Commercial rooftop solar projects pass through distinct professional stages. Feasibility and detailed design are adjacent in the timeline but different in purpose, liability, and cost. Treating a feasibility layout as build-ready design — or commissioning full design before feasibility confirms viability — are two of the most expensive mistakes estates teams make.
This guide clarifies the boundary for UK commercial PV.
What feasibility delivers
Feasibility answers strategic and early technical questions:
- Is the roof worth pursuing economically?
- What estimated capacity fits the usable area?
- What are approximate generation and financial outcomes?
- What screening-level flags exist for structure, wind, grid, and planning?
- What is the recommended next step?
Deliverables are dossier-style: executive summary, concept layout, financial model, risk flags, and verdict. Stage1Energy’s site assessment is explicitly early-stage feasibility — not design, procurement, or construction documentation.
The concept layout shows where panels could sit at plausible density. It does not specify module part numbers, inverter models, string lengths, cable routes, protection settings, or mounting fixings.
For section detail, see what is in a solar feasibility report. Review our example report for feasible depth at this stage.
What detailed design delivers
Detailed design answers construction and compliance questions:
- Exact equipment specification and compatibility
- String configuration and DC/AC cable sizing
- Inverter placement, switchgear, and earthing
- Mounting system selection with load paths into structure
- Roof penetration or ballast layout coordinated with membrane warranty
- Single-line diagrams, schematics, and as-built record
- Compliance with BS 7671, MCS requirements where applicable, and manufacturer warranties
Design is iterative with surveys. Structural calculations, roof condition reports, and DNO connection terms feed back into layout and equipment choices.
Liability sits with designers, installers, and certifying bodies — not with the feasibility author.
Side-by-side comparison
| Element | Feasibility | Detailed design |
|---|---|---|
| Purpose | Go / no-go decision | Build and certify |
| Layout | Indicative zones | Dimensioned arrays |
| Equipment | Generic or benchmark | Specified makes and models |
| Structure | Screening flags | Calculations and sign-off |
| Grid | G98/G99 screening | Formal application and offer |
| Financials | NPV, payback model | Firm quote and contract price |
| Typical cost | £1,000–£2,500 fixed fee | Within install contract or separate design fee |
| Stage1Energy | Yes — £1,250/site | Out of scope |
Why the boundary matters
Cost sequencing. Detailed design and surveys can exceed £10,000 before install contract. Feasibility tests the case first — before commissioning that spend.
Board clarity. Committees approve feasibility-backed business cases. They should not think concept layouts are frozen scope for capex sign-off.
Installer tenders. Feasibility gives tenderers a consistent capacity range and constraints. They refine in their design proposals. Starting from three incompatible installer designs without feasibility recreates chaos — see feasibility study vs installer quote.
Liability. Feasibility authors screen risks; they do not certify buildability. Misreading flags as approval creates false confidence.
Our methodology states explicitly where feasibility stops.
Typical project sequence
- Screen — free screening if needed.
- Feasibility — independent dossier and verdict.
- Confirmatory surveys — structure, roof, grid enquiry.
- Tender — installers quote against feasibility brief.
- Detailed design — winning contractor or designer produces build pack.
- Install and commission — construction, testing, handover.
Feasibility sits at step two. Design intensifies at step five, though outline design may appear in tender submissions at step four.
When to order a solar feasibility study covers timing before steps three onward.
How feasibility layouts feed design
A good feasibility layout reduces design rework by documenting:
- Excluded zones (plant, setbacks, fragile areas)
- Preferred mounting approach if known (ballast vs penetration)
- Capacity capped by economics or export, not only geometry
- Access and maintenance considerations
Designers still revalidate everything on site. Feasibility shrinks the solution space; it does not replace site verification.
Generation estimates from feasibility inform design tool inputs but will be refined with specified module performance and final shading analysis — see how accurate solar generation estimates are.
When organisations skip feasibility
Skip only when prior evidence is strong: identical successful install on a sister building, confirmed structural capacity, known load and grid position, and board appetite already aligned.
Even then, a light independent check before major capex is prudent. The savings from avoided design on a weak site exceed another feasibility fee.
Stage1Energy’s role
Stage1Energy provides fixed-fee feasibility dossiers for UK commercial rooftops. We do not produce detailed design — by design. That independence keeps the verdict focused on whether to proceed, not on selling a build specification.
Use feasibility output to brief designers and installers. Use design output to contract and construct. Keeping the stages separate keeps accountability clear.
For structural confirmation between feasibility and design, read feasibility vs structural survey. For the full service overview, visit commercial solar feasibility.
Handover between feasibility and design teams
When appointing designers or installers, issue a short handover note extracted from the dossier: verdict, estimated capacity range, excluded roof zones, grid and planning flags, financial assumptions, and explicit data gaps. Designers should sign off that they have read feasibility limitations before proposing deviations.
If design capacity exceeds feasibility recommendations, require written justification — usually improved load data, confirmed grid headroom, or revised roof access after survey. Without that discipline, install contracts drift from the business case the board approved, and post-project reviews become arguments about scope rather than performance.