Quick answer: requirement L3 sets the target from the ground floor area — about 70.7 kWh a year for every m² in SAP 10.3. The roof then decides the panel count. With 440 W panels at 35°, a 45 m² ground floor needs 9 panels facing south or 11 facing east or west; 60 m² needs 12 or 14; 80 m² needs 15 or 19.
Panels needed by ground floor area
The target comes from Approved Document L 2026, paragraph 5.73(a): an array at least matching the annual output of 0.22 kWp/m² panels over 40% of the ground floor area, facing south-east to south-west, pitched at 45° and not overshaded. The table turns that into panels for a roof with all its panels on one slope.
| Ground floor | Target, kWh/yr | South | SE or SW | East or west | North |
|---|---|---|---|---|---|
| 30 m² | 2,120 | 6 | 6 | 7 | 9 |
| 35 m² | 2,474 | 7 | 7 | 8 | 11 |
| 40 m² | 2,827 | 8 | 8 | 10 | 12 |
| 45 m² | 3,181 | 9 | 9 | 11 | 14 |
| 50 m² | 3,534 | 10 | 10 | 12 | 15 |
| 55 m² | 3,887 | 11 | 11 | 13 | 17 |
| 60 m² | 4,241 | 12 | 12 | 14 | 18 |
| 70 m² | 4,948 | 13 | 14 | 16 | 21 |
| 80 m² | 5,655 | 15 | 16 | 19 | 24 |
| 90 m² | 6,361 | 17 | 18 | 21 | 27 |
| 100 m² | 7,068 | 19 | 20 | 23 | 30 |
| 120 m² | 8,482 | 23 | 24 | 28 | 36 |
The target reads "south-east to south-west" as south-east or south-west, as most tools do. Read as due south, the benchmark yields 854 kWh per kWp instead of 803, so the target is 6.4% higher — one more panel in 5 of these 12 rows on a south-facing roof. The calculator checks a design against both readings until the approved software settles it. Each panel is 1.76 × 1.13 m (1.99 m², frame included), so the 9 panels for a 45 m² ground floor facing south cover 17.9 m² of roof.
The target follows the floor; the panel count follows the roof
The benchmark array is fixed — it is not your roof — and SAP 10.3 runs Part L compliance on UK-average weather (Appendix U). So the target depends only on the ground floor area: not on the region, and not on the way your roof faces. What the roof decides is how much PV it takes to hit that target. SAP's Appendix M gives each kWp an annual output of 0.8 × the solar radiation on the panels × an overshading factor, and the benchmark works out at 803 kWh per kWp:
| Slope faces | kWh/yr per kWp | kWp needed vs benchmark |
|---|---|---|
| South | 865 | 93% |
| South-east or south-west | 820 | 98% |
| East or west | 714 | 113% |
| North-east or north-west | 591 | 136% |
| North | 542 | 148% |
A south-facing slope out-performs the benchmark, so it needs a little less than the benchmark's kWp. East and west need about 13% more, and SAP treats them alike, so an array split across an east slope and a west slope needs the same number of panels as one facing either way. North needs about half as much again. For a roof facing south, south-east or south-west the pitch hardly matters — between 30° and 40° those columns of the first table barely move. East, west and north slopes do better shallower: a north slope at 30° needs about a tenth fewer panels than one at 40°.
Bungalows need twice the panels
Because the target follows the ground floor, the same floor space inside can need very different arrays. Three homes of 90 m² each:
| Home | Ground floor | Target, kWh/yr | South | East or west |
|---|---|---|---|---|
| Three-storey townhouse | 30 m² | 2,120 | 6 | 7 |
| Two-storey house | 45 m² | 3,181 | 9 | 11 |
| Bungalow | 90 m² | 6,361 | 17 | 21 |
The bungalow has about twice the roof to put its panels on, so it is rarely the one that runs out of space. The townhouse is the tight one in a different way: a small roof, often with dormers or rooflights in it. A room in the roof adds floor space but not ground floor area, so it does not raise the target — but dormers take roof that panels could have used.
Bigger panels, fewer of them
Requirement L3 counts output, so it is the kWp that matters, not the number of modules. For a 50 m² ground floor (3,534 kWh a year):
| Panel | South | East or west |
|---|---|---|
| 400 W | 11 | 13 |
| 440 W | 10 | 12 |
| 500 W | 9 | 10 |
The benchmark's 0.22 kWp/m² is roughly a modern 440 W panel (440 W on 1.99 m² is 0.221 kWp/m²). MHCLG's FAQ defines a panel's efficiency as its kWp divided by the area of the whole panel, frame included but not the flashings — so a higher-output panel only helps on a tight roof if it delivers more per square metre.
What counts as the ground floor area
- Measured to the inside face of the external walls — the notes to equations 5.1 to 5.3 in Approved Document L.
- Certain unheated spaces within the dwelling are included, as defined in section 8.05 of the SAP 10 conventions.
- Unheated spaces outside the projected main roof area are not — MHCLG's FAQ gives a detached garage as the example. Its roof does not count as roof area either, but panels on it can count towards the output if it is within the curtilage.
For flats the area comes from the whole building instead — the gross internal area divided by the number of storeys. That has its own page: solar PV for flats.
If the panels won't fit
Dormers, rooflights, hips and party walls can leave too little roof for the full array. Then paragraph 5.73(b) accepts an array covering the reasonably practicable roof area — the usable part of every slope, north-facing ones included, after the Appendix B offsets — and paragraph 5.77 sets a floor of 720 kWh a year below which no PV may be needed. Paragraph 5.76 expects the design to try first: higher-output panels, other orientations, moving vents and features. An array that falls short is flagged on the BREL report, and Building Control will want the Appendix B evidence.
Frequently asked questions
How many solar panels does a new build need?
It depends on the ground floor area and on the roof. Requirement L3 sets a target of about 70.7 kWh a year for every m² of ground floor. With 440 W panels on a 35° roof, a 45 m² ground floor needs 9 panels facing south or 11 facing east or west; 60 m² needs 12 or 14; 80 m² needs 15 or 19.
How many kWp does a new build need?
The benchmark is 0.088 kWp per m² of ground floor — 40% of the area at 0.22 kWp/m² — so 3.96 kWp for 45 m² and 5.28 kWp for 60 m². What the roof actually needs depends on its direction: at 35°, about 93% of the benchmark facing south, 113% facing east or west and 148% facing north.
Do bungalows need more solar panels than houses?
Yes. The target follows the ground floor area, not the total floor area, so a 90 m² bungalow needs twice the output of a two-storey house with the same 90 m² inside: 6,361 kWh a year against 3,181 kWh. It also has about twice the roof to put it on.
Does it depend on where in England the house is built?
No. SAP 10.3 works out Part L compliance — the targets, the DER and DPER, and the requirement L3 output — on UK-average weather (Appendix U), so the same house needs the same array in Cornwall and in Cumbria. The panels will generate more in the south-west than the calculation says, but the calculation is what Building Control checks.
Do I need a battery to meet requirement L3?
No. Requirement L3 asks for generation, and a battery is optional. SAP 10.3 counts a battery in your own dwelling's figures but does not copy it into the notional dwelling that sets the targets. Approved Document L asks that any storage is sized for the site and has controls that work without the occupant stepping in (paragraphs 5.64 and 5.67).
Can I use fewer, bigger panels?
Yes. Requirement L3 counts the array's annual output, not the number of panels. For a 50 m² ground floor on a south-facing roof, 9 panels of 500 W do the job of 10 at 440 W. Higher-wattage panels are often physically larger, so the roof area saved is smaller than the drop in the count.
What if the roof cannot take enough panels?
Then paragraph 5.73(b) applies: the array has to match 0.22 kWp/m² panels covering the reasonably practicable roof area — every slope, after the Appendix B offsets. If the whole usable roof cannot produce 720 kWh a year, paragraph 5.77 treats it as an exceptional circumstance. Either way, Building Control will want the Appendix B evidence.
Sources: Approved Document L, Volume 1: Dwellings, 2026 edition (revised 8 September 2026) — paragraphs 5.64–5.78 and equations 5.1–5.3 · SAP 10.3 specification, version 12-06-2026 (BRE) — Appendix M1, Appendix U, Table M1 and Appendix R · Approved Document L frequently asked questions (MHCLG, 8 September 2026). Figures computed with the same SAP 10.3 arithmetic as the calculator.
Next: the solar PV calculator for your own roof · solar PV for flats · when the roof can't fit it · what PV does in SAP.