For architects working on FHS-compliant new builds — whether one-off self-build commissions or volume housebuilder house-type design work — solar PV is no longer an end-of-design appendage. The 40% PV rule, paired with the tightened air permeability, U-values and HEM compliance route, means that decisions made at RIBA Stage 2 cascade into the buildable spec at Stage 4. Here is how the design conversation unfolds stage by stage.
RIBA Stage 2 — Concept Design
Three architectural decisions made at Stage 2 substantially affect FHS compliance: (1) roof geometry — south-facing pitched roofs at 25-40° are optimal for PV; complex roofscapes, mansards and shallow pitches all reduce PV efficiency and may force larger arrays; (2) glazing ratio — large south-facing glazing improves passive solar gain but increases summer overheating risk under TM59; (3) plot orientation — the orientation of the long axis of the plot determines whether the main roof can face south. A sensible Stage 2 target is a single south-facing roof pitch of 25–40° large enough for the 40% PV area plus some headroom, with south-facing glazing kept in check to manage overheating under Approved Document O.
RIBA Stage 3 — Spatial Coordination
Stage 3 is where the PV array sizing becomes definitive. Once the ground floor area is fixed, the 40% rule gives the minimum panel area; once the roof geometry is fixed, the maximum installable array is set. The trade-off conversation: if the planned roof can't accommodate the 40% requirement (e.g. complex hipped roof on a small plot), options are (a) redesign the roof; (b) move PV to a flat-roof element or outbuilding; (c) use BIPV on smaller available roof areas (efficiency loss but better fit); (d) accept a smaller-than-40% array and compensate through enhanced fabric. Options (c) and (d) both require careful HEM modelling at Stage 4.
RIBA Stage 4 — Technical Design
Stage 4 is the SAP/HEM modelling stage. The PV array spec must align with the inverter sizing, the battery (if any), the ASHP heat-loss design, and the fabric spec. Common Stage 4 issues we see: (1) inverter undersized for future battery addition — fix by specifying a hybrid inverter with battery-ready capacity; (2) PV array spans two roof orientations without per-panel optimisers — fix by specifying optimisers on each panel; (3) MVHR duct routing conflicts with structural members — fix by coordinating the M&E layout early with the structural engineer. We produce SAP and HEM compliance models as part of Stage 4 deliverables for partner architects.
RIBA Stage 5 — Manufacturing & Construction
During Stage 5 the PV procurement happens. For volume housebuilder work this is typically a framework call-off; for self-build, a procurement decision between volume installer and architect-led bespoke installer. Architect involvement at this stage: confirm panel make and model is on the consented spec (planning conditions sometimes mandate specific products), confirm in-roof tray detail matches roof tiling specification, sign off the inverter location and battery location, witness commissioning.
RIBA Stage 6 — Handover and Close Out
Stage 6 deliverables for FHS solar include: as-built O&M manual, MCS certificate, EPC, monitoring app onboarding documentation, warranty schedule. On architect-led custom builds, ask for a plain-English summary alongside the technical pack.
Common architect pitfalls
Four issues we see repeatedly: (1) Roof geometry decided before PV sizing — leads to forced compromises at Stage 3. Bring PV requirement into Stage 2 conceptual review. (2) Glazing ratios optimised for daylight without TM59 overheating check — leads to fabric remediation at Stage 4. (3) Inverter/battery space allocated without service heat-dissipation calculation — leads to plant room rework at Stage 5. (4) BIPV specified without confirming product availability against programme — leads to procurement delays. The simplest avoidance: engage the PV designer at Stage 2 alongside the structural engineer.
