Quick answer: requirement L3 asks for an array whose annual output matches a fixed benchmark — panels of 0.22 kWp/m² over 40% of the ground floor area, facing south-east to south-west, pitched at 45° and not overshaded (Approved Document L, paragraph 5.73(a)). SAP 10.3 works out compliance on UK-average weather, so the target is about 70.7 kWh a year per m² of ground floor: 3,181 kWh for a 45 m² ground floor. Your roof decides what it takes to get there — 3.68 kWp on a south-facing 35° slope, 4.46 kWp facing east or west.
Requirement L3 calculator
Start with the building and the panels you have in mind. The target, and what it takes on each orientation, update as you type; then add your roof slopes below to check a design.
What it takes on one roof slope
| Slope faces | kWh/yr per kWp | kWp needed | Panels | vs benchmark |
|---|---|---|---|---|
| South | 865 | 3.68 | 9 | 93% |
| South-east or south-west | 820 | 3.88 | 9 | 98% |
| East or west | 714 | 4.46 | 11 | 113% |
| North-east or north-west | 591 | 5.39 | 13 | 136% |
| North | 542 | 5.87 | 14 | 148% |
Check your roof
Add each slope that could carry panels, with the number of panels you plan on it. If the roof is tight, add each slope's usable area — the part panels can cover after the Appendix B offsets (600 mm from the ridge, 500 mm from eaves, verges and windows, 750 mm from a party wall's centre line, 300 mm from vents) — and fill it in for every slope, north-facing and shaded ones included, to test the reasonably practicable route.
Overshading is the share of the southern sky blocked by buildings, trees or the land (SAP Table M1). If an MCS overshading figure exists for the site, the SAP uses that instead.
SAP 10.3 Appendix M1 and Appendix U at UK-average weather, overshading from Table M1, the benchmark from Approved Document L paragraphs 5.73–5.74. The BREL report from the assessor's approved software is the final word; this is the same arithmetic, done early. It does not size for requirement L2 — the DNO's export limit — or decide the reasonably practicable roof area for you.
How the L3 target is worked out
Approved Document L 2026 gives two ways for a house to meet requirement L3 (paragraph 5.73). The first, and the one to design for, is an annual output at least equal to that of a benchmark array with all of these characteristics:
- a peak power of 0.22 kWp per m² over 40% of the ground floor area — equation 5.1, PPDWELLING = APVD × EFFPV;
- orientated south-east to south-west;
- a pitch of 45°;
- not overshaded.
The benchmark is fixed. It is not your roof: whichever way your slopes face, the target is what that array would produce. SAP 10.3 turns it into kilowatt-hours with Appendix M — 0.8 × kWp × the annual solar radiation on the array × an overshading factor — and it runs Part L compliance on UK-average weather, not the site's own (Appendix U); paragraph 5.72 ties L3 to the same methodology as the DER and DPER. So the target does not change with the region either. The benchmark yields 803 kWh a year per kWp, which is 70.7 kWh a year for every m² of ground floor.
Your array then has to produce at least that, worked out the same way for its own orientation, pitch and overshading. A south-facing slope at 30–35° out-yields the benchmark, so it needs slightly less than the benchmark's kWp; anything facing east, west or north needs more. An array that falls short is highlighted on the BREL report (paragraph 5.78).
One detail the guidance leaves open: "south-east to south-west" spans three of SAP's eight orientations. Read as south-east or south-west — how Part L 2021 described the notional dwelling's array — the benchmark yields 803 kWh/kWp; read as due south it yields 854, about 6% more. The calculator checks a design against both and says when one clears only the first.
A worked example
A house with a 45 m² ground floor:
- Benchmark: 45 × 0.4 × 0.22 = 3.96 kWp — 18.0 m² of panel, nine 440 W modules.
- Target: 3.96 kWp × 803 kWh/kWp = 3,181 kWh a year (3,384 if the benchmark is read as due south).
- South-facing roof at 35°: each kWp gives 865 kWh, so the array needs 3.68 kWp — still nine 440 W panels.
- East–west roof at 35°: each kWp gives 714 kWh, so it needs 4.46 kWp — eleven panels. Ten, five a side, produce 3,140 kWh: 41 kWh short. The eleventh takes it to 3,454.
- The same east–west roof with modest overshading: 5.57 kWp — thirteen panels.
When the roof can't fit it
If the benchmark cannot be reached, paragraph 5.73(b) accepts an array covering the reasonably practicable roof area instead. Its target is the output of 0.22 kWp/m² panels over that area, on the slopes you actually have. Three things shape the area:
- Every slope counts. MHCLG's FAQ says north-facing and heavily overshaded slopes cannot be left out because they generate less — SAP already discounts them.
- The Appendix B offsets set what is usable: 600 mm from the ridge; 500 mm from eaves, verges, windows and doors; 750 mm from a party wall's centre line; 300 mm from vents and flues; 1,000 mm from an automatic opening vent — measured to the edge of the panel. Lay out real, commercially available panels; there is no prescribed panel size.
- A detached garage sits outside both the 40% and the roof area — but panels on it, within the curtilage, count towards the output.
Before settling for that, paragraph 5.76 expects the design to try harder: higher-output panels, other orientations, moving vents and features. If even the whole usable roof cannot produce 720 kWh a year, paragraph 5.77 treats it as an exceptional circumstance in which no PV may be needed. Either way the BREL report flags the shortfall, and Building Control will want the Appendix B evidence: roof diagrams with and without the panels, a statement of why more cannot fit, and the calculations signed by a suitably qualified person — which includes an On Construction Domestic Energy Assessor.
Flats
For a building containing dwellings, paragraph 5.74 works from the building instead of a single ground floor:
- 5.74(a), the whole building: 40% of AGRND, where AGRND = the gross internal area ÷ the number of storeys (equation 5.2);
- 5.74(b), each dwelling: 40% of AGRNDDWELL = (the flat's floor area ÷ GIA) × AGRND (equation 5.3) — which comes to the flat's floor area ÷ storeys;
- 5.74(c): the reasonably practicable roof area, as for a house.
Both (a) and (b) use the same fixed south-east-to-south-west, 45°, unshaded benchmark. A 1,200 m² block of 4 storeys has an AGRND of 300 m² and a whole-building target of 21,204 kWh a year; each of its 60 m² flats has a target of 1,060 kWh. The exceptional-circumstances floor becomes 720 kWh a year for each dwelling divided by the number of storeys (5.77(b)). The electricity has to be available to the residents (5.75) — MHCLG's FAQ confirms supply to lifts, corridors and other communal spaces counts.
What requirement L3 does to the SAP
Under Part L 2021 the notional dwelling carried a fixed array — 40% of the ground floor area ÷ 6.5, facing south-east or south-west — so a house without PV started behind its target. SAP 10.3 changes that. The notional dwelling's PV is now the same as the actual building's, up to 40% of the ground floor area ÷ 4.5 (Appendix R, Table R1). Below that cap the target moves with your array, so PV no longer buys headroom against the TER and TPER, and a smaller array on the reasonably practicable route does not have to be made up in fabric; above the cap, the extra generation does improve the DER and DPER. Requirement L3 is what now makes you fit the array.
Requirement L2 still applies on top: the system must be appropriately sized for the site and its infrastructure, and MHCLG's FAQ names the DNO's export limit as a factor. Get the G98 notification or G99 application in early — the correspondence is compliance evidence. More on that in MHCLG's FAQ on requirement L3, explained.
Frequently asked questions
How many solar panels does a new build need from 2027?
It depends on the ground floor area and on the roof. The requirement L3 target works out at about 70.7 kWh a year for every m² of ground floor. A 45 m² ground floor needs 3,181 kWh a year: nine 440 W panels on a south-facing roof at 35°, eleven if they face east or west. A 60 m² ground floor needs 4,241 kWh: 12 panels facing south, 14 facing east or west. The calculator on this page works it out for your own floor area, roof and panels.
Are solar panels mandatory on new homes in England?
Yes, for dwellings built under Approved Document L 2026. Requirement L3 applies from 24 March 2027 unless the building is covered by the transitional provisions — a building notice, initial notice or full plans in before 24 March 2027, with work started before 24 March 2028. It does not apply to a "relevant building" under regulation 7(4) — broadly one with a storey at least 18 m above ground that contains dwellings — nor where no system could produce a reasonable output, nor where an equivalent output is available to the building from an on-site system elsewhere. The requirement is for on-site renewable electricity; in practice that is roof-mounted PV.
Does the L3 target depend on which way my roof faces?
No. The benchmark array in paragraph 5.73(a) is fixed — south-east to south-west, a 45° pitch, not overshaded — and SAP 10.3 runs Part L compliance on UK-average weather, so the target depends only on the ground floor area. Your roof decides how much PV it takes to reach the target: slightly less than the benchmark on a south-facing slope, about 13% more facing east or west, and more again facing north.
Does a north-facing roof count?
For the reasonably practicable roof area, yes: MHCLG's FAQ says north-facing and heavily overshaded slopes cannot be left out just because they generate less, because SAP already discounts their output. Panels on a north slope count towards the output at their SAP yield, which at 35° is about 63% of a south-facing panel's.
What if the roof is too small for the full array?
Then paragraph 5.73(b) applies: the array must match the output of 0.22 kWp/m² panels covering the reasonably practicable roof area — the usable area of every slope after the Appendix B offsets. Paragraph 5.76 expects you to try higher-output panels, other orientations and moving vents or features first. If even the whole usable roof cannot produce 720 kWh a year, paragraph 5.77 treats it as an exceptional circumstance. Either way the BREL report flags the shortfall and Building Control will want the Appendix B evidence.
Can the panels go on a detached garage?
Yes, if the garage is within the curtilage of the dwelling. MHCLG's FAQ says a detached garage's floor area is not part of the ground floor area and its roof is not part of the reasonably practicable roof area — so it does not change the target — but panels on it can count towards the output.
Does PV still improve the SAP result?
Less than it used to. Under SAP 10.3 the notional dwelling's PV is the same as the actual building's, up to 40% of the ground floor area ÷ 4.5 (Appendix R, Table R1). Below that cap the target moves with your array, so PV no longer buys headroom against the TER and TPER; above it, the extra generation does improve the DER and DPER. Requirement L3 is what makes you fit the array.
How accurate is this calculator?
It uses SAP 10.3's own equations and tables — Appendix M1, Appendix U at UK-average weather and the Table M1 overshading factors — and equations 5.1 to 5.3 of Approved Document L. The result that counts is the BREL report from the assessor's approved software. The one open point is how "south-east to south-west" is read, which the calculator shows both ways. Treat the answer as a design-stage estimate.
Sources: Approved Document L, Volume 1: Dwellings, 2026 edition (revised 8 September 2026) — paragraphs 5.69–5.78, equations 5.1–5.3 and Appendix B · SAP 10.3 specification, version 12-06-2026 (BRE) — Appendix M1, Appendix U (Tables U3–U5), Tables M1 and R1 · Approved Document L frequently asked questions (MHCLG, 8 September 2026).
Next: the Future Homes Standard, requirement by requirement, or SAP for new builds — design stage, as built and the EPC.