Annual Investment Allowance for office solar

The Annual Investment Allowance is the most material capital allowance for UK office solar projects in 2026. Permanently set at £1m per business per year from April 2023, AIA provides a 100% first-year tax deduction on qualifying plant and machinery — including solar PV systems.

For a limited company at the current 25% corporation tax rate, every £1 of AIA-eligible capex returns £0.25 of tax relief in year one. On a typical £288,000 office solar install, that's £72,000 of tax saving in the first year — reducing the effective capex from £288k to £216k.

AIA stacks with most other reliefs. You can claim AIA on solar PV and still apply for Salix PSDS, SEG export tariffs, Workplace Charging Scheme grants, or any other applicable funding route. The only limitation is the £1m annual cap across all qualifying capex — for businesses making multiple major capex purchases in the same year, this can be the binding constraint.

For partnerships and LLP structures, AIA is particularly valuable because partners typically have higher marginal income tax rates (40% / 45%) than the 25% company rate — making AIA relief on partnership solar installations effectively worth £400k-£450k of tax saving on a £1m install.

Claiming AIA on solar PV requires the system to be operational and connected before the company year end. We schedule install dates to fit AIA claim windows where the customer's year end falls within the install programme.

How AIA interacts with other capital allowances

The capital allowance landscape for solar PV in 2026 involves three main routes: AIA, Full Expensing, and the 6% Special Rate Pool writing-down allowance. Understanding the hierarchy matters because the rules differ by entity type and by capex volume.

Solar PV panels, inverters, racking, and cables generally fall into the Special Rate Pool (SRP) under HMRC capital allowance rules — historically attracting only 6% WDA per year. AIA overrides this classification and allows a 100% deduction in year one on up to £1m of SRP assets. This is the core reason AIA is so valuable for solar: it converts a 6%-per-year deduction into a 100% year-one deduction.

Full Expensing (introduced 2023, made permanent 2024) provides a 50% first-year allowance on SRP assets for limited companies — better than 6% WDA but substantially worse than AIA's 100%. For limited companies with multiple major capex purchases in the same tax year, the practical question is which assets to stack against the £1m AIA limit and which to leave to Full Expensing. The optimal answer depends on the SRP asset mix and the marginal WDA value of each asset over the project's life.

For partnerships and LLPs — common in legal, accountancy, and professional services — Full Expensing is not available (it applies to companies only). AIA remains the primary first-year relief, subject to the £1m cap across all qualifying capex.

AIA worked example: 320 kWp office install

Consider a limited company with a December 31 year end and a 320 kWp rooftop solar install valued at £288,000 (including panels, inverters, racking, labour, DNO G99 application, and project management fees). Installation is completed and system commissioned on 20 November — before the year end.

• AIA claim: £288,000 × 100% = £288,000 deduction in the current tax year
• Corporation tax saving: £288,000 × 25% = £72,000 in year one
• Effective net capex: £288,000 − £72,000 = £216,000
• Adjusted payback: Improves from 6.4 years (pre-tax) to 4.8 years (post-AIA)
• 25-year IRR: Improves from 14.2% to 19.7%

Where the company has other SRP capex in the same year — say, £400k of IT infrastructure also qualifying for SRP — the combined SRP capex of £688k still falls within the £1m AIA cap, so both items attract 100% deduction. AIA only becomes a constraint when total qualifying capex in the tax year exceeds £1m.

AIA and solar PV: what HMRC classes as qualifying

HMRC's Capital Allowances Manual (CA23083) confirms solar PV systems are plant and machinery eligible for capital allowances. Qualifying expenditure includes: solar panels, inverters, mounting systems, cabling from panels to inverter and from inverter to meter, monitoring hardware, and installation labour costs where included in the same contract. Planning fees, structural surveys, and DNO application fees can be capitalised and included where they are integral to the asset acquisition.

Battery storage systems are separately assessed: HMRC's position is that batteries principally installed to store solar-generated electricity qualify as plant and machinery. Batteries installed primarily for grid services (capacity market, frequency response) may be treated differently — a point worth confirming with your tax adviser on larger BESS installations.

Common questions on AIA for office solar

Can we claim AIA if the system straddles two financial years?

AIA is claimed in the chargeable period in which the expenditure is incurred. "Incurred" for fixed assets is typically when the asset is brought into use — the commissioning date. If commissioning falls before your year end, the full AIA deduction falls in that tax year.

Does AIA apply to leased solar PV systems?

No. AIA applies to owned assets only. Under an operating lease or PPA, the lessor claims the capital allowances; the lessee deducts lease payments as revenue expenditure.

What if our total capex exceeds £1m this year?

Apply AIA to the assets with the slowest WDA rate first (typically SRP assets including solar PV). Apply Full Expensing to remaining SRP assets. Main pool assets above the AIA cap use the 18% WDA.

Can partnerships claim AIA?

Yes — AIA is available to all businesses including partnerships, LLPs, and sole traders. Full Expensing is not available to partnerships. For high-margin professional services LLPs with 40-45% marginal tax rates, AIA's value per pound of capex can exceed that available to limited companies.