5 corporate tax planning strategies most mid-market businesses overlook

Mid-market finance teams routinely miss reliefs they're entitled to. HMRC data for tax year 2023 to 2024 shows total R&D claims fell 26% to 46,950, with smaller and first-time claimants disproportionately affected. Patent Box reliefs are similarly concentrated. Large companies receive 95% of the total, while mid-market businesses are largely absent.

At 25% Corporation Tax, every £1,000 of unclaimed relief costs you £250 in tax you didn't need to pay. For mid-market businesses with profits in the marginal relief band, that effective rate climbs to approximately 26.5%, taking the cost of every unclaimed £1,000 up to £265. Behind every relief claim is a documentation question. Can the transaction-level data your finance team holds support what you want to claim?

This piece covers five places where mid-market underclaiming hides: marginal relief calculations, the merged R&D scheme, Patent Box, capital allowances changes before April 2026, and the spend data infrastructure that supports every one of those claims. This is general guidance for UK finance teams. Tax treatment depends on your specific circumstances, so consult a qualified tax adviser before acting on the strategies covered here.

How the current UK tax framework creates both opportunity and risk for mid-market

The Corporation Tax structure has rewarded precision since 1 April 2023. You pay 19% on profits up to £50,000 (small profits rate) and 25% on profits above £250,000 (main rate), with marginal relief applying in the band between those thresholds. The Corporate Tax Roadmap, published by HM Treasury in October 2024, caps the headline rate at 25% for the duration of this Parliament. It also maintains full expensing, the £1 million Annual Investment Allowance (AIA), and R&D reliefs at their current generosity.

HMRC is intervening earlier and collecting more compliance yield. In 2024 to 2025, HMRC secured £48.0 billion in compliance yield, exceeding its £45.4 billion target. Upstream compliance activity, where HMRC intervenes before errors are filed, grew from 24% to 43% of the compliance yield target between 2019 to 2020 and 2024 to 2025. The practical implication for finance teams: capture evidence as transactions happen, because year-end reconstruction makes claims harder to support and HMRC's recent enforcement campaigns show where mid-market is most exposed.

Strategy #1: Marginal relief errors are triggering HMRC letters

HMRC's most visible recent campaign targets marginal relief. If you've ever counted your associated companies for marginal relief purposes and quietly wondered whether dormant subsidiaries should be included, you're not alone. This grey area catches more mid-market groups than any other single calculation error.

Both the £50,000 lower limit and £250,000 upper limit must be divided by the number of associated companies, including the company itself. Dormant subsidiaries, holding companies, special purpose vehicles, and overseas entities all count.

For a group with four associated companies, the marginal relief band compresses from £50,000 to £250,000 down to £12,500 to £62,500. A company that believed it qualified for the 19% small profits rate may actually owe tax at the 25% main rate, or at the approximately 26.5% effective marginal rate within the band.

HMRC is writing to companies it suspects have failed to declare the correct number of associated companies. The Institute of Chartered Accountants in England and Wales (ICAEW) confirmed the campaign in August 2025, and those letters require a response within 30 days.

ICAEW's TAXguide 02/23 explains that certain dormant or non-trading associated companies are disregarded when determining the associated company count for corporation tax thresholds. If you've understated the count, it's worth reviewing prior CT returns within the amendment window. You may also want to model whether dissolving genuinely dormant entities would improve your tax rate position before acting. Can you confidently list every associated company in your group right now, including the ones nobody thinks about?

Strategy #2: R&D tax credits under the merged scheme

The transition to the merged Research and Development Expenditure Credit (RDEC) scheme, effective for accounting periods beginning on or after 1 April 2024, caught many mid-market finance teams off guard. If you're still operating on assumptions from the old SME scheme, your claims may be wrong.

The merged scheme offers a 20% above-the-line credit on qualifying R&D expenditure. For companies paying CT at 25%, the net cash benefit is 15%.

A separate system, Enhanced R&D Intensive Support (ERIS), for loss-making SMEs, provides up to 27% where qualifying R&D expenditure represents at least 30% of total expenditure. That threshold was reduced from the original 40%, so more companies qualify than many finance teams realise. Note that ERIS applies only to companies meeting the SME definition used for R&D relief, which is expressed using euro thresholds: fewer than 500 staff, with either turnover below €100 million or balance sheet total below €86 million. These limits are set in euros because the definition is inherited from EU rules. But if your accounts are in sterling, convert at the exchange rate on your balance sheet date. Larger mid-market businesses claim under the merged scheme regardless of R&D intensity.

The pre-notification deadline most new claimants miss

If you haven't claimed R&D relief in the previous three accounting periods, or if you're claiming for the first time, you must notify HMRC of your intention to claim within six months of the end of the accounting period. Miss this deadline and you forfeit the claim entirely. HMRC has no statutory discretion to extend it.

Why HMRC scrutiny shouldn't stop valid claims

Documentation has become the practical battleground. Roughly 2,100 R&D claims were amended by HMRC's Fraud Investigation Service in 2022 and 2023, with over 73% of queried claims subsequently amended. ICAEW has observed that HMRC's compliance strategy "is disrupting compliant claims, hindering genuine R&D investment and disproportionately affecting small companies." Every claim now requires a mandatory Additional Information Form with technical narrative and cost breakdown.

A Spring 2026 advance assurance pilot is opening to any SME planning to claim, including those with previous claims. If your R&D involves complex or high-risk elements, this is worth monitoring.

Your R&D qualifying expenditure (staff costs, subcontractor costs, consumables) needs to be tracked and attributed to specific qualifying projects as the expenditure is incurred. The difference between a defensible R&D claim and a contested one usually shows up in the cost attribution evidence, not in the technical narrative. The same documentation logic determines who can credibly claim Patent Box.

Strategy #3: Patent Box, and the 10% rate most mid-market businesses miss

Only around 1,650 companies elect into Patent Box. The relief stays concentrated among larger businesses: an estimated £1,977 million was claimed in 2023 to 2024, with large companies receiving 95% of it. The top 145 companies, by relief value, account for 92% of all Patent Box claims.

Patent Box allows you to apply a 10% effective Corporation Tax rate to profits derived from patented inventions, against the standard 25% main rate. That's a saving of up to 15 percentage points on qualifying profits. A single patented component can bring all revenue from a product within Patent Box.

Why nexus tracking deters mid-market claimants

Many mid-market finance teams avoid Patent Box because nexus tracking has to start when the intellectual property (IP) is created. You need to track the R&D expenditure that generated the IP from the point of IP creation. You must also elect into Patent Box, either in the computations accompanying your CT return or separately in writing to HMRC, within two years of the end of the relevant accounting period. That two-year window is a hard deadline.

If you're already claiming R&D tax credits, you should almost certainly be assessing Patent Box eligibility as well. The reliefs are designed to work together. R&D credits reduce the cost of innovation. Patent Box reduces the tax on the profits that innovation generates. Yet mid-market companies claiming R&D credits are, by the data, overwhelmingly failing to also capture Patent Box.

Why the new transfer pricing exemption doesn't help with Patent Box

The new UK-to-UK transfer pricing exemption, which applies for chargeable periods beginning on or after 1 January 2026, carves out transactions interacting with Patent Box. If you hold a Patent Box election, you must continue to maintain arm's-length intercompany IP pricing regardless of the broader exemption.

Both R&D credits and Patent Box share the same underlying requirement: traceable, project-level cost data captured as work happens, not reconstructed at year-end.

Strategy #4: Capital allowances decisions before April 2026

The main pool Writing Down Allowance rate (WDA, the annual deduction on certain capital assets) drops from 18% to 14% from 1 April 2026 for companies. For accounting periods straddling that date, a blended rate applies.

If you have significant main pool balances (vehicles, general equipment, and legacy IT that don’t qualify for full expensing), the pre-April 2026 window is an active planning period. After 1 April 2026, it closes permanently.

Full expensing versus AIA: which applies to your purchases

Full expensing provides 100% first-year relief on qualifying main rate plant and machinery with no monetary cap. But it only applies to new, unused assets purchased by companies, and excludes cars, second-hand assets, and leased assets. The AIA, now permanently set at £1 million, generally covers most new and used plant and machinery and is available to most businesses. But be aware that this is capped, and it excludes cars, gifted or previously-owned items, and certain mixed partnerships or trusts.

For mid-market groups, the AIA may also need to be shared between companies under common ownership. Plan that allocation deliberately so the group doesn't default into suboptimal claims.

Structures and Buildings Allowance: likely underclaimed since 2018

The Structures and Buildings Allowance (SBA) provides 3% straight-line annual relief on eligible construction, renovation, and conversion costs for non-residential buildings and structures. It applies to expenditure incurred on or after 29 October 2018, but it isn't automatic. You must maintain an allowance statement per building.

Many mid-market companies that built or refurbished premises between October 2018 and now have never claimed SBA. In Freeport or Investment Zone special tax sites, the enhanced rate is 10%. The cumulative effect across several years and multiple properties adds up.

The capital allowances decisions due before April 2026 are documentation problems as much as tax problems. The records you can produce (purchase dates, asset categorisations, emissions data) determine which pool each asset lands in, and which rate it attracts.

Strategy #5: Treat your spend data as tax infrastructure

Every relief covered above depends on transaction-level data. Asset class for capital allowances, project attribution for R&D, nexus tracking for Patent Box, associated company count for marginal relief. Poor expense categorisation makes each of those harder to substantiate, and harder claims either don't get filed or don't survive scrutiny.

Whether a purchase qualifies for full expensing at 100%, AIA, or the reduced 14% WDA from April 2026 depends entirely on how you categorise it. Cars with CO₂ emissions up to 50g/km go to the main pool; those above 50g/km go to the special rate pool at 6% WDA. Capturing emissions data and asset classification at the point of purchase makes the correct claim possible.

The same principle applies to VAT reclaim. HMRC's record-keeping rules require digital transaction-level records, and expenses with mixed business and personal use must have the disallowable portion separately identified at transaction level. HMRC's 2025 tax gap publication reported upward revisions to estimated missing VAT for 2022 to 2023 of around £5 billion, confirming the regulator's focus on transaction-level evidence.

Better transaction-level data makes substantiating relief claims easier. Spendesk is a spend management platform consolidating company cards, expense management, accounts payable, procurement, and budgeting. It captures transaction data with automated categorisation and receipt matching at the point of spend.

According to Spendesk, customers can achieve up to 98% receipt collection. That gives the finance team a documentation base built up over the year, rather than a year-end reconstruction exercise. It also provides a continuous audit trail if HMRC asks for evidence.

Closing the gap between reliefs available and reliefs claimed

Every £1,000 of unclaimed relief still costs £250 in tax at the 25% main rate, or up to £265 in the marginal relief band. The five gaps covered above share the same root cause: associated company tracking, the merged R&D scheme's pre-notification window, Patent Box nexus traceability, capital allowances classification before April 2026, and the spend data behind all four. Relief claims are won or lost on the records your finance team can produce, not on the strength of the technical argument.

The businesses that close that gap aren't the ones with the most aggressive tax planning. They're the ones whose transaction data is categorised, attributed, and substantiated as work happens. Year-end reconstruction can't manufacture a defensible R&D project narrative, a Patent Box nexus trail, or an associated company count that survives an HMRC letter. A structured planning calendar built around when reliefs are identified, documented, and claimed beats one built around statutory deadlines alone.

As a practical next step, review your associated company count, pre-notification deadlines for any first-time R&D claim, Patent Box eligibility, and capital expenditure plans before April 2026. The earlier each one moves from a once-a-year reconciliation to a continuous data trail, the smaller the gap between what you're entitled to and what you actually claim. That's what shifts a finance team from reactive filing to proactive planning.

See how Spendesk approaches spend data capture at the point of transaction.

Frequently asked questions about corporate tax planning

Which review should happen first if a finance team can't tackle everything at once?

Start with the area where a mistake would be both immediate and expensive. For some teams, that will be associated company counts because an error changes the tax rate applied to profits already earned. For others, it will be an R&D claim with a live pre-notification deadline. A practical way to prioritise is to rank issues by two factors: whether a deadline can close off the claim entirely, and whether the numbers involved are large enough to change your tax position materially.

What should a finance team gather before speaking to a tax adviser?

Pull together the records that determine whether a claim can be supported, not just whether it looks attractive in theory. That includes a current associated company list, details of any prior R&D claims, transaction-level spend data, asset classifications, emissions data for cars, and the documentation needed to trace R&D costs or Patent Box nexus. The stronger that pack is before the conversation starts, the easier it is to move from high-level review to action.

Where does data quality most often affect the final tax position?

Usually at the point where tax treatment depends on classification rather than intention. If transaction data is incomplete or captured too late, the business may struggle to distinguish between assets that qualify for full expensing and those that fall into other allowances. The same goes for identifying mixed-use VAT items properly, and for attributing R&D costs to the right projects. Weak categorisation tends to show up first in the claims that rely on detailed transaction evidence rather than broad year-end estimates.

When is it worth reviewing prior returns instead of focusing only on the next filing?

When you have reason to think a past assumption was wrong and the return is still within the amendment window. The clearest example in this article is an understated associated company count for marginal relief. If that count was wrong, the issue may not be limited to the next period. Reviewing earlier returns can help you identify whether the tax cost has already crystallised and whether corrective action is still available before the window closes.