Annual Investment Allowance
100% deduction in year one on the first £1m of qualifying capital spend per accounting period.
100% of qualifying expenditure, capped at £1m per accounting period (the limit was made permanent at £1m from April 2023).
Permanent — no current end date. £1m annual limit per group.
In detail
The Annual Investment Allowance is the workhorse of UK capital allowance regime — a generous, broadly-available 100% year-one deduction on qualifying capital spend. For commercial solar, AIA is the most valuable allowance available where headroom exists. The reason FYA matters is that most trading businesses already have committed capital expenditure that absorbs their £1m AIA allocation: vehicle fleets, plant replacement, IT infrastructure, leasehold improvements. By the time solar PV gets considered, AIA capacity is often already allocated. In those cases, FYA is the next-best relief — 50% in year one is materially better than the 6% WDA that would otherwise apply. Where AIA capacity does exist, allocating it to solar typically gives the best after-tax economics: 100% year-one relief beats 50% FYA + 6% WDA on the residual. We model both scenarios in any project where it matters.
Who qualifies
Most UK businesses — companies, sole traders, and partnerships of individuals (but not partnerships with corporate members). Group companies share a single AIA limit.
What it does
Provides 100% tax relief in year one on qualifying plant and machinery expenditure, including commercial solar PV.
Worked example
On a £200,000 system, claiming AIA gives £200,000 deducted from taxable profits in year one. At 25% corporation tax, that's £50,000 of corporation tax saved — twice the year-one saving from FYA alone. However, £200,000 of AIA capacity is consumed, leaving £800,000 of headroom for other capital spend in the same accounting period.
Tax treatment / process
- Identify qualifying expenditure across all capex in the accounting period
- Allocate the AIA to the spend that benefits most (typically: highest-value, longest-life, or special-rate items)
- Claim on the corporation tax computation
- Any spend above the AIA limit goes into the relevant pool (special-rate pool for solar, main rate pool for general plant)
Pitfalls to watch
- Group companies share a single AIA — careful allocation is essential
- Connected businesses may be required to share the limit
- The limit applies per accounting period, not per asset — concentrating capex in one year can waste headroom
- Where AIA is available, claiming AIA on solar gives 100% year-one relief vs FYA's 50% — AIA wins if you have headroom
- Many trading businesses already use their AIA on other capital expenditure (vehicles, machinery, IT) — the FYA fills the gap
Best paired with these finance structures
Capital Purchase
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Green Loans
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Finance Lease
Functionally similar to a loan — you pay over time, the asset hits your balance sheet, and you keep …
Asset Finance
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Frequently asked questions
What is the Annual Investment Allowance (AIA) for commercial solar?
Is AIA or the 50% FYA better for commercial solar?
Does the AIA apply to solar battery storage?
Can a public-sector body claim AIA on solar PV?
Annual Investment Allowance for commercial solar — detailed guide
The Annual Investment Allowance (AIA) provides 100% first-year tax relief on qualifying capital expenditure (plant and machinery) up to £1,000,000 per year. For commercial solar installations, the AIA is typically the most practical route to maximum first-year tax relief — simpler to apply than the 50% FYA and available to a broader range of business structures.
AIA eligibility for commercial solar
| Entity type | AIA eligible? | Notes |
|---|---|---|
| Limited company (CT payer) | Yes | Claims on CT600 in accounting period of expenditure; £1m limit applies per group/common control |
| Limited liability partnership (LLP) | Yes | AIA apportioned between LLP and individual partners for income tax purposes |
| Sole trader / individual (income tax) | Yes | Claims on Self Assessment; AIA applies to business portion of expenditure |
| Trust | Limited | Trusts can claim AIA on trading assets; investment trusts have restricted access |
| Charity (non-commercial) | No | Charities are generally exempt from corporation tax on charitable activities; no tax benefit from AIA |
| Local authority / NHS Trust | No | Public sector — not corporation tax entities |
£1m annual limit: planning for large installations
The £1m AIA limit per annum creates a planning consideration for large commercial solar installations:
£1m limit applies per group
Where a business is connected with other businesses under common control, the £1m AIA is shared across all connected entities — not £1m per company. A corporate group with 5 trading subsidiaries all under common ownership has a single £1m AIA to allocate across all 5 companies' capital expenditure. This can create a constraint on large solar programmes — where the solar cost across connected entities exceeds £1m in a single tax year, the FYA (50%, no monetary limit) becomes the more practical allowance for the excess.
Straddling year-ends: phasing expenditure
For installations close to (but above) the £1m AIA threshold, phasing the installation to straddle a year-end can effectively double the AIA available — £1m in year 1 and up to £1m in year 2. However, the "brought into use" test applies — assets cannot be claimed in a year they are not in use. Phasing the installation into two distinct commissioning stages (e.g., phase 1 in December, phase 2 in January of the following year) is the commercially clean solution, but requires installer co-operation and increases project management complexity.
AIA and the pool: what happens to expenditure above £1m
Capital expenditure above the AIA limit is not wasted — it enters the main pool at 18% WDA per annum (or special rate pool at 6% for some long-life assets and integral features). For a £1.5m solar installation, the AIA claims £1m in year 1; the remaining £500k enters the main pool. Year 2 WDA on £500k at 18% = £90,000 allowance. Year 3 WDA on remaining £410k at 18% = £73,800 allowance. The full cost is eventually relieved, just spread over 15+ years.
AIA vs FYA: combined usage strategy
The AIA and 50% FYA can be used in combination within the same accounting period. The optimal combination depends on the size of the installation and other capital expenditure in the period:
Installation under £1m: use AIA (100% relief)
For a solar installation costing under £1m, use the full AIA — claiming 100% relief in year 1 is materially better than the 50% + 18% WDA trail of the FYA. Only use FYA if the AIA limit has already been used by other capital expenditure in the same period.
Installation between £1m–£2m: AIA on first £1m, FYA on remainder
For a £1.5m installation, claim AIA on £1m (100% relief) and FYA on the remaining £500k (50% relief = £250k allowance in year 1, remaining £250k enters main pool). Total year-1 allowance: £1.25m of £1.5m cost = 83% of cost claimed in year 1.
Installation above £2m: AIA on first £1m, FYA on remainder
For a £3m installation: AIA on £1m = £1m allowance; FYA on remaining £2m = £1m allowance (50%); remaining £1m enters main pool at 18% WDA. Total year-1 allowance: £2m of £3m = 67% of cost. Remaining £1m relieved over subsequent years. With a 25% CT rate, year-1 tax relief on a £3m installation is approximately £500,000 (25% × £2m).
Run the numbers on your project
We build the after-tax model with the right reliefs applied — no missed deductions, no double-counted benefits.
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