Equation 5.3, step by step
Every dwelling type gets its allocated ground floor area, its panel area and its capacity, with the arithmetic shown.
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Renewables, design stage
PV Apportionment Calculator
Equation 5.3 of Approved Document L Volume 1, 2026 edition: dividing a building's ground floor area between the dwellings inside it, so each dwelling's share of the requirement L3 benchmark can be stated separately.
The point is the comparison. Paragraph 5.74 lets you size the array for the building as a whole or for each dwelling. Where a block has stairs, corridors, plant rooms and bin stores, the dwellings do not account for the whole gross internal area — so the two routes do not give the same answer, and the tool shows the gap.
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Both routes side by side
The whole-building benchmark against the sum of the individual allocations, and the communal area that separates them.
The per-dwelling threshold
720 kWh a year divided by the storeys, which is what paragraph 5.77(b) tests the exceptional circumstances route against.
The building 400 m² footprint
Equation 5.2 takes the building's ground floor area as the gross internal area divided by the number of storeys, so a taller block with the same floor area gets a smaller benchmark.
The dwellings 16 dwellings
One row per dwelling type, with how many of that type the building contains. Leave the rest blank.
0 of 100 rows used
The apportionment 0 kWp allocated
| Dwelling type | No. | A_DWELL m² | A_GRNDDWELL m² | Panel m² | kWp each | kWp total |
|---|
A_GRNDDWELL = (A_DWELL ÷ A_TOTAL) × A_GRND. Panel area is 40% of that, and capacity is the panel efficiency times the panel area.
The panels 0.22 kWp/m²
Project and your details Needed for the PDF
The name and company are printed on the PDF report and are needed before it can be produced. Everything else here is optional and is printed only if you supply it.
Method, data and scope
Paragraph 5.74 lets a building containing dwellings demonstrate a reasonable output for the building as a whole, for each dwelling separately, or against the reasonably practicable roof area. The three are alternatives, not a hierarchy, and they do not give the same answer where a building has communal space. Whichever is used, paragraph 5.75 requires the generated electricity to be available to residents of the dwellings, which an apportionment on paper does not by itself achieve. The 2026 edition takes effect on 24 March 2027.
Help
How to use this tool
Start here, then keep the guide open alongside your work. Expand a section for the information you need.
What this tool does
Equation 5.3 divides a building’s ground floor area between the dwellings inside it, so each dwelling’s share of the requirement L3 benchmark can be stated separately. The point is the comparison it makes visible: where a building has communal space, the two routes in paragraph 5.74 do not give the same answer.
1. The building
Gross internal area — the total floor area of all spaces, dwellings and communal areas together — and the number of storeys as defined in section 2.08 of the SAP 10 conventions. Equation 5.2 divides one by the other to give the building’s ground floor area.
2. The dwellings
One row per dwelling type with its floor area and how many of that type the building contains. Six rows is usually enough; where a scheme has more, group the types that share a floor area. Floor areas are measured to the internal face of the external perimeter wall.
3. Read the gap
The difference between the sum of the dwelling areas and the gross internal area is the communal space: stairs, corridors, plant rooms, bin and cycle stores. That area is allocated to no dwelling, so route (b) asks for less capacity than route (a). Where the dwellings account for the whole gross internal area, the two routes agree exactly.
4. Which route to use
Paragraph 5.74 offers three: an output for the building as a whole, an output for each dwelling from its allocated share, or an output matching an array over the reasonably practicable roof area. They are alternatives rather than a hierarchy, but the difference between them is a difference a building control body will notice.
What an apportionment does not settle
Paragraph 5.75 requires the generated electricity to be available to residents of the dwellings, which in a block may mean supplying individual dwellings, communal spaces, or both. How the array is physically arranged, how the generation is metered and how the electricity reaches each dwelling are all outside this calculation and all relevant to compliance.
PDF, saving and support
The PDF needs your name and company. It shows the per-dwelling table, the split between dwelling and communal area and both route benchmarks. Save inputs writes a .briarypvapp file. For problems, email toolsuite@briary.co.uk.
Working on a block of flats?
Send the areas schedule and the roof layout and we will work the L3 apportionment through with you.