When the UK Government published Approved Document L 2026 on 24 March 2026 it introduced a new functional requirement: every new dwelling must include on-site renewable electricity generation, with the deemed-to-satisfy benchmark set at solar PV covering 40% of the dwelling's ground floor area. This page explains the rule in detail, walks through three worked examples and covers the exemptions.
Where the 40% rule comes from
The figure was set by the FHS Impact Assessment as the level which produces a meaningful contribution to the dwelling's Dwelling Emission Rate without imposing an array size that exceeds typical roof geometry. The Government's analysis showed that 40% coverage delivers, on average, around 4 MWh/year of generation for a typical 2-storey 3-bed home — comfortably more than the home's on-site electricity demand once ASHP heating and EV charging are factored in.
How the calculation actually works
Ground floor area is the heated floor area of the lowest storey, measured in square metres. The PV requirement is panel area, not roof area, equal to 0.4 × ground floor area. For a typical 3-bed semi at 42.5 m² ground floor, the panel area requirement is 17 m² — roughly 9 standard 425W panels totalling 3.4 kWp. Our FHS calculator automates this for any plot.
When the 40% can be relaxed
The Approved Document recognises that geometry will not always allow 40% coverage. Flats in tall blocks, plots in heavy shade, mansards in Conservation Areas and listed-curtilage infill plots are the four documented categories. In all cases the developer must install a "reasonable amount" and document the technical justification in the SAP/HEM compliance file. "Reasonable" is interpreted in practice as the maximum geometrically feasible array.
Worked examples for three common house types
3-bed semi (85 m² total, 42.5 m² ground floor): 17 m² panel area → 3.4 kWp → ~3,570 kWh/yr.
4-bed detached (140 m² total, 65 m² ground floor): 26 m² panel area → 5.2 kWp → ~5,460 kWh/yr.
5-bed executive (220 m² total, 95 m² ground floor): 38 m² panel area → 7.6 kWp → ~7,980 kWh/yr. All figures assume south-facing roofs at 35° pitch in the South Midlands region.
Common misconceptions about the rule
The rule is panel area, not roof area — many designers initially overstate the requirement. It is also per dwelling, not per plot — for blocks of flats, ground floor area is shared between dwellings on an attributable basis. Finally, the rule applies to new dwellings only — there is no retrofit requirement for existing homes.