The AIA £1m Cap for Property Investors 2026/27: Allocation Strategy and the Associated-Companies Sharing Rules Under CAA 2001 ss.51A-51N

The Annual Investment Allowance looks deceptively simple on paper: a property company spends up to £1 million on plant and machinery in an accounting period, claims the full amount as a first-period deduction against profits, and the AIA does its work. The complexity is structural, not arithmetical. A property HoldCo with three operating SPVs does not get £4 million of AIA between the four entities; under CAA 2001 ss.51E and 51G, where the companies are under common control and related to one another, a single £1 million cap is shared across the group and the allocation is decided period by period. Misallocate, and the cap-protected relief disappears not into the next entity but out of the group entirely.

This page walks the AIA framework as it stands for 2026/27, with the permanent £1 million cap from Finance (No. 2) Act 2023 s.8, the single-allowance-per-company rule at CAA 2001 s.51B, the parent-subsidiary sharing at s.51C, the related-companies sharing at s.51E plus the s.51F control definition and the s.51G shared-premises and similar-activities NACE-classification tests, the allocation mechanics at s.51K, the long-chargeable-period apportionment at s.51M-N, the interaction with HoldCo extraction structures, and a worked HoldCo-plus-three-SPV allocation scenario.

The £1 million cap is permanent

CAA 2001 s.51A(5) reads verbatim: "The maximum allowance is £1,000,000." The figure was made permanent at that level by Finance (No. 2) Act 2023 s.8, with the substitution into s.51A(5) effective from 11 July 2023 and applying for chargeable periods beginning on or after 1 April 2023. The pre-FA(No.2)2023 AIA cap structure was a permanent £200,000 baseline with successive temporary uplifts to £1 million; that structure was replaced. Anyone describing the £1 million as "temporary" or quoting £200,000 as the current cap is reading pre-2023 material.

AIA-qualifying expenditure covers most plant and machinery: main-rate plant (which would otherwise go to the main pool at 18 per cent reducing-balance WDA) and special-rate plant (integral features under s.33A, long-life assets under s.90, thermal insulation under s.28). The 100 per cent first-period write-off applies up to the £1 million cap. Above the cap, the excess rolls into the relevant pool. Cars are explicitly excluded by CAA 2001 s.38B; the dedicated FYA route at s.45D applies to low-emission cars. Used and second-hand assets are AIA-eligible (unlike full expensing under s.45S, which requires unused and not second-hand plant and is companies-only).

Single AIA per company: s.51B

CAA 2001 s.51B sets the baseline: a company is entitled to a single AIA across all its qualifying activities. A company carrying on both a UK property business and a trading qualifying activity does not get two AIAs; it gets one, allocable as the company decides between the two activities. The rule prevents same-company double-counting and is the foundation for the related-companies sharing rules that layer on top.

For individuals and partnerships, the equivalent single-AIA principle applies under s.51B's wider drafting. A sole-trader landlord with a commercial property and a separate trade does not get two AIAs; the £1 million headroom is shared across the activities. For partnerships, the AIA is at partnership level, shared between partners on the profit-sharing ratio for the relevant qualifying expenditure period.

Parent-subsidiary sharing: s.51C

CAA 2001 s.51C requires that a parent company and its 51 per cent subsidiaries share a single AIA between them for any chargeable period that is the same chargeable period across the parent and subsidiary (broadly: aligned accounting periods). The single £1 million is allocated between the parent and the subsidiaries as the group decides; there is no requirement for equal split. A property HoldCo with three 100 per cent property SPVs files four corporation tax returns but works with a single £1 million AIA across the four companies.

The s.51C parent-subsidiary trigger looks at formal share-ownership: the parent must hold (directly or indirectly) more than 50 per cent of the ordinary share capital of the subsidiary. The 51 per cent threshold is the standard one for corporate groups in UK tax legislation, mirroring the CTA 2010 s.1154 definition of a 51 per cent subsidiary. Where the threshold is not met, the s.51C route does not apply, but the s.51E common-control + related-companies route may still apply, capturing 'brother-sister' structures under common individual ownership.

Common-control + related-companies sharing: s.51E

The operationally important sharing rule for property structures is CAA 2001 s.51E. The provision requires that two or more companies under common control AND related to one another share a single AIA between them, regardless of formal parent-subsidiary structure. The classic case is the brother-sister structure: an individual founder owns 100 per cent of HoldCo A and 100 per cent of HoldCo B, with no parent-subsidiary chain, but the two companies are both letting commercial premises. They share a single £1 million AIA under s.51E, not s.51C.

"Control" for s.51E is defined at CAA 2001 s.51F by reference to CTA 2010 ss.450-451: a person controls a company if they have power to ensure the affairs of the company are conducted in accordance with their wishes, whether through ownership of share capital, through articles-conferred powers, or through any other means. Two companies are under common control where the same person (or the same group of persons acting together) controls both. Connected persons under CTA 2010 s.1122 are aggregated for the control test: spouses' and civil partners' interests are aggregated, as are interests of certain relatives, partners and associated trusts.

"Related to one another" for s.51E is defined at CAA 2001 s.51G by either the shared-premises test or the similar-activities test, both of which appear below.

The s.51G shared-premises test

CAA 2001 s.51G(2) sets out the shared-premises test: two companies are related to one another for AIA-sharing purposes if they carry on qualifying activities at premises that are common to both of them, or at substantially the same premises. The bar is low. A HoldCo and an SPV operating from the same registered office and the same workspace meet the test. A HoldCo and an SPV occupying different floors of the same building meet the test. A HoldCo and an SPV at separate addresses but sharing administrative or back-office facilities likely meet the test (HMRC's interpretive position is that "substantially the same premises" extends to functionally-shared facilities).

For property structures with a common founder operating from one office, the shared-premises test is typically met across the entire HoldCo + SPV ecosystem, even where each SPV holds a different commercial property at a different physical address. The relevant premises for the test is where the qualifying activity is managed and conducted, not where each SPV's property asset is located.

The s.51G similar-activities test (NACE Rev 2 first-level classification)

CAA 2001 s.51G(3) sets out the similar-activities test: two companies are related to one another for AIA-sharing purposes if more than 50 per cent of each company's turnover from qualifying activities is derived from activities falling within the same first-level classification under NACE Rev 2 (the EU Statistical Classification of Economic Activities, retained in UK statutory drafting post-Brexit).

NACE Rev 2 first-level classifications relevant to property structures include: Section L ("Real estate activities", covering buying and selling of own real estate, renting and operating of own or leased real estate, and real estate activities on a fee or contract basis); Section F ("Construction", covering construction of buildings, civil engineering, and specialised construction activities); and Section M ("Professional, scientific and technical activities", which catches some property-adjacent advisory firms). For pure property-investment SPV structures, all of the SPVs typically fall within Section L; the similar-activities test is met across the group, even where individual SPVs hold different property types (offices, warehouses, retail, residential).

The bar is the 50 per cent threshold on turnover, not 100 per cent. An SPV that derives 60 per cent of its turnover from commercial property rents (Section L) and 40 per cent from incidental services (e.g. parking facility licensing, Section H "Transportation and storage") is still primarily Section L for the s.51G test. The threshold is applied entity by entity; the test is met as long as each related entity is primarily Section L.

Allocation mechanics: s.51K

CAA 2001 s.51K governs how the single £1 million AIA is allocated within a related-companies group. The principles are: the group decides the split between the related companies before the relevant returns are filed; the allocation must total no more than £1 million across the group; the allocation is binding for the period and cannot be varied retrospectively once the returns are submitted.

The practical mechanic is that the corporate finance function projects each related entity's qualifying expenditure in the period and allocates AIA headroom to maximise relief. The optimal allocation typically loads AIA into the entity with the highest marginal-rate-of-tax-saved-per-£-of-AIA: where one SPV is small-profits-rate (currently 19 per cent main, 25 per cent main from £50k of augmented profits), another is in marginal-relief territory (effective rate around 26.5 per cent for the £50k-£250k band), and a third is solidly at the 25 per cent main rate, loading AIA into the marginal-relief entity often delivers the highest absolute tax saving per pound of AIA spent. The allocation question is dynamic and changes year on year; treating it as a financial-year planning question, not a year-end accident, is the operating discipline.

The allocation is filed via the corporation tax computation for each related company, with the group documentation typically prepared as a stand-alone allocation memorandum referenced from each computation. HMRC's expectation is that the documentation is contemporaneous (prepared in or before the period) and consistent across the related companies' returns. Misallocation is permanent loss of cap-protected relief, because the rule is one cap not four, and a £400k AIA "claim" in one SPV with the remaining £600k taken by another SPV is the maximum group-level allocation for the period; it cannot be uplifted by adjusting the split.

Long-chargeable-period apportionment: s.51M-N

CAA 2001 s.51M (Long chargeable periods) and s.51N (corresponding provisions for shared-AIA groups) apportion the £1 million cap pro rata where a chargeable period is not 12 months long. A 9-month accounting period gives an AIA of £1m x 9/12 = £750,000. An 18-month period (where allowed; typically only by election to lengthen) gives £1m x 18/12 = £1,500,000. For shared-AIA groups, the apportionment runs at group level: a group with a 9-month aligned period gets a single £750,000 AIA shared across the related companies.

Where related companies within a sharing group have misaligned accounting periods, s.51M-N becomes complex, generally requiring period-by-period apportionment for each related company within the relevant overlap windows. The pragmatic discipline is to align periods deliberately: most property HoldCo + SPV groups choose a common year-end and stick to it, even where the individual SPVs were incorporated on different dates, precisely to keep the AIA allocation tractable. Year-end changes to align periods are permitted under Companies Act 2006 s.392 with HMRC notification, subject to the usual constraints on how often the year-end can be changed.

The interaction with HoldCo extraction structures

A HoldCo plus property SPV structure typically combines AIA allocation considerations with dividend extraction mechanics, intra-group lending, and exit-route planning. The single-AIA constraint adds a dimension to the HoldCo group corporate-tax computation: AIA decisions in year one affect the pool position in subsequent years, which interacts with full expensing on top of AIA (full expensing covers excess unused main-rate spend) and with the dividend extraction flow upward into the HoldCo and out to shareholders.

The cross-bucket sibling page on HoldCo dividend conduit mechanics covers the extraction side in depth, including the intra-group transfer of capital-allowance-claimed plant under s.45BB (the full-expensing intra-group transfer carve-out) and the Sch A1 connected-company restrictions on subsequent disposals; the AIA allocation strategy in this page should be read in conjunction with that companion piece for the full HoldCo-plus-SPV planning frame.

Worked HoldCo + three SPV allocation scenario

Consider an anonymised property holding structure: HoldCo (UK Ltd, no operating activity, holds the three SPVs as 100 per cent subsidiaries), Property SPV 1 (commercial office unit in regional city, £180k of integral-features spend in the period), Property SPV 2 (small warehouse, £45k of removable plant spend), Property SPV 3 (mixed-use building with commercial ground floor and residential flats above, £620k of qualifying commercial-only refurbishment in the period). All four entities share a common 31 March accounting year-end. The founder owns 100 per cent of HoldCo, which holds 100 per cent of each SPV.

Group AIA position: the three SPVs are 100 per cent subsidiaries of HoldCo (s.51C parent-subsidiary trigger met); they are also under common control with HoldCo and related to one another via the s.51G similar-activities test (all operating in NACE Section L "Real estate activities") plus the s.51G shared-premises test (all operating from the same founder's London office). The group is therefore subject to single-AIA sharing under both s.51C and s.51E; single £1 million AIA across the four companies for the period.

Aggregate qualifying expenditure: £180k (SPV 1) + £45k (SPV 2) + £620k (SPV 3) + nil (HoldCo) = £845k, comfortably inside the £1 million cap. Optimal allocation absorbs the full £845k under AIA: £180k to SPV 1, £45k to SPV 2, £620k to SPV 3, £nil to HoldCo. Each SPV writes off its qualifying spend in full in the period, with no main-pool or special-rate pool roll into subsequent periods on the AIA-covered portion. £155k of AIA headroom remains unused at group level; the headroom does not roll forward (AIA is use-it-or-lose-it per period).

The misallocation failure mode: suppose the SPVs' accountants do not coordinate, and each SPV claims the full applicable AIA on its own return without reference to the group cap. SPV 1 claims £180k, SPV 2 claims £45k, SPV 3 claims £1,000,000 (capping its £620k qualifying spend short, wait, capping at £1m means the SPV claims its full £620k; this is the favourable misallocation). The aggregate claimed is £180k + £45k + £620k = £845k, still within £1m cap. No actual harm done in this specific scenario because aggregate spend is below the cap; the misallocation matters when aggregate spend exceeds £1m.

Substitute a scenario where SPV 3's qualifying refurbishment is £1,400,000 not £620,000. Aggregate qualifying spend across the group is £180k + £45k + £1,400k = £1,625k. The single-£1m AIA absorbs only £1,000,000 of this; £625k must roll into the relevant pools (special-rate pool for integral features at 6 per cent reducing-balance WDA, or main pool at 18 per cent). The group allocation question is now binding: load the £1m AIA into the SPV with the highest marginal-rate-of-tax-saved-per-£-of-AIA, and let the remaining £625k roll into the pool in another SPV. Misallocation here costs years of accelerated relief.

Practical mistakes to avoid

The AIA-plus-association-rules cluster generates recurring errors in practice. Recognising them is the cheapest way to defend a claim.

Treating each SPV as having its own £1m AIA. A HoldCo + three SPV structure that meets the s.51C or s.51E + s.51G tests has a single £1m cap, not £4m. The default mental model among non-specialist accountants is per-entity caps; the legislation is otherwise. Reading the SPV-by-SPV AIA claims back against the group cap at year-end planning is the protective discipline.

Citing the AIA cap as £200,000 or "temporarily £1m". The £1m cap is permanent from 1 April 2023 under F(No.2)A 2023 s.8. Material that frames the cap as £200,000 baseline with £1m temporary uplift is pre-2023 commentary.

Treating AIA as available on cars. Cars are excluded from AIA by CAA 2001 s.38B. The dedicated FYA route at s.45D applies to low-emission cars; other cars go to the main pool or special-rate pool by emissions threshold. Cars do not consume AIA headroom.

Claiming AIA on residential-let in-unit plant. CAA 2001 s.35 dwelling-house restriction blocks plant-and-machinery allowances on plant in a dwelling-house where the qualifying activity is an ordinary UK property business. AIA cannot rescue spend that is out of qualifying expenditure under s.35. The HMO common-parts narrow exception is the operational depth question covered in bucket sibling C7.

Ignoring the long-chargeable-period adjustment. A 9-month period gets £750k AIA, not £1m. A property SPV in its first short period after incorporation often has an accounting period of less than 12 months; the AIA available is apportioned, not full.

Allocating AIA after the returns are filed. The s.51K allocation is binding for the period and cannot be varied retrospectively. Group allocation decisions need to be made before the returns go out, with contemporaneous documentation supporting the chosen split.

Failing to align accounting periods across the related-companies group. Misaligned periods make the s.51M-N apportionment complex and create allocation traps where the cap can be partly lost across overlapping period segments. Aligning periods is administrative housekeeping with material AIA-protection value.

Forgetting that shared-premises operates broadly. The s.51G shared-premises test is met by SPVs operating from the same office, the same registered address, or substantially the same functionally-shared facilities. Geographic dispersion of the property assets themselves does not defeat the test; what matters is where the qualifying activity is managed.

Sources and statutory references

1. Capital Allowances Act 2001, section 51A (Entitlement to AIA; maximum £1,000,000).
2. Capital Allowances Act 2001, section 51B (Single AIA per company across qualifying activities).
3. Capital Allowances Act 2001, section 51C (Parent + 51 per cent subsidiary single AIA sharing).
4. Capital Allowances Act 2001, section 51E (Common-control + related-companies single AIA sharing).
5. Capital Allowances Act 2001, section 51F (Control definition via CTA 2010 ss.450-451).
6. Capital Allowances Act 2001, section 51G (Related-companies tests: shared-premises and similar-activities NACE Rev 2 first-level classification).
7. Capital Allowances Act 2001, section 51K (Allocation mechanics within shared-AIA groups).
8. Capital Allowances Act 2001, section 51M + section 51N (Long-chargeable-period apportionment for AIA and for shared-AIA groups).
9. Finance (No. 2) Act 2023, section 8 (AIA £1m permanent from 1 April 2023; substitution effective 11 July 2023).
10. HMRC Capital Allowances Manual, CA23080 (AIA general guidance), CA23210 (associated-companies sharing).

Related reading on this site: bucket sibling pages C1 (the four-axis CAA 2001 decision framework pillar); C2 (disposal mechanics); C3 (SBA depth); C5 (full expensing carve-outs and intra-group transfer); A7 (HoldCo extraction mechanics: forward-link, see manager back-patch at wave merge). Adjacent existing pages on AIA: AIA Capital Allowances: legacy entry-level (note: pre-FA(No.2)2023 framing; should be updated post-launch).