HMRC has quietly updated its guidance on changes and issues affecting the Corporation Tax online service, and it matters more than it might sound for small limited companies. This is not a headline-grabbing tax reform, but it is exactly the sort of back-office detail that can trip up a business that files its CT600 on autopilot or assumes its software will handle every new rule cleanly.
The update brings together several problem areas, including new capital allowance workarounds, quirks affecting some R&D claims, a filing issue for certain creative industry claims and a warning that one loans-to-participators rate change will not be fully reflected in the online service until April 2027.
Why this matters to smaller companies
For many SMEs, Corporation Tax is not managed by a large in-house finance team. It is handled by a founder, finance manager or external accountant working against deadlines with limited time to double-check edge cases. That is why guidance like this matters. Even when the underlying tax policy is known, the filing route does not always catch up immediately.
HMRC says companies claiming the new 40% first-year allowance on qualifying plant or machinery expenditure incurred on or after 1 January 2026 can still file before the CT600 form is formally updated. Until April 2027, businesses should use existing capital allowance boxes instead. That may sound manageable, but it is the kind of detail that can be missed if someone relies too heavily on habit or assumes the software will present a dedicated box automatically.
There is a similar issue for some companies claiming the extended 100% first-year allowance for zero-emission cars. If the accounting period starts on or after 1 April 2026 and the return is filed before April 2027, HMRC says the claim may need to go through the broader “other allowances and charges” boxes rather than the usual vehicle-specific ones.
R&D and director-loan complications are easy to overlook
The guidance also flags a validation issue affecting some merged R&D expenditure credit claims where a company is exempt from the PAYE cap. HMRC says affected companies need to make sure specific figures align and also state the exemption in their computations or in an attached PDF. For smaller innovation-led firms, that is important because an otherwise valid claim could still become messy if the supporting filing detail is not handled properly.
Another point worth noting is the change to the loans-to-participators tax rate, which rose from 33.75% to 35.75% for loans made or benefits conferred on or after 6 April 2026. HMRC says its online service will not reflect that change until 6 April 2027. If the new rate applies and a company files before then, it may need to amend the return after that date. For owner-managed businesses where director loan accounts are part of normal cash management, that is not a detail to leave buried in year-end paperwork.
There is also a software angle behind all this
HMRC is also moving towards more standardisation in Corporation Tax computations, starting with accounts adjustments and capital allowances. In most cases, businesses will not need to do anything themselves because software providers are expected to implement the changes. Even so, small companies should not take that as a reason to switch off completely. If your accountant or software provider has not reviewed the latest HMRC technical changes, it is sensible to ask whether your filing workflow is ready for them.
That is especially true for firms making less common claims, buying qualifying equipment, running low-emission vehicle fleets, or dealing with R&D relief. These are exactly the situations where a filing can be technically possible but still need a workaround or manual explanation.
What small businesses should do now
First, do not assume the online form fully reflects every new Corporation Tax change already in force. If your company has unusual allowances, R&D claims or director-loan issues, check HMRC’s current service guidance before filing.
Second, ask your accountant or software provider whether any of your upcoming CT600 submissions rely on temporary box workarounds, extra PDF explanations or a later amendment once HMRC updates the service.
Third, keep better notes around capital expenditure and director loan movements this year. Good records will matter if you need to justify how a claim was entered or revisit a return later.
The takeaway
HMRC’s latest update is a reminder that tax compliance problems do not always begin with the tax rules themselves. Sometimes they start when the filing system lags behind the policy or handles a new case awkwardly. For small limited companies, the safest move is simple, check the current guidance before submitting the next Corporation Tax return and make sure your filing method matches HMRC’s temporary workaround where needed.
Sources
- HMRC, Changes and issues affecting the Corporation Tax online service, updated 16 April 2026
- HMRC, New first-year allowance and main rate of writing-down allowances, published 26 November 2025
- HMRC, Corporation Tax technical specifications in XBRL and iXBRL format, updated 20 November 2025
- HMRC, Research and Development (R&D) tax relief: the merged R&D expenditure credit scheme and enhanced R&D intensive support, GOV.UK
