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 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; 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.
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 PV covering the equivalent of 40% of the ground floor area, or — where that is not achievable — an array covering the “reasonably practicable roof area”. The government’s consultation response of March 2026 added the floor: where the available roof cannot produce 720 kWh a year — “roughly equivalent to less than 2 PV panels” — it is appropriate to install no panels at all.
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. Then run that peak power through Appendix M1 of SAP 10.3 to get the 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 panel’s orientation, pitch and region, and by an overshading factor. So the benchmark array is assessed as if it were on the actual roof — and the target moves with the roof. A south-facing slope at a typical pitch gives the highest target; the same benchmark array on an east–west roof gives a lower one. 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.
For a building containing one or more dwellings, the ground floor area in equation 5.1 is replaced by AGRND from equation 5.2 or AGRNDWELL from equation 5.3 “as appropriate” — the FAQ does not spell out which applies when, so read the two equations against your building before you start.
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, 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. 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.
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 SAP pass/fail test in itself, but it is not separate from the SAP either. The SAP 10.3 notional dwelling already carries PV sized at 40% of the ground floor area divided by 4.5 — the same 0.22 kWp/m² — so the target emission and primary energy rates assume that array is there. Where a scheme takes the reasonably practicable route and installs less, the DER and DPER still have to be met, and the difference has to come from fabric, airtightness, calculated junction Ψ-values and the heat pump’s seasonal performance. Where a scheme installs more than the benchmark because the roof allows it, the SAP result improves — but the L2 sizing test and the export limit set the ceiling.
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) · 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)