Amending a CT600 after it has been filed

A Company Tax Return can be corrected after HMRC has accepted it. You change the figures, then send the whole return again marked as amended; HMRC replaces the original with it. This page covers when you can do that, how to do it in Tax Optimiser, and the two cases that catch people out: a return first filed in other software, and a period that needed two returns.

How long you have

A company can amend its return up to 12 months after the filing deadline. The filing deadline is normally 12 months after the end of the period, so in practice you have about two years from the period end.

You can amend more than once inside that time. After it, HMRC refuses the amendment online: see After the window has closed.

Before you start

• Amend only a return HMRC has accepted. If the first attempt was rejected, nothing was filed: fix the problem and send it as a new return. CT600 submission errors explains each rejection.
• Do the accounts need to change too? If the correction changes the accounts and they have been filed at Companies House, those need amending as well. That is a separate filing: see Amending accounts.
• Is it a loss carried back? Carrying a later loss back to this period is a claim, and it has its own guide: Carrying a trading loss back.

Step 1: change the figures

Open the period. If it was locked after filing, click Unlock period on the banner at the top of the computation first; a locked period cannot be amended. Then make the correction where the figure comes from: the Trial Balance for an accounting figure, or Corp Tax Calculations for a tax adjustment, capital allowance or relief. Check the new Corp tax payable on the computation before going further.

The original return is not touched by this. Its accounts and computation were frozen when it was sent, and they stay on the filed return as a record of what HMRC received.

Step 2: Submit as amended

Choose Corp Tax Submission. Because a return has been accepted, this opens the filed return rather than the wizard: its status is Complete, with the date, the IR mark (HMRC’s receipt) and downloads of exactly what was sent.

Click Submit as amended and confirm. Tax Optimiser keeps the original as history, creates a new return for the period and opens the submission wizard with This is an amended submission already ticked.

From here the wizard is the same as for a first filing: confirm with the client, review the accounts, review the computation and the CT600, and send. HMRC receives the complete return, not just the changes. There is no second submission fee; the fee paid for the period covers amendments.

If the day the amended return is declared should appear in box 980, set it before you send: see CT600 declaration date.

The amended setting on the computation

Submit as amended also sets Is this an amended return? to Yes on the General tab of the computation. Under it, Original accepted by HMRC confirms that the return being amended was filed from Tax Optimiser.

While that setting is Yes, Corp Tax Submission goes straight to the wizard for the amended return. Once HMRC accepts it, the period has two returns on record, the original and the amendment.

If the original was filed somewhere else

When the original return was filed in other software or by a previous accountant, Tax Optimiser has no filed return to show and no Submit as amended button. Tell it yourself:

1. Enter the period and its figures as usual.
2. On the General tab of the computation, set Is this an amended return? to Yes and save. The note under it reads Amending a return filed elsewhere.
3. Choose Corp Tax Submission. The wizard opens with the amended box ticked.

The period dates must match the original return exactly, or HMRC cannot find the return to amend.

Periods with two returns

A period of account longer than 12 months is filed as two returns, and HMRC treats each one separately. Tax Optimiser works out new or amended for each return from its own history: a return HMRC has already accepted goes as amended, and one it has not goes as new. So if the first return was accepted and the second was rejected, sending again amends the first and files the second for the first time, without you choosing.

The Is this an amended return? setting only decides the matter when neither return has been filed from Tax Optimiser.

After the window has closed

Once the last day to amend has passed, Submit as amended is greyed out and the General tab shows Amendment window closed. HMRC would reject the return with CTISS002, so Tax Optimiser does not send it.

The correction then goes to HMRC another way:

• too much tax paid: check whether to claim overpayment relief, which can be claimed up to four years after the end of the accounting period;
• too little tax paid: tell HMRC about the underpaid tax as soon as you can;
• anything else: write to the Corporation Tax office with the UTR, the period and the corrected figures.

HMRC’s own summary is at Company Tax Returns: making changes.