— reviewed against the regs as they stand.
Solar PV calculator — free in your browser
How much PV your roof needs for requirement L3
The Part L 2026 target from the ground floor area — equation 5.1, worked in SAP 10.3 — then the kWp and the panels your actual roof slopes need to reach it, with the reasonably practicable roof area route and the 720 kWh floor.
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Why PV dominates new-build compliance
Solar photovoltaic panels generate electricity from daylight, directly reducing a dwelling’s modelled energy demand and CO₂ emissions. Under Part L 2021, the SAP 10 notional dwelling includes an assumed PV array — roughly 4kW for an average house — so a dwelling without PV starts behind its target and must make the difference up elsewhere. In practice, PV (or a heat pump) is how most SAP 10 passes are achieved.
PV is also the most common answer to planning conditions requiring a percentage CO₂ or energy demand reduction from renewables.
From 2027: a requirement, not just a SAP lever
Under Approved Document L 2026 — the Future Homes Standard, for work notified from 24 March 2027 — PV stops being optional. Requirement L3 asks for an array whose annual output matches a fixed benchmark: panels of 0.22 kWp/m² over 40% of the ground floor area, facing south-east to south-west, pitched at 45° and unshaded. In SAP 10.3 that is about 70.7 kWh a year per m² of ground floor. A south-facing roof reaches it with slightly less than the benchmark’s kWp; an east–west roof needs around 13% more. Where the roof genuinely cannot take it, the reasonably practicable roof area applies instead, down to a floor of 720 kWh a year. The solar PV calculator works out the target and what your roof needs, and MHCLG’s FAQ on requirement L3 is explained here. In more depth: how many panels by ground floor area, solar PV for flats, the reasonably practicable roof area and the evidence Building Control asks for.
The SAP side changes with it. The SAP 10.3 notional dwelling’s PV is the same as your array, up to 40% of the ground floor area ÷ 4.5 — so below that cap, PV no longer buys headroom against the target. Requirement L3 is what makes you fit it.
What affects the SAP contribution
Array size (kWp), orientation and pitch (south-facing ~35° is ideal; east/west loses ~15–20%), and overshading. SAP 10 allows MCS overshading data instead of defaults, and now recognises battery storage and PV diverters (which dump surplus generation into the hot water cylinder). For flats, only dwellings directly connected to an array get the benefit — landlord-supply arrangements no longer count for everyone.
Practical notes
Panels are typically roof-mounted on-roof or integrated (in-roof trays look cleaner on new builds and avoid raised fixings). No moving parts, 25-year+ panel warranties, minimal maintenance. Tell us early if PV is in the design — sizing it precisely against the SAP calculation avoids paying for panels you don’t need.