Found a CT600 mistake after filing? What to check next

In short: You can usually amend a filed Company Tax Return within 12 months of its filing deadline, not 12 months from the day you submitted it: for a filing deadline of 31 December 2026, the window generally runs to 31 December 2027 (Finance Act 1998, Schedule 18, paragraph 15). Work from the return HMRC actually holds, correct the accounts and computation behind it, and send an amended return. Outside that window, HMRC describes separate routes for overpayments and underpayments (GOV.UK: Company Tax Returns: making changes).

First establish what was actually filed, then identify what needs correcting. A change to your bookkeeping does not automatically amend the Company Tax Return already held by HMRC.

Is this an amendment, a rejected return or a missing receipt?

An accepted return with a mistake needs an amendment, a rejected return needs correcting before it is filed, and an uncertain submission needs a status check before you send it again. If the return was never submitted, a bookkeeping change amends nothing: update and review the draft filing pack instead. Match your situation to its first action below.

Do not press "submit" repeatedly because the result is unclear. Ask the provider to establish the existing submission's status. That is a practical control against duplicate or contradictory filing attempts, not a statement about every product's retry behaviour.

How do you correct a CT600 after filing, step by step?

Work from the return HMRC actually holds, check the time limit, schedule every change, correct the whole filing pack, send the amendment through a route that supports it, then check HMRC acknowledged it. The usual window is 12 months from the filing deadline, and the correction must run through the records, accounts and computation, not just one CT600 box.

Step 1: Find the CT600 you filed

Gather the actual filed return, accounts, computation and submission evidence. Work from the filed versions, not a similarly named draft: a change to your bookkeeping does not automatically amend the Company Tax Return already held by HMRC. If the software timed out and you are not sure anything was filed, ask the provider to establish the existing submission's status before sending anything again.

Step 2: Check you are within the amendment window

You can usually amend a Company Tax Return within 12 months of its filing deadline, which is not necessarily 12 months from the day you submitted it. Outside that window HMRC describes different routes for overpayments and underpayments, so a late correction should not be treated as an ordinary software amendment (see the time limits below). Where a claim or election is involved, check its own conditions and deadlines carefully.

Step 3: List every change in a before-and-after schedule

Give the schedule five columns: affected item, originally filed amount, corrected amount, explanation and supporting document. Add a note identifying any related accounts or tax-computation change. Do not assume every accounting correction changes Corporation Tax by a fixed percentage: the effect on tax depends on the transaction and its treatment, so work it out for each item.

Step 4: Correct the records, accounts and computation together

Make the correction at its source. For example, if an invoice was duplicated in the bookkeeping and both copies affected the filed computation, remove the duplicate in the records and recalculate the relevant figures. It is not enough to reduce one CT600 box while leaving the supporting computation inconsistent. Review separately whether the accounts themselves are wrong, whether only the tax computation needs changing and whether a separate Companies House correction is needed.

Step 5: Send the amendment through a route that supports it

HMRC's amendment guidance explains the available routes. If you previously filed through HMRC's now-closed service, the closure guidance sets out routes for changes and rejected returns. Check that your replacement provider supports your historic amendment: being able to file a new return does not prove that it does. In Taxley, a return it filed and HMRC accepted is amended from its filing page. Do not press "submit" repeatedly because the result is unclear.

Step 6: Check HMRC acknowledged the amendment

The correction is complete when you hold the acknowledgment HMRC sent for the amended return. Record who approved the change, when it was sent and what acknowledgment was received, and keep the original filed pack, the corrected pack and the explanation together. Then review any effect on tax already paid, instead of assuming the payment position updates itself.

Can you amend a return in Taxley?

Yes, if you filed the return with Taxley and HMRC accepted it. Press "Amend this return" on the filing page, correct the figures, and Taxley files a complete replacement return marked as amended, normally within 12 months of the filing deadline. An amendment costs the standard fee for a return of that kind.

Before you file it, "See what changed" compares the tax and every changed box and figure with the accepted return. The amendment goes to HMRC. If that period's accounts aren't at Companies House yet, you can file the amended accounts there from the same page; Taxley doesn't file revised accounts for a period Companies House already has, so correct those there separately. Taxley can't amend a return filed another way, including through HMRC's closed service.

How long do you have to amend a CT600?

You can usually amend a Company Tax Return within 12 months of its filing deadline: for a filing deadline of 31 December 2026, the window would generally run to 31 December 2027. This is an illustration of the time limit, not a determination of your company's filing deadline.

Outside that window, HMRC describes different routes for overpayments and underpayments. If tax has been underpaid, its guidance says to notify HMRC using the online disclosure service as soon as possible. A late correction should not be treated as an ordinary software amendment without checking the route. Read GOV.UK: Company Tax Returns: making changes.

Does changing the CT600 also change Companies House accounts?

Not automatically: review separately whether the accounts themselves are wrong, whether the tax computation alone needs changing and whether any separate Companies House correction is needed. For a new return rather than an amendment, take the 30-second check to see whether Taxley fits your company.

What records should you keep after correcting a return?

Keep the original filed pack, the corrected pack and the explanation, and record who approved the change, when it was sent and what acknowledgment was received. Review any effect on tax already paid instead of assuming the payment position updates itself.

Next step: if Taxley filed the return, open it and press "Amend this return". For a return filed another way, ask Taxley whether your scenario is supported, stating the period and original filing route. Confirm the tax treatment separately before amending.

Frequently asked questions

What do you need to prepare an amendment?

The actual filed return, accounts, computation and submission evidence, plus a before-and-after schedule of every change. Work from the filed versions, not a similarly named draft.

Can I quietly fix the error in next year's return?

Do not assume that is correct. Identify the period affected and obtain advice about the proper correction: within 12 months of the filing deadline the usual route is an amendment to that period's return, and outside it HMRC describes different routes for overpayments and underpayments.

Does filing an amendment guarantee there is no penalty?

No. The circumstances matter, and an amendment does not by itself settle whether a penalty applies. Seek advice where the error is material, repeated or may involve insufficient care; if tax has been underpaid, HMRC's guidance says to use the online disclosure service as soon as possible.

What if my software timed out while filing?

Check the submission's status before sending anything again. Ask the provider to establish whether the existing submission reached HMRC, because pressing "submit" repeatedly when the result is unclear risks duplicate or contradictory filing attempts. If nothing was filed, update and review the draft filing pack instead.

Can I amend a return filed through HMRC's closed service?

Not through that service. HMRC's closure guidance sets out routes for changes and rejected returns, and a replacement provider must support your historic amendment — being able to file a new return does not prove that it does. Taxley amends only returns it filed (GOV.UK closure guidance).

General information, not personalised tax or accounting advice.