The 40% ground floor area PV rule was designed for houses. Applying it to flats requires interpretation. Part L 2026 addresses the case in two ways — through an attributable share calculation and through specific Higher-Risk Building (HRB) provisions for taller blocks.
The attributable share calculation
For each flat in a block, the PV requirement is calculated as 40% of the flat's notional ground floor area — calculated by dividing the building's total roof area by the number of dwellings. This typically produces a much smaller per-flat requirement than for a house, because the roof is shared.
Where the PV is physically located
A communal roof array, sized to meet the aggregated requirement across all flats in the block, with electricity allocated either through embedded supply, communal meter or PPA. The flats themselves get the SAP/HEM credit on a pro-rata basis.
Higher-Risk Buildings (HRB) over 18 m
For HRB (blocks over 18 m), Approved Document L 2026 acknowledges that rooftop PV split many ways produces minimal per-dwelling benefit and may not be economically viable. HRB provisions allow a "reasonable" array (typically dimensioned for communal landlord supply rather than to dwelling-attributable shares) with the residual savings credited through tighter fabric.
BTR and PRS implications
Build-to-Rent and Private Rented Sector developments often have unified ownership of common parts, simplifying solar PV ownership. PPA models — operator owns the array and sells electricity to tenants — work well for BTR but require careful tenancy agreement drafting.
Heat networks and flats
Most large blocks of flats use heat networks (district heating or block-scale CHP) rather than per-dwelling heat pumps. Heat networks meeting the Heat Network Technical Assurance Scheme (HNTAS) standard count as compliant FHS heating systems.