How to amend a CT600

Short version. Once a Corporation Tax return is filed with HMRC for a period, you can’t file a second “new” return for the same period. HMRC rejects it as a duplicate (error 3304). To change what’s filed, you file an amendment, which updates the existing return instead of creating a new one. If the change also affects your annual accounts, we file revised accounts to Companies House at the same time. Start from the filing’s page with Amend this filing.

Amendment, new filing, or re-file: which do I need?

• Amendment is right when a return for this period was already filed and accepted (by us or elsewhere) and you want to correct or update the figures.
• New filing is right when no return has been filed for this period yet. If you try to amend when HMRC has no original on record, you’ll see error 1611.
• Re-file is right when your submission was rejected, because HMRC holds nothing to amend. Fix the problem and submit again as a normal filing. See CT600 filing errors, explained.

Amending a filing made with SimpleCompanyTax, step by step

This is the simple case. We already hold your original return, so we pre-fill everything.

1. Open the company, then open the accepted filing for the period you need to change and choose Amend this filing.
2. You’ll see a confirmation page first: a read-only summary of what was originally filed and whether the HMRC amendment window for the period is still open (see Deadlines). Nothing is created until you proceed.
3. Choose Proceed. We start an amendment pre-filled with your accepted figures and take you to the review page. You’ll see an amber “Amendment” marker throughout, so you always know you’re editing an amendment rather than a new return.
4. Change the figures that need correcting, exactly as you would on an original filing.
5. In the amendment panel, briefly describe what was wrong and why you’re correcting it, using at least 16 characters. For example: “Capital allowances were understated. A £2,400 equipment purchase was missed.” This is kept with the filing record.
6. Check the destination toggles. Based on what you changed, we pre-select whether the amendment goes to HMRC, to Companies House, or both (see the next section). You can override the defaults.
7. If revised accounts are going to Companies House, enter the date the directors re-approved the revised accounts. Companies House requires revised accounts to be formally approved again on or after the day they were revised.
8. Submit. We send HMRC an amended return, so it updates what’s on file rather than duplicating it, and we send Companies House a set of revised accounts where applicable. You can track acceptance on the filing page as usual.

HMRC, Companies House, or both?

Not every correction affects both regulators, and filing an unnecessary revision creates needless public record changes. We suggest destinations from what actually changed:

• Tax-only changes. Capital allowances, disallowable expenses, brought-forward losses and similar tax adjustments live only in your CT600 and tax computation. Default: HMRC only, because your published accounts are unchanged.
• Balance-sheet-only changes. For example a corrected debtors or creditors figure. Default: Companies House only (revised accounts), because your tax numbers are unchanged.
• Profit & loss changes. Turnover, cost of sales, expenses. These flow into both the accounts and the tax computation. Default: both.

Revised accounts replace the original set on the public register (a “replacement” revision). The revised accounts include the statements required by the Companies (Revision of Defective Accounts and Reports) Regulations 2008, which we add automatically.

Deadlines

• HMRC. By law (Finance Act 1998, Schedule 18, paragraph 15) you can amend a Company Tax Return up to 12 months after the statutory filing deadline. The filing deadline is 12 months after the end of the accounting period, so in practice the amendment window closes 24 months after the period end. Example: a period ending 31 March 2025 can be amended until 31 March 2027. We show the window status before you start, and we won’t submit an HMRC amendment past it. After that, write to HMRC or call them on 0300 200 3410.
• Companies House. Revised accounts have no equivalent deadline. You can file revised accounts even after the HMRC window has closed (the accounts side of the toggle stays available).

Amending a CT600 that was filed elsewhere

If the original return was filed another way (HMRC online, an accountant, or other software), we don’t hold it, but you can still file the amendment through us. From your company page, choose Amend filing made elsewhere, confirm the company and the accounting period of the original, and complete the return with the corrected figures. The most important thing: HMRC matches your amendment to the original using two facts, and they must be exactly right.

• UTR (Unique Taxpayer Reference): the 10-digit company tax reference HMRC holds for this company.
• Accounting period start and end dates: they must match the dates of the return HMRC already has, to the day.

If either doesn’t match, HMRC can’t find the original and returns error 1611 (“no original return found”). Double-check the UTR and the exact period dates against your HMRC online account or the original return, then resubmit. If they’re correct and it still fails, call the HMRC Online Services Helpdesk on 0300 200 3600.

What it costs

Amendments and revised accounts are included free while you have an active subscription. Correct a filing as many times as you need within the amendment window. Without an active subscription, amending works like any other filing: subscribe first, then amend.

This article is general guidance, not tax advice. If you’re unsure whether a correction is needed, for example whether an error is material enough to revise your accounts, speak to an accountant.