Solar system design covers everything before installation: array sizing, a 3D layout with shading analysis, SAP 10.3 compliance modelling, inverter and battery specification, and the documents building control needs. On a new build it is done by the installer's design team or an independent designer, working with the architect from RIBA Stage 2.
What a design package should include
- 1Plot or scheme assessment. A review of the drawings, the ground floor area, roof orientation and a shading study from satellite imagery and neighbouring buildings.
- 2Array sizing. The FHS 40% ground-floor-area rule as the starting point, plus any over-size you choose for household demand, an EV or a future battery.
- 33D solar PV design. Panel placement on a roof model, an annual and monthly generation estimate, and the shading impact through the year.
- 4SAP 10.3 modelling. The Part L 2026 compliance calculation, done by an accredited energy assessor, with the array entered exactly as designed. (The Home Energy Model has been postponed and is not yet an approved route.)
- 5Inverter and battery specification. A hybrid inverter sized to the array and any future battery, and a battery size matched to household load and tariff.
- 6Building control documents. The SAP calculation, panel and inverter data sheets, and the mounting system details, ready for the local authority or registered building control approver.
RIBA stages and the right starting point
When solar design starts decides how much it can shape the building:
| RIBA Stage | Decisions still open | What solar design can still do |
|---|---|---|
| 2 — Concept Design | Roof geometry, glazing ratio, plot orientation | Shape architectural decisions for the best array |
| 3 — Spatial Coordination | M&E layout, inverter location, service routing | Coordinate PV with the other building services |
| 4 — Technical Design | Panel make and model, mounting system | Specify the system within the existing design |
| 5 — Manufacturing and Construction | Installation programme only | Little room left: the system must fit what is already designed |
For self-builders, Stage 2 is the time to start if at all possible: there is far more room to optimise the roof for the array while the design is still open.
Custom solar systems for non-standard projects
A standard 3-bed semi with a south-facing pitched roof is straightforward, but architect-led projects often are not. Common non-standard cases include flat-roof self-builds with ballasted or tilted arrays; replacement dwellings in National Landscapes using BIPV slate; modular (MMC) homes where the array is fitted in the factory; homes designed well beyond the FHS minimum; mixed-use schemes with shared arrays; and infill plots in conservation areas. The method is the same; the constraints differ.
For volume housebuilders, design is done once for each house type and repeated across the plots of that type, so the design cost per plot falls as the plot count rises.
Solar roof design: getting the roof right first
Solar roof design is where the biggest generation and appearance decisions are made, and on a new build they are made at the drawing board rather than retrofitted onto a finished roof. Three roof-design variables set the outcome: orientation and pitch (a south-facing roof at 30–40° maximises annual yield, while an east-west split often suits the plot better at the cost of some output), the integration method, and how the array reads from the street. Working the solar roof design out with the architect lets the panels complement the elevation rather than fight it.
The integration choice is the heart of solar roof design. In-roof panels sit flush in the tile line and are the usual choice for a clean modern roofline; solar roof tiles (BIPV) make the array far less visible for conservation areas and heritage plots; and the architectural solar design guide covers bespoke geometry, colour-matched panels and solar canopies. Each route should be checked for both generation and appearance before a tile is ordered.
Designing the roof and the PV together also resolves the practical clashes that derail retrofit: rooflight and dormer positions, valley and hip losses, panel setbacks from ridges and verges, and cable routes to the inverter, all fixed on the drawing before the roof is built.
Tools and outputs to ask for
Designers commonly use PVsyst for production modelling and shading analysis, OpenSolar for 3D plot-level layout, SketchUp for visualisation and approved SAP software, such as Elmhurst Design SAP, for the compliance calculation. Ask for a PDF design report, the 3D model in SKP or DWG, the production report and the SAP calculation.
To request a quote, use the form below. For an initial cost estimate sized to your plot, use the FHS PV calculator.