Quick answer: if the array meets the benchmark — paragraph 5.73(a) for a house, 5.74(a) or (b) for flats — the BREL report shows it. If it doesn't, Appendix B paragraph B9 asks for (a) roof diagrams with and without the proposed panels, showing what restricts them; (b) a statement of why lower provision is being installed and why the design can't be changed; and (c) the calculations, signed by a suitably qualified person — which includes a qualified On Construction Domestic Energy Assessor.
When the evidence is needed
Paragraph B9 asks for evidence to go to the building control body in two circumstances:
- Lower output than the benchmark — an array that doesn't reach the 5.73(a), 5.74(a) or 5.74(b) standard, usually because it covers the reasonably practicable roof area instead;
- No array at all — the exceptional circumstances of paragraph 5.77, where the whole usable roof can't produce 720 kWh a year (720 kWh per dwelling divided by the number of storeys, for flats).
Paragraph 5.78 makes the link: any array that misses those standards is highlighted on the BREL report, and the evidence for the reasonably practicable area or the lower provision goes to Building Control as Appendix B describes. A shortfall on the BREL with nothing behind it is the thing to avoid.
The three items
1. Roof diagrams, with and without the panels
B9(a) asks for roof diagrams with and without the proposed photovoltaic panels, clearly demonstrating any design restriction on them; paragraph B11 points to Diagrams B1 to B3 as examples. In practice that means:
- Every slope of the main roof, dimensioned — north-facing and shaded slopes included, because MHCLG's FAQ says their lower output is unlikely to justify leaving them out;
- Every feature that takes space: ridge, eaves, verges, party walls, dormers, rooflights and windows, vents and flues, automatic opening vents — and on a flat roof, plant, lift overruns, hatches, pipes and the access route;
- The offsets drawn on: Table B1 for pitched roofs (600 mm from the ridge; 500 mm from eaves, verges, parapets, 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), Tables B2 and B3 for flat roofs — measured to the edge of the panel;
- The panel layout, with the panel dimensions used. There is no prescribed size; MHCLG's FAQ expects commercially available residential panels in most cases.
2. A statement
B9(b) asks for a statement outlining why lower provision is being installed and why any modification to the design cannot be made. Paragraph 5.76 says what modifications are expected first — higher-performance panels, other panel orientations, and other changes such as moving roof vents or architectural features — so the statement should deal with each. Paragraph B10 adds the test: show that the installation provides the maximum capacity (kWp) and annual generation (kWh) possible for the roof layout and orientation, given its pitch, windows, architectural features, HVAC plant and maintenance access routes.
3. The calculations, signed
B9(c) asks for any calculations made in support of paragraphs 5.73(a), 5.74(a) and 5.74(b), signed by a suitably qualified person — and says a qualified On Construction Domestic Energy Assessor is one. They are done in the same version of the approved methodology as the dwelling's emission and primary energy rates (paragraph 5.72) — SAP 10.3 for schemes under the 2026 edition. Set out:
- the benchmark target — 0.22 kWp/m² over 40% of the ground floor area, south-east to south-west, 45°, unshaded (the calculator works it out);
- the reasonably practicable roof area and its target — 0.22 kWp/m² over that area, on the slopes as they are;
- the proposed array's annual output, slope by slope;
- for exceptional circumstances, the 720 kWh check.
Where it fits with the BREL report
Appendix B asks for two BREL reports for every new dwelling (paragraph B3): a design-stage report before work begins, with the targets, the dwelling's rates and a list of specifications, and an as-built report with any changes to that list. The as-built report is signed by the assessor (B4) and by the developer (B5), and the software produces a schedule of the changes between the two. The roof is designed before work starts, so the L3 evidence belongs with the design-stage submission — and needs updating if the array that goes on the roof isn't the one that was drawn.
Photographs of the PV
The photographic evidence of paragraph B7 covers building services, and item 6 includes low or zero carbon technology equipment within or on the building: photograph the plant and equipment identification labels, including make, model and serial number (6a) — for a PV system, the inverter and the panels. Paragraph B8 asks for digital photographs good enough to audit, with geo-location on, and file names carrying the plot number and the detail reference — P1/6a for plot 1. Our photographic evidence guide covers taking the rest of the set.
Requirement L2 and the DNO
Requirement L3 doesn't replace L2. A system installed to meet L3 must also be appropriately sized for the site and the available infrastructure, with controls (paragraph 5.64 asks for a size appropriate to the site, the infrastructure and the on-site demand). MHCLG's FAQ lists the factors, including the export limits set by the District Network Operator, and accepts that an appropriately sized system's output may differ from the L3 arithmetic — leaving the judgement to Building Control on the facts. If an export limit shaped the array, the DNO correspondence is part of the evidence: the G98 notification for a system up to 16 A per phase, or the G99 application above that.
The checklist
- Roof plan of every slope, dimensioned, with the Table B1 (and B2/B3) offsets marked
- The same plan with the proposed panel layout, and the panel dimensions used
- Panel datasheet — kWp and the whole-panel area, frame included (the FAQ's definition of efficiency)
- Statement: why the array is smaller, and what was tried under paragraph 5.76
- SAP 10.3 calculations: the benchmark target, the reasonably practicable target, the proposed output (and the 720 kWh check, if relevant) — signed by the assessor
- DNO correspondence, if an export limit affected the size
- Photographs of the inverter and panel labels, geo-located and named by plot and detail
- An updated set if the as-built array differs from the design
Frequently asked questions
Who can sign the requirement L3 calculations?
A suitably qualified person. Appendix B of Approved Document L (paragraph B9) says that includes a qualified On Construction Domestic Energy Assessor — the assessor who produces the SAP and the BREL report.
Do I need Appendix B evidence if the array meets the target?
No. Paragraph B9 asks for it in two circumstances: to support lower output than the 5.73(a), 5.74(a) or 5.74(b) standard, and in the exceptional circumstances of paragraph 5.77. A design that meets the benchmark shows it on the BREL report, which goes to Building Control with the photographs for every new dwelling anyway.
What does the BREL report show for requirement L3?
Paragraph 5.78 says any array that does not achieve the 5.73(a), 5.74(a) or 5.74(b) standard will be highlighted on the BREL report, and that the evidence supporting the reasonably practicable roof area or the lower provision should be given to the building control body as Appendix B describes.
Can the DNO export limit reduce the size of the array?
It is a relevant factor. A system installed for requirement L3 must also meet requirement L2 — appropriately sized for the site and the available infrastructure — and MHCLG's FAQ lists the export limits set by the District Network Operator among the factors, accepting that an appropriately sized system's output may differ from the L3 arithmetic. Whether reasonable provision has been made depends on the facts, so keep the DNO correspondence with the evidence.
Is the 720 kWh exceptional-circumstances floor automatic?
No. Paragraph 5.77 applies where it can be demonstrated, by following paragraph 5.73 (5.74 for flats) and Appendix B, that there is not enough roof for an array producing 720 kWh a year — 720 kWh for each dwelling divided by the number of storeys, for flats. Paragraph B9 asks for the same evidence: roof diagrams, a statement and signed calculations.
Sources: Approved Document L, Volume 1: Dwellings, 2026 edition (revised 8 September 2026) — requirement L3, paragraphs 5.64, 5.72–5.78 and Appendix B, paragraphs B1–B11 and Tables B1–B3 · Approved Document L frequently asked questions (MHCLG, 8 September 2026).
Next: how much of the roof counts · the solar PV calculator · MHCLG's FAQ on requirement L3, explained · SAP for new builds.