blocks than for houses. And once a block has a storey 18 m or more above ground, requirement L3 stops applying altogether — see the exemptions below.
Fitting it on a flat roof
Most blocks have a flat roof, where panels sit on low frames rather than at 45°. SAP 10.3 gives low-tilt arrays a decent yield against the benchmark's 803 kWh per kWp:
| Arrangement | kWh/yr per kWp | kWp vs benchmark |
|---|---|---|
| Flat on the roof (0°) | 760 | 106% |
| Facing south, tilted 10° | 812 | 99% |
| Facing south, tilted 15° | 832 | 97% |
| East–west, tilted 10° | 757 | 106% |
So the kWp isn't the problem; the area is. Block B needs about 28.0 kWp in an east–west arrangement at 10° — 64 panels of 440 W. Rows of tilted panels need gaps between them so they don't shade each other, east–west arrays pack tighter than south-facing rows, and Appendix B takes space out for access and plant:
- Table B2, flat roof offsets: 100 mm from soil vent pipes and rainwater pipes; 200 mm from guttering and cable trays; 1,000 mm from lift overruns, HVAC plant and access hatches.
- Table B3, maintenance access routes — minimum distance to the roof edge: 2,000 mm with no edge protection; 500 mm behind a parapet; 300 mm with a man-safe system or railings.
- Table B1 still applies to roof edges, party walls, vents and flues, and 1,000 mm round an automatic opening vent — the smoke vent over a stair core.
Measure the offsets to the edge of the panel, as MHCLG's FAQ says. A parapet buys roof back: 500 mm instead of 2 m all round the edge.
When the roof can't take it
Then paragraph 5.74(c) applies — an array covering the reasonably practicable roof area at 0.22 kWp/m², after the offsets, on every part of the roof that can take panels. Paragraph 5.76 expects higher-output panels, other layouts and moving plant first. The floor is low: under 5.77(b), Block B would need the roof to be unable to make 2,880 kWh a year (180 kWh for each of its 16 flats) — about 9 panels on a south-facing 10° frame. Anything short of the 5.74(a) or (b) standard is flagged on the BREL report, with the Appendix B evidence to back it up.
When requirement L3 doesn't apply
- Relevant buildings. L3 does not apply to a "relevant building" for the purpose of regulation 7(4) — broadly, one with a storey at least 18 m above ground level that contains dwellings.
- No reasonable output possible on account of the building's design or surroundings — paragraph 5.77(b) gives the 720 ÷ storeys test above as one such case.
- Equivalent output from elsewhere on site — an on-site system that is not on the building or within its curtilage, designed so the electricity is available to the residents.
In a mixed-use building, Volume 1 still governs requirement L3 for the dwellings (paragraph 0.19), and equation 5.2's ATOTAL is the gross internal area of all spaces within the building.
Frequently asked questions
Which equation do I use for a block of flats?
Equation 5.2 gives the building a ground floor area — its gross internal area divided by the number of storeys — and the whole-building target is the output of 0.22 kWp/m² over 40% of that (paragraph 5.74(a)). Equation 5.3 gives each flat its share of that area, in proportion to its floor area, for a flat-by-flat target (5.74(b)). Paragraph 5.74 asks for one of the routes, not both; where the roof cannot take either, the reasonably practicable roof area applies (5.74(c)).
Does electricity for the communal areas count towards requirement L3?
Yes. Paragraph 5.75 asks that the generated electricity is available to the residents, and says in a building containing dwellings that can mean individual dwellings and communal spaces used for the residents' benefit. MHCLG's FAQ repeats it. A landlord's supply serving lifts, corridors and plant is a legitimate design.
Are tall blocks of flats exempt from requirement L3?
Requirement L3 does not apply to a "relevant building" under regulation 7(4) of the Building Regulations — broadly, a building with a storey at least 18 m above ground level that contains dwellings. Below that it applies to blocks of any height.
Do taller blocks need more solar PV?
Not for the building: the target follows the ground floor area the building is given, which is roughly its footprint, whatever the height. Each flat's share shrinks as storeys are added — a 60 m² flat's target is 1,060 kWh a year in a four-storey block and 530 kWh in an eight-storey one.
What is the minimum for a flat if the roof is too small?
Paragraph 5.77(b) treats it as an exceptional circumstance where, following paragraph 5.74 and Appendix B, the roof cannot take an array producing 720 kWh a year for each dwelling divided by the number of storeys — 180 kWh a flat in a four-storey block. Below that, no PV may be required; above it, the array covering the reasonably practicable roof area is still needed.
Can the panels go on a bike store or car port?
Yes, if it is within the curtilage of the building — requirement L3 allows the system on the building or within the boundaries of its curtilage (see also paragraph 5.71). MHCLG's FAQ says a block's curtilage may include service areas, courtyards and other spaces that serve or support the building, decided case by case.
Sources: Approved Document L, Volume 1: Dwellings, 2026 edition (revised 8 September 2026) — requirement L3, paragraphs 0.19 and 5.69–5.78, equations 5.2 and 5.3, Appendix B Tables B1–B3 · SAP 10.3 specification, version 12-06-2026 (BRE) — Appendix M1 and Appendix U · Approved Document L frequently asked questions (MHCLG, 8 September 2026). The blocks are illustrative; figures use the calculator's SAP 10.3 arithmetic.
Next: the solar PV calculator · how many panels a house needs · the evidence for Building Control · the Future Homes Standard.