Quick answer: where the full benchmark array won't fit, paragraph 5.73(b) of Approved Document L accepts an annual output at least equal to that of 0.22 kWp/m² panels covering the reasonably practicable roof area (5.74(c) for flats). Every slope counts, north-facing and shaded ones included. Take the Appendix B offsets off each slope — 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 — measured to the panel edge. If even that can't make 720 kWh a year, paragraph 5.77 treats it as an exceptional circumstance.
How many panels fit — and what the roof has to make
Add every slope of the main roof as a rectangle: its width along the eaves and its length from the eaves to the ridge, measured on the slope. The Table B1 offsets come off each edge, and the calculator lays out whole panels in rows — upright or on their side, whichever fits more.
For rectangular slopes — gables and mono-pitches. A hipped or irregular slope needs a drawing. Take off panels that clash with rooflights (500 mm), vents and flues (300 mm) or an automatic opening vent (1,000 mm) in "Panels lost". Output per SAP 10.3 Appendix M1 at UK-average weather; the target compares with the south-east-to-south-west reading of the benchmark. Then put each slope's panels into the main calculator to check the whole design.
What paragraph 5.73(b) asks for
Requirement L3 asks for a system "capable of generating a reasonable output taking account of the building's design and surroundings". Paragraph 5.73 gives a house two ways to show it. The first, 5.73(a), is the fixed benchmark: 0.22 kWp/m² over 40% of the ground floor area, facing south-east to south-west, pitched at 45°, unshaded. The second, 5.73(b), is an annual output at least equal to that of a photovoltaic array covering the reasonably practicable roof area with a panel efficiency of 0.22 kWp/m². Flats have the same option at 5.74(c). MHCLG's FAQ fills in what goes into the area:
- Every slope. North-facing and heavily overshaded slopes can't be left out because they generate less — SAP already discounts their output.
- The developer identifies it and justifies it to the building control body.
- No prescribed panel size — lay it out with panels appropriate to the design, which in most cases means commercially available residential panels.
- Offsets are measured to the edge of the panel, and they are primarily for sizing the area; different installation offsets may be appropriate if the panels still cover it.
- Not a detached garage. The phrase means the roof of the dwelling — though panels on a garage within the curtilage can count towards the output.
The Appendix B offsets
| Roof feature | Minimum offset, mm |
|---|---|
| Pitch top or ridge | 600 |
| Roof edge (eaves, verge, parapet) | 500 |
| Party wall (measured from the centre line) | 750 |
| Vent or flue | 300 |
| Window or door | 500 |
| Automatic opening vent | 1,000 |
Flat roofs have their own offsets for pipes, plant, lift overruns and hatches (Table B2) and access routes to the roof edge (Table B3) — both on the solar PV for flats page. Paragraph B10 lists what typically limits a roof: its pitch, windows, architectural features, HVAC plant and maintenance access routes.
Approved Document L's own examples
Appendix B (paragraph B11) draws three layouts that make the most of a roof. Taking each diagram's foundation area as the ground floor area, here is how they compare with the benchmark, if the panels faced south or east–west at 35°:
| Diagram | Foundation | PV area | kWp | L3 target, kWh | Facing south | East–west |
|---|---|---|---|---|---|---|
| B1Semi-detached or end of terrace, hipped roof | 39 m² | 16 m² | 3.52 | 2,757 | 3,045 | 2,512 |
| B2Semi-detached or end of terrace, pitched roof with gable | 39 m² | 20 m² | 4.40 | 2,757 | 3,807 | 3,140 |
| B3Mid-terrace, pitched roof with dormer windows | 35 m² | 8 m² | 1.76 | 2,474 | 1,523 | 1,256 |
kWp is the PV area at 0.22 kWp/m². Diagram B2 prints 4.44 kWp, which is 20 m² at 1 kWp per 4.5 m². Outputs in kWh a year, SAP 10.3, unshaded.
The two semi-detached layouts carry more panel than the benchmark's 40% of the ground floor — but on an east–west roof the hipped one (B1) still falls short of the target, because the target is output, not area. The mid-terrace with dormers (B3) fits only 8 m² against the 14.0 m² the benchmark assumes, so it is a reasonably practicable roof area case whichever way it faces. Even facing north, its 1.76 kWp would make 954 kWh a year — well above the 720 kWh floor, so PV is still required.
Before settling for less
Paragraph 5.76 says that where the benchmark can't be met, the design should be changed to get as much as it can: higher-performance panels, other orientations and other modifications — moving roof vents or architectural features to make room. Paragraph B10 puts it as demonstrating that the installation provides the maximum capacity (kWp) and annual generation (kWh) possible for the roof layout and orientation. A soil vent pipe moved off the main slope, or a rooflight moved to the back, can be worth a row of panels.
The 720 kWh floor
Paragraph 5.77 recognises exceptional circumstances in which a house can't take a system capable of a reasonable output: where, following 5.73 and Appendix B, there isn't enough roof for an array producing 720 kWh a year. For flats it is 720 kWh for each dwelling divided by the number of storeys. It is a low bar:
| Slope faces | kWp | 440 W panels |
|---|---|---|
| South | 0.83 | 2 |
| East or west | 1.01 | 3 |
| North | 1.33 | 4 |
A roof that can take a handful of panels on any slope is above it, so the floor is for roofs that are almost all window, plant or party wall — or so heavily shaded that SAP's overshading factor wipes them out.
What Building Control will want
An array that misses the 5.73(a) standard is highlighted on the BREL report (paragraph 5.78), and Appendix B (paragraph B9) asks for the evidence: roof diagrams with and without the proposed panels, a statement of why lower provision is being installed and why the design can't be changed, and the calculations signed by a suitably qualified person — which includes a qualified On Construction Domestic Energy Assessor. More on putting that together: requirement L3 evidence for Building Control.
Frequently asked questions
What is the reasonably practicable roof area?
It is the part of the roof that can realistically carry panels once the Appendix B offsets, features and access are allowed for. Where the full benchmark array will not fit, paragraph 5.73(b) (5.74(c) for flats) asks for an annual output at least equal to that of 0.22 kWp/m² panels covering that area. The developer identifies it and justifies it to Building Control.
Do north-facing and shaded roof slopes count?
Yes. MHCLG's FAQ says the lower output of a north-facing or heavily overshaded slope is unlikely to be sufficient justification to leave it out, because SAP already discounts the output for orientation and overshading. The slope has to be included; it simply contributes less.
Does a detached garage roof count?
Not towards the reasonably practicable roof area, which MHCLG's FAQ says means the roof of the dwelling or building. But panels on a detached garage within the curtilage can count towards the output.
What panel size should I use to lay out the area?
There is no prescribed size. MHCLG's FAQ says to use dimensions appropriate to the building's design, which in most cases will mean commercially available residential panels.
Where are the Appendix B offsets measured from?
From the roof feature to the edge of the panel. MHCLG's FAQ adds that the offsets are primarily intended for sizing the reasonably practicable roof area, do not cover every circumstance, and that different installation offsets may be appropriate if the panels still cover that area, meet the other regulations and follow the manufacturer's instructions.
What happens if the whole roof can't produce 720 kWh a year?
Paragraph 5.77(a) treats it as an exceptional circumstance in which it may not be possible to install a system capable of a reasonable output, so no PV may be required — but only where it is demonstrated by following paragraph 5.73 and Appendix B. In SAP 10.3, 720 kWh a year takes about 0.83 kWp facing south at 35° or 1.33 kWp facing north, so it is a rare roof that falls below it.
Sources: Approved Document L, Volume 1: Dwellings, 2026 edition (revised 8 September 2026) — requirement L3, paragraphs 5.73–5.78, Appendix B paragraphs B9–B11, Table B1 and Diagrams B1–B3 · SAP 10.3 specification, version 12-06-2026 (BRE) — Appendix M1, Appendix U and Table M1 · Approved Document L frequently asked questions (MHCLG, 8 September 2026).
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