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How much PV your roof needs for requirement L3
The Part L 2026 target from the ground floor area — equation 5.1, worked in SAP 10.3 — then the kWp and the panels your actual roof slopes need to reach it, with the reasonably practicable roof area route and the 720 kWh floor.
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Quick answer: MHCLG’s FAQ on requirement L3, published on 8 September 2026, confirms that the L3 target for a house is the annual output of a benchmark PV array with a peak power of 0.22 kWp/m² × 40% of the ground floor area (equation 5.1 of Approved Document L1), converted to kWh through Appendix M1 of SAP 10.3. Paragraph 5.73(a) fixes the rest of that benchmark — facing south-east to south-west, pitched at 45°, not overshaded — so the target depends only on the ground floor area, and your roof decides how much PV it takes to reach it. The FAQ also confirms that panel efficiency is kWp divided by the whole panel including its frame; that north-facing and overshaded roof slopes cannot be left out of the “reasonably practicable roof area” just because they generate less; that a detached garage is excluded from both the 40% and the roof area, but may still carry panels; and that a system installed for L3 must also be “appropriately sized for the site and available infrastructure” under requirement L2 — which is where the DNO’s export limit comes in. Our solar PV calculator works out both numbers for your own floor area and roof.
The rule the FAQ is interpreting
From 24 March 2027 (24 September 2027 for higher-risk building work, and subject to the transitional provisions), requirement L3 of Schedule 1 to the Building Regulations applies whenever a dwelling or a building containing dwellings is erected. A system for on-site renewable electricity generation must be installed on the building or within the boundaries of its curtilage, designed so that the electricity is available to the residents and so that it produces a reasonable output given the building’s design and surroundings. The requirement does not apply to a “relevant building” under regulation 7(4) — broadly a building with a storey at least 18m above ground that contains dwellings, an institution or rooms for residential purposes, the same buildings caught by the combustible-materials ban — nor where no reasonable output is possible, nor where an equivalent output is available to the residents from an on-site system that is not on the building or in its curtilage.
Approved Document L Volume 1 (2026) says the requirement is met by following paragraphs 5.69 to 5.78. The core of that guidance is paragraph 5.73 for a dwelling-house and 5.74 for a building containing dwellings: an array whose output is at least that of a benchmark array — PV covering the equivalent of 40% of the ground floor area, facing south-east to south-west at 45° and not overshaded — or, where that is not achievable, an array covering the “reasonably practicable roof area”. Paragraph 5.77 sets the floor: where the available roof cannot produce 720 kWh a year — “roughly equivalent to less than 2 PV panels”, as the government’s consultation response of March 2026 put it — it is an exceptional circumstance in which no panels may be required. For flats the floor is 720 kWh per dwelling divided by the number of storeys.
That is the framework. The FAQ is MHCLG’s first published set of answers on how it works in practice, and it was needed.
The arithmetic
The FAQ walks through equation 5.1. Take the dwelling-house’s ground floor area, multiply by 40%, and multiply by 0.22 kWp per square metre — the peak power of PV “with an efficiency of 0.22kWp per m² installed over an area equivalent to 40% of the dwelling-house’s ground floor area”. That gives the installed peak power of the benchmark array, and paragraph 5.73(a) fixes the rest of it: orientated south-east to south-west, a pitch of 45°, not overshaded. Then run that array through Appendix M1 of SAP 10.3 to get its annual output in kWh, and that kWh figure is what the system you actually install must be designed to achieve.
For a house with a 45 m² ground floor: 45 × 0.4 × 0.22 = 3.96 kWp. At the benchmark efficiency that is about 18 m² of panel — around nine 440 W modules of the usual 1.76 × 1.13 m size, which come out at almost exactly 0.22 kWp/m² by the FAQ’s own definition of efficiency: the panel’s kWp divided by the area of the whole panel including the frame, excluding flashings.
SAP’s Appendix M formula multiplies the peak power by 0.8, by the annual solar radiation for the array’s orientation and pitch, and by an overshading factor. SAP 10.3 runs Part L compliance on UK-average weather (Appendix U), and paragraph 5.72 ties the L3 calculation to the same methodology as the DER and DPER. Because the benchmark’s orientation, pitch and shading are fixed, the target does not move with your roof or your region: it works out at about 70.7 kWh a year for every square metre of ground floor — 3,181 kWh for the 45 m² house. What your roof changes is how much PV it takes to get there. On a south-facing slope at 35° the array needs 3.68 kWp, slightly less than the benchmark. On an east–west roof at the same pitch it needs 4.46 kWp, about 13% more: ten panels, five a side, produce 3,140 kWh and fall 41 kWh short, where eleven clear it. Because the requirement is expressed as an annual output rather than an area, a developer can meet it with fewer, more efficient panels, or with a better-oriented slope, and the FAQ frames the test for any on-site renewable electricity system — not just PV — as achieving that annual output. Our solar PV calculator does the sum for your own floor area, roof slopes and panels.
One detail is left open. “South-east to south-west” covers three of SAP’s eight orientations. Read as south-east or south-west, the benchmark gives 803 kWh a year per kWp; read as due south, 854 — about 6% more. Which reading the approved software takes will show on the BREL report; until you have seen it, design to clear both.
For a building containing one or more dwellings, the FAQ says to use AGRND from equation 5.2 or AGRNDDWELL from equation 5.3 “as appropriate”, and paragraph 5.74 sets out the choice. Route (a) sizes the whole building from AGRND, its gross internal area divided by the number of storeys. Route (b) gives each dwelling its own target from AGRNDDWELL, its share of AGRND by floor area — which comes to the flat’s floor area divided by the number of storeys. Route (c) is the reasonably practicable roof area. Routes (a) and (b) both use the same fixed south-east-to-south-west, 45°, unshaded benchmark.
What the “reasonably practicable roof area” has to include
This is the part of the FAQ that will change designs. Where the 40% benchmark cannot be achieved, paragraphs 5.73(b) and 5.74(c) require an array covering the reasonably practicable roof area — its target is the output of 0.22 kWp/m² panels over that area, on the slopes you actually have — and the developer is responsible for identifying that area “and justifying that assessment to the relevant building control body”. Two questions in the FAQ ask whether north-facing slopes and heavily overshaded slopes can be left out. The answer to both is no.
The reasoning is the same in each case: the approved methodology already discounts the output of a panel for its orientation and its overshading, so the lower generation from a north slope or a shaded slope “is unlikely to be sufficient justification to exclude it”. A north-facing slope raises the required output by less than a south-facing slope of the same size would — but it raises it, and it has to be covered. If you have been assuming that a north slope simply drops out of the calculation, it does not.
Appendix B of Volume 1 gives offset distances from roof edges and features — 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. The FAQ says they are “primarily intended for determining the size of the reasonably practicable roof area”, are not intended to cover every circumstance, and that different installation offsets may be appropriate provided the panels still cover the reasonably practicable area, comply with other regulatory requirements and follow the manufacturer’s instructions. Offsets are measured from the roof feature to the edge of the panel. There is no prescribed panel dimension for laying out the array; housebuilders should use dimensions “consistent with commercially available residential solar PV panels” and appropriate to the building’s design.
An array that misses the benchmark is highlighted on the BREL report (paragraph 5.78), and Appendix B lists the evidence Building Control will expect: roof diagrams with and without the panels, a statement of why more cannot be installed, and the calculations signed by a suitably qualified person — which includes an On Construction Domestic Energy Assessor.
Garages, curtilage and flats
Three of the questions deal with the edges of the building.
A detached garage does not count towards the 40%: unheated spaces outside the projected main roof area are excluded from the ground floor area, which the FAQ says aligns with the SAP 10 conventions’ treatment of attached garages for the notional dwelling. Its roof is not part of the reasonably practicable roof area either, because that phrase refers to the roof of the dwelling. But you can put panels on it to meet the required output, provided it is within the curtilage and the other conditions of L3 are met. A detached garage is therefore an option, not an obligation, and not a way to shrink the target.
Curtilage is given its usual planning meaning: land “so closely and directly associated with a building as to form ‘part and parcel’ of it” — gardens, garages and driveways for a house; service areas, courtyards and other supporting space for a block of flats — judged case by case on layout, ownership and use.
For buildings containing dwellings, paragraph 5.75 requires the system to be designed so that generated electricity is available to the residents. The FAQ confirms that supply to communal spaces used for the residents’ benefit counts, as does supply to individual dwellings. A landlord’s-supply connection serving lifts, corridors and plant is a legitimate design.
The L2 question: appropriately sized for the site
Requirement L2 has been in Part L since 2021 and says that where an on-site electricity generation system is installed, it must be appropriately sized for the site and available infrastructure, have effective controls, and be commissioned so that it produces the maximum reasonable electricity in the circumstances. The FAQ’s first two questions deal with how L2 and L3 interact, and the answer is worth reading twice. A system installed to meet L3 must also comply with L2. Having considered the site-specific factors, “it is possible that the output of a system appropriately sized for the site and available infrastructure may differ from the output of a system calculated in accordance with paragraphs 5.73 and 5.74”. The factors listed are the on-site energy demand, the capacity and nature of the available infrastructure “including export limits set by the District Network Operator”, the relationship between export limits, generation and demand, and other characteristics of the site. Whether reasonable provision has been made “will depend on the facts of the particular case”.
In plain terms: the DNO’s export limit is a relevant factor in sizing, and the FAQ acknowledges that an appropriately sized system may not match the L3 arithmetic. It does not say the export limit overrides L3, and it leaves the judgement with the building control body on the facts. The practical consequence is that the DNO correspondence — the G98 notification for systems up to 16 A per phase, or the G99 application above that — becomes compliance evidence. Get it early, keep it, and put it in the pack.
What this means for the SAP
The L3 output is not a DER or DPER test, but it is not separate from the SAP either: it is worked out in the same approved software, and a shortfall shows on the same BREL report. What has changed is the notional dwelling. Under Part L 2021 it carried a fixed array; the SAP 10.3 notional dwelling carries PV that is the same as the actual building’s, up to 40% of the ground floor area divided by 4.5 — the same 0.22 kWp/m² (SAP 10.3 Appendix R, Table R1). Below that cap, the target moves with your array. PV no longer buys headroom against the TER and TPER, and a scheme that takes the reasonably practicable route and installs less is not penalised for it in the DER and DPER — the fabric, airtightness, calculated junction Ψ-values and the heat pump’s seasonal performance carry that comparison either way. Where a scheme installs more than the cap because the roof allows it, the SAP result improves — but the L2 sizing test and the export limit set the ceiling. Requirement L3 is what now makes you fit the array.
Design the array and the SAP together, from the first iteration, using the actual roof geometry and the DNO’s answer. It is a great deal cheaper than discovering at the as-built stage that a north slope should have been covered, or that the array on the drawings will never be allowed to export.
Sources: Approved Document L: Energy and greenhouse gas emissions, frequently asked questions (MHCLG, 8 September 2026) · Approved Document L (2026), Volume 1: Dwellings, revised 8 September 2026 (MHCLG) — paragraphs 5.69–5.78 and Appendix B · SAP 10.3 specification, version 12-06-2026 (BRE) — Appendix M1, Appendix U and Table R1 · The Future Homes and Buildings Standards: consultation response (MHCLG, March 2026), paragraphs 3.10–3.16 · Amendments to Approved Documents L and F, Circular 04/2026 — letter (MHCLG, 8 September 2026)
Corrected 29 September 2026: an earlier version of this article said the L3 target moves with the orientation of the actual roof and with the site’s region, and that a shortfall on the reasonably practicable route has to be made up in the fabric. Neither is right. Paragraph 5.73(a) fixes the benchmark array’s orientation, pitch and shading, SAP 10.3 works on UK-average weather, and the SAP 10.3 notional dwelling’s PV follows the actual array up to its cap.