Before your next CT600: why HMRC’s computation format update matters
If your company files a Company Tax Return, HMRC’s move towards standardised Corporation Tax computations is worth understanding before your next CT600. The first phase, published in July 2025, introduced a prescribed structure for accounts adjustments and capital allowances, 2 areas where computations often differ between software products.
This does not change how much Corporation Tax you owe. It changes how the workings behind your tax position are structured, tagged and submitted. HMRC’s direction is clear: company tax return data should become cleaner, more consistent and easier for software and HMRC systems to read.
Your CT600 is only one part of the return. Alongside the form, most companies submit statutory accounts and a tax computation in iXBRL format. The computation is the bridge between accounting profit and taxable profit. It adds back items such as depreciation and certain entertaining costs, deducts non-taxable or differently taxed items, and replaces depreciation with capital allowances where relevant. Our guide to small company accounts explains the accounts side that feeds into the computation.
Where the standardisation is heading
HMRC published its consultation on modernising and standardising company tax returns on 10 March 2026, with responses due by 2 June 2026. The consultation builds on the July 2025 work and sets out a staged move towards a fully prescribed format for Corporation Tax computations.
| Stage | Timing | What it involves |
|---|---|---|
| Phase 1 standard format | Published July 2025 | Prescribed structure for accounts adjustments and capital allowances |
| Consultation | 10 March to 2 June 2026 | Views on timeline, enforcement and online amended returns |
| Collaborative development | April to September 2026 | HMRC works with software firms and advisers on detailed specifications |
| Final prescription | By end of September 2026 | Full computation format issued to developers and stakeholders |
| Build and test | October 2026 to September 2027 | Software developers implement and test the new requirements |
| Live pilot | October 2027 to September 2028 | Real returns are tested against the new format before full enforcement |
There is another practical change to watch. HMRC has proposed making online filing of amended company tax returns mandatory from 1 April 2027, subject to limited exceptions. If you have previously corrected a return by letter, that route is likely to become much more restricted.
Why it matters for your business
The practical point is that HMRC wants more structured data. That is much easier to deliver if your bookkeeping and year-end preparation are tidy before the Corporation Tax work starts. If your records are a scramble at the year end, a standard format will not fix the problem. It will expose the gaps faster.
Need Expert Accounting Advice?
If you are unsure about tax, bookkeeping, payroll, property accounts or business finances, speak to the team at FHP Accounting for clear, practical guidance.
Good year-end accounts are the best preparation. Your accountant needs clear records for income, expenses, payroll, directors’ loan accounts, fixed assets, capital allowances, loan interest, dividends and any disallowable costs. The cleaner the source data, the easier it is to produce a computation that fits HMRC’s required structure.
It is also worth separating the 2 Corporation Tax dates that often catch directors out. The CT600 is normally due 12 months after the end of the accounting period. The Corporation Tax payment is usually due 9 months and 1 day after the accounting period ends. The standardisation work does not change either deadline. If tax is paid late, interest can build quickly, as we explain in our note on HMRC late payment interest.
This is not Making Tax Digital for Corporation Tax. HMRC confirmed in its 2025 digital roadmap that it does not intend to introduce MTD for Corporation Tax. If you have been expecting quarterly Corporation Tax updates like MTD works for income tax, that is not what this is. The change is about the format and quality of the annual return, not the filing frequency.
Who should pay attention
Any limited company that files a CT600 should pay attention, but the change is especially relevant where computations are more detailed. If you hold property through a company or special purpose vehicle, capital allowances, finance costs, disallowable expenses and property income adjustments need to be shown clearly.
The same applies if you run residential property management accounting through a company, or operate as a landlord through a limited company. Property records, loan interest, repairs, improvements and capital items need to be separated properly before the computation is prepared.
From 1 April 2026, companies that previously used HMRC’s free online filing service need commercial software to file annual accounts and Company Tax Returns, except in limited cases. You can check the current form and guidance on the GOV.UK CT600 page.
Get your next return right
We are accountants in Nottingham who prepare Company Tax Returns and computations day in, day out. Whether you want an outsourced finance department to handle the whole process or you are setting up your first company, get in touch and we will make sure your next CT600 is accurate, on time and ready for whatever format HMRC settles on.
I lead FHP Accounting, an accountancy practice specialising in Commercial and Residential Property Accounting. Our goal is to make the administration of running property portfolios easier for landlords, managers, and investors — allowing you to focus on what you do best, while we take care of everything behind the scenes.
Need Expert Accounting Advice?
If you are unsure about tax, bookkeeping, payroll, property accounts or business finances, speak to the team at FHP Accounting for clear, practical guidance.
