HMRC has released the updated CT600 (2026) Version 3 - the form you'll use to file your Company Tax Return if you hold properties in a limited company structure (GOV.UK, 1 April 2026). From a portfolio perspective, this matters because it affects how you report your annual corporation tax liability to HMRC, particularly if you're running a single-let SPV (Special Purpose Vehicle) or multi-property holding company. The 2026 update introduces preparation for a potential Northern Ireland devolved corporation tax rate - but you won't complete those sections unless and until the Northern Ireland Executive formally introduces one.

The CT600 form was last updated on 1 April 2026 to align with potential changes to corporation tax rules in Northern Ireland (GOV.UK, 2026). The form now includes new boxes to capture data for a devolved Northern Ireland rate - but here's the key point: you should not enter any data in the Northern Ireland-specific boxes unless that rate actually comes into effect. This is a preparatory change only.

The core structure of the form remains consistent with previous versions. You'll still need to declare your taxable profit, corporation tax liability, and relevant adjustments. The supporting pages (CT600A through CT600N) cover specific scenarios: research and development relief, group relief, foreign companies, partnerships, and other specialist areas.

The guidance emphasises that this is now a paper-filing option only. Your first choice should always be to file digitally using commercial software approved by HMRC. You can only use the paper CT600 form if you have a reasonable excuse for not filing online - for example, significant IT problems that prevent digital submission.

If you're holding property in a limited company, your filing deadline remains 12 months from the end of your company's accounting year (HMRC guidance, 2026). Corporation tax itself is due nine months from year-end, but the return paperwork has the longer window.

Here's what's changed operationally: HMRC now expects all companies to file digitally by default. If you do choose to submit a paper return, you must include a WT1 form (a declaration explaining your reasonable excuse for not using software) alongside the CT600. Your accountant will handle this in practice, but it's worth knowing the requirement.

The address for posting paper returns is: Corporation Tax Services, HMRC, BX9 1AX.

If you're submitting late or amending a previous return, the paper route might feel necessary - but be aware that digital filing is faster and generates an immediate acknowledgement from HMRC. From a portfolio perspective, if you're managing multiple properties across one holding company, cleaner data flow means faster confirmation of your tax position, which matters for refinance conversations with lenders.

Corporation tax rates for 2026 remain at 19% for profits under £50,000 and 25% for profits above £250,000, with marginal relief for profits in between (Office for National Statistics and HMRC guidance). Whether you file on paper or digitally, your liability calculation doesn't change - but the form itself now anticipates future Northern Ireland divergence.

If your property portfolio sits within a limited company and you're based in Northern Ireland, speak to your accountant about whether this update affects your planning. For most England-based and Wales-based investors, the CT600 (2026) Version 3 is a standard annual requirement with no material change to your filing process.

The release of Version 3 signals that HMRC is preparing its infrastructure for potential devolved tax rates. If you hold property across multiple UK nations or run a group structure with Northern Ireland interests, this is worth a conversation with your tax adviser to confirm your filing strategy.