Solar PV: new build vs retrofit on UK commercial property
UK commercial solar deployment splits between new-build integration (where solar is part of the development consent) and retrofit (where solar is added to existing commercial property). Each has distinct economics, planning considerations and grant eligibility.
Side-by-side comparison
| Feature | New build | Retrofit |
|---|---|---|
| Planning route | Part of development consent | Permitted development (mostly) |
| Structural requirement | Designed for solar load | Cat 2 survey often needed |
| Cost £/Wp | £0.55-£0.85 (integrated) | £0.65-£1.20 (separate) |
| Re-roof savings | N/A (new roof) | Avoided if integrated with re-roof |
| DNO complexity | Designed in | Existing supply assessment required |
| Planning requirement | London Plan SI2 for major commercial | Permitted development typically |
| Delivery time | Part of build programme | 2-6 months separate project |
| Tenant arrangement | Designed in lease | Lease variation often required |
| Best grant fit | London Plan compliance | Full Expensing, IETF, PSDS |
Which one for your business? Real scenarios
New commercial development in Greater London
Recommendation: New build integration required. London Plan Policy SI2 effectively mandates solar PV on major commercial developments. Plan integrated from RIBA Stage 2.
Existing commercial property, end-of-life roof
Recommendation: Integrated re-roof + solar retrofit. Capture re-roof economics. Lifetime asset extension + solar in single capital programme.
Existing commercial property, modern roof in good condition
Recommendation: Standard retrofit. Cat 1 structural check, mounting design, deploy in 2-6 months. Most common UK commercial solar pattern.
Listed building or heritage property
Recommendation: Conservation-sensitive retrofit on ancillary buildings. Heritage architect input essential. Listed Building Consent or faculty jurisdiction add 6-18 months.
Tenant on long lease without solar in original design
Recommendation: Lease variation + landlord-funded install with rent uplift. PPA structure can preserve flexibility for both parties.
Key deciding factors
- New build integration is 20-30% cheaper per Wp than retrofit because of shared scaffolding, access, design and project management.
- Greater London commercial developments effectively require solar integration under London Plan Policy SI2 — material design implication.
- End-of-life roof retrofit captures combined re-roof + solar economics; sequential installation pays for solar twice.
- Existing tenants with long leases need lease variation for solar retrofit; PPA structures can work without variation.
- Modern commercial property (post-2000) typically has adequate structural capacity for solar; pre-2000 often needs Cat 2 survey.
- Heritage and listed buildings require specialist conservation engagement — add 6-18 months to programme.
Comparison FAQs
Is new build solar cheaper than retrofit?
Yes — typically 20-30% cheaper per Wp because of shared scaffolding, access, design and project management.
Does London Plan require solar on new commercial?
Effectively yes for major developments. Policy SI2 requires renewable energy contribution to operational energy demand. Solar PV is the standard route.
Should I solar an old roof or refurbish first?
Combined re-roof + solar typically saves 15-25% versus sequential installation. End-of-life roof + solar should be integrated.
Can tenants block landlord solar retrofit?
Most modern leases require tenant consent for roof works. Lease variation is the cleanest route; PPA structures can sometimes work without variation.
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