Capital Allowances on Commercial Solar: The 2026 Tax Relief Guide

A £200,000 quote for the warehouse roof has just landed on your desk. The salesperson cheerfully explained that “Full Expensing” wipes the lot off your tax bill, payback is two years, you’d be mad not to. Then your accountant rang back and said, gently, that solar doesn’t actually qualify for Full Expensing. So which is it? Are you about to make a £200,000 mistake, or has your accountant misread the rules?

The answer is: neither, but the salesperson was using the wrong words for the right outcome. Commercial solar can absolutely deliver close to 100% first-year tax relief on systems below £1 million, and substantial relief on bigger ones. It just doesn’t get there via the route the industry keeps shouting about. This guide explains what actually applies, with real worked numbers, in plain English. If your accountant reads this they’ll nod. If your installer reads this they may quietly update their slide deck.

01What capital allowances actually do for you

Before we get into the specifics, a quick orientation. When a business buys equipment, you can’t simply deduct the cost from your taxable profits the way you would a phone bill or a delivery invoice. Equipment is “capital expenditure” and it gets its own tax treatment, called capital allowances. The system exists so you spread the deduction across the asset’s useful life, or in some cases, take it all in year one.

For solar, the question is which allowance applies, because different allowances move tax relief from year 25 (where it’s worth almost nothing in present-value terms) to year one (where it cuts your immediate Corporation Tax bill). The difference between a fast and slow allowance on a £500,000 system is roughly £100,000 of cash, in your pocket, this year versus over two decades.

02The Full Expensing myth (and why it spreads)

If you’ve spoken to more than one solar installer about a commercial system, there’s a good chance at least one of them mentioned Full Expensing. You’ll see it in PDF brochures, sales decks, finance broker emails. The problem is the policy doesn’t apply to solar, and acting on the claim could put a business in an awkward spot at tax-return time.

The confusion is understandable because both AIA and Full Expensing produce the same headline result for spend under £1 million: a 100% deduction in year one. The salesperson who promised your warehouse install would be “fully expensed” probably meant it sincerely, in the sense that you’d get full first-year relief, and you might. But the route is the AIA, and that distinction starts to matter the second your project goes over the AIA threshold.

Why does it matter? Three reasons. First, your tax return needs to claim the right allowance, and HMRC takes a dim view of incorrect filings. Second, AIA has a £1m annual cap that includes all qualifying capital spend, not just solar, so if you’re also buying machinery or fitting out an office, the cap fills up fast. Third, the rules around disposing of an asset later differ between Full Expensing and the 50% FYA, and getting the original claim right protects you from balancing-charge surprises down the line.

03Your three actual routes to tax relief

Once you’ve parked the Full Expensing red herring, the picture clarifies. There are three real allowances available for solar, and which one (or which combination) gives you the best result depends on your business structure and how much you’re spending.

The strategy nearly writes itself. Use the AIA first to soak up as much spend as you can, up to £1 million. Anything above that, if you’re a limited company, claim the 50% FYA. The residual (the un-relieved portion after FYA) drops into the special rate pool and grinds away at 6% a year for a long time. If you’re a sole trader or partnership the FYA isn’t available, so anything over £1m goes straight to the 6% special rate pool.

04Three worked examples

Numbers move this conversation faster than any rules summary. The table below shows three plausible commercial solar projects, the route they’d use, and the year-one cash impact at the 25% Corporation Tax main rate. (Companies with profits below £50,000 pay 19% and the savings scale accordingly.)

The £1.5m example is worth pulling apart because it shows the hybrid strategy in action. The first £1m gets AIA (100% deduction). The remaining £500,000 sits in special-rate territory: a limited company claims 50% of that as the First Year Allowance (£250,000 deduction in year one), and the other £250,000 enters the special rate pool to be written down at 6% per year. Add it up and £1,250,000 of the £1,500,000 is deducted in year one. At 25% Corporation Tax, that’s £312,500 of cash flow back to the business immediately. The remaining £250,000 dribbles relief at roughly £15,000 a year thereafter.

For context on what these system sizes actually cost in real installations, our commercial solar pricing guide breaks down £/kWp by system size, and the warehouse-specific guide covers the extra considerations for industrial roofs (structural surveys, insurance, fire-suppression sign-off).

05What actually counts as part of your solar install

Capital allowances aren’t only for the panels themselves. The whole installed system, plus a fair chunk of the work around it, falls within the qualifying spend. This is where a good accountant earns their fee, because the more you can correctly classify as plant and machinery, the bigger your year-one relief.

One subtlety to flag. If your solar project involves significant structural work to the building itself (reinforcing roof trusses, replacing cladding so the panels can be mounted properly), that structural element typically falls under the separate Structures and Buildings Allowance at 3% per year, rather than the much faster solar allowances. Make sure your installer’s quote breaks these costs out clearly, because lumping them together costs you tax efficiency.

06The business rates exemption you might be missing

Capital allowances are only one part of the tax picture. The other one, often missed in the spreadsheet, is the 100% business rates exemption on the rateable-value uplift attributable to rooftop solar. This was introduced in April 2023 and runs until 2035, which means a business installing solar in 2026 still has a clear 9-year window of exemption ahead.

In plain terms: if your local valuation office would otherwise have increased your property’s rateable value because of the new solar installation (because it’s a building improvement), they can’t. Your business rates bill stays exactly where it was. The exemption applies whether the system is bought outright or financed through a Power Purchase Agreement, and it sits in addition to the capital allowance reliefs covered above.

For larger sites this is far from trivial. A 500 kWp installation could otherwise add £15,000-£30,000 a year to the rateable value, and at the standard multiplier that means thousands a year in avoided business rates over the lifetime of the system. It’s a meaningful component of the total return that doesn’t show up on most installer ROI spreadsheets.

07Common mistakes (and how not to make them)

That last point is worth lingering on. Capital allowances are essentially a deferral of tax in the long run, not a forgiveness of it. If you eventually sell the panels for more than their tax-written-down value (or sell the building they’re attached to with the system included in the price), HMRC can claim some of that relief back as a balancing charge. It rarely changes whether solar is worth doing, but it absolutely changes how the numbers look if you’re planning to sell up in three or four years.

Further reading on commercial solar

If you’re early in the buying process, our breakdown of commercial solar panel costs covers the £/kWp benchmarks for office, warehouse and industrial sites. For sector-specific reads, check our guides on solar for offices, farms, and warehouses. If you’re weighing up Capex against a leased model, our overview of solar financing options compares outright purchase, asset finance and PPAs side by side, and the landlord solar ROI calculator handles the specific case of commercial landlords looking at portfolio-wide installs.