The 40% ground floor area rule explained

What requirement L3 actually says

The new functional requirement is short, and worth reading rather than paraphrasing:

L3. (1) When a building is erected which is or contains one or more dwellings, a system for on-site renewable electricity generation must be installed on the building or within the boundaries of the curtilage of the building.

(2) The system installed for the purpose of this requirement must be — (a) designed to enable generated electricity to be available for the use of residents of the dwellings; (b) capable of generating a reasonable output taking account of the building’s design and surroundings.

Three things follow that most coverage misses.

It does not say “solar”. It says on-site renewable electricity generation. Approved Document L confirms that this “can be generated by photovoltaic panels, or another renewable technology”, with other technologies held to an equivalent output. Solar is what will be built in practice, because on a house nothing else is close.

It does not have to be on the roof. The system may be within the curtilage of the building, and there is a further exemption where equivalent output comes from an off-site system that still makes electricity available to residents.

“Reasonable output” is the whole argument. That phrase is what paragraphs 5.73 and 5.74 exist to define — and the definition is where the 40% appears.

The formula

For a house, Approved Document L gives equation 5.1:

So the arithmetic is: ground floor area × 0.4 × 0.22 = target kWp. Or, more simply, ground floor area ÷ 11.4.

Panel counts assume 400–440 W modules and are ours, not the document’s.

Approved Document L’s own worked example, in Appendix B, is a house with a 39 m² foundation area, giving a 16 m² photovoltaic area and an installed capacity of 3.52 kWp. The arithmetic checks out: 39 × 0.4 = 15.6, rounded to 16, times 0.22.

Flats work differently, and much less generously

For a building containing dwellings, the ground floor area is not the footprint you might expect. Equation 5.2 defines it as:

A_GRND = A_TOTAL ÷ number of storeys

where A_TOTAL is the gross internal area of the whole building. Equation 5.3 then allocates a share of that to each dwelling in proportion to its floor area.

The effect is straightforward once you see it: the taller the block, the smaller the target per dwelling, because the same roof is divided between more homes. A six-storey block gets a sixth of the notional area a single-storey footprint would imply. That is not a loophole — it is a recognition that a block of flats has one roof and many households — but it does mean a flat in a new development will have far less solar attributed to it than a house, and this is worth knowing before anyone tells you your new flat comes with solar.

The same division runs through the exceptional-circumstances threshold below.

The output target, not the panel count, is what binds

This is the part that matters and the part that gets lost.

The notional array is specified as south-east to south-west, 45 degrees, not overshaded. That is close to a best case for a British roof. Compliance is an annual output “at least equal to” what that array would produce, calculated with the approved methodology.

So on a real house:

• a roof facing east–west produces materially less per kWp than the notional array, so more capacity is needed to reach the same output
• a shallow pitch, a shaded plot, or a roof broken up by vents and windows all push the same way
• a north-facing main roof cannot get there at all on that pitch — see north-facing roofs

Paragraph 5.76 says exactly this: where the standard cannot be met, efforts should be made to maximise output, including higher performance panels, alternative orientations, and modifications such as repositioning roof vents or architectural features to fit more panels.

Our own cross-check on generation

Compliance is modelled with the approved methodology, which is not what we use. But it is reasonable to ask what those capacities actually generate, so here is our own figure from our PVGIS dataset — clearly labelled as ours.

Our dataset holds 35° and 60° pitches, which bracket the notional 45°. At 35° facing south, a kWp yields 692 kWh a year in Lerwick and 1,042 kWh in Brighton, with a median across our 25 locations of 856 kWh.

Two honest caveats. These are south-facing, unshaded figures — the notional case, not your case. And they are PVGIS at 20% system losses, not the approved methodology, so they will not match a BREL report.

When a developer can install less, or nothing

Paragraph 5.77 sets the exceptional case, and it is a low bar in absolute terms:

• for a house, where it can be demonstrated there is insufficient roof area for an array generating 720 kWh a year
• for a building containing dwellings, insufficient roof area for 720 kWh per dwelling divided by the number of storeys

For context, 720 kWh is roughly 0.8 kWp of south-facing panels on our figures — two panels. A house that cannot fit two panels’ worth of output is a genuinely unusual house.

Requirement L3 also does not apply to a building that is a “relevant building” for the purposes of regulation 7(4) of the Building Regulations 2010, or where equivalent renewable output reaches residents from a system outside the curtilage.

The evidence trail is specific. Under Appendix B, a lower provision needs roof diagrams with and without the proposed panels, a statement explaining why more cannot be fitted and why the design cannot change, and supporting calculations signed by a suitably qualified person — a qualified On Construction Domestic Energy Assessor is named. And under 5.78, any array falling short of the standard is flagged in the BREL report.

That last point is the useful one for a buyer. The shortfall is written down, in a document that exists for your home.

The offsets that decide how much fits

Appendix B also sets minimum clearances, which quietly determine how much roof is actually usable:

On flat roofs, further offsets apply to plant, lift overruns and access hatches (1,000 mm), guttering and cable trays (200 mm), and soil vent and rainwater pipes (100 mm). Maintenance access routes need 2,000 mm from an unprotected roof edge, falling to 300 mm where there are railings or a man-safe system.

A terrace with party walls both sides loses 750 mm from each — which is why the mid-terrace comes out as the most expensive dwelling type to bring up to the standard in the government’s own impact assessment.

What to do with this if you are buying

1. Ask for the ground floor area and do the division by 11.4 yourself.
2. Ask what capacity is specified, in kWp, and compare.
3. Ask whether the BREL report flags a shortfall under paragraph 5.78. If it does, ask for the Appendix B evidence — the roof diagrams and the statement.
4. Ask which way the roof faces. The target assumes south-east to south-west.
5. Ask who owns the panels, which is a separate question the regulations do not touch.

More on all of that in buying a new build.