How to Change Your Company Year End (AA01): Shortening, Extending and What Happens with HMRC

Every UK limited company has a year end, which Companies House calls the accounting reference date (ARD). Many directors keep the date Companies House gives them automatically. Others want something different: to line up with the tax year, to avoid a busy season, to match a parent company, or to stop their first period running past 12 months.

You change your year end with form AA01. It's free and quick to file, but the change ripples into your Companies House deadline, your Corporation Tax accounting periods and your payment dates. Get the order wrong and you can end up with HMRC expecting a return for a period that no longer exists.

This guide covers the rules for shortening and extending, when you can't change your ARD, how to file AA01, what happens with HMRC, and a worked example.

The accounting reference date is the day and month your company's financial year ends each year, such as 31 March or 30 June. When a company is incorporated, Companies House sets it automatically to the last day of the month in which the first anniversary of incorporation falls.

Each period of account ends on the ARD, and the next one starts the following day. You can find your current ARD and next accounts due date on the company's free Companies House register page.

For more on how the first period works, see our guide to first year accounts for a new limited company.

Common reasons directors file an AA01 include:

• Keeping the first period to 12 months. An automatic first period is often a few days or weeks longer than a year. For Corporation Tax that means two returns for one set of accounts. Shortening the first period to exactly 12 months avoids this.
• Aligning with the tax year. A 31 March year end matches the Corporation Tax financial year, and a 5 April year end matches the personal tax year, which can make planning dividends and salary simpler.
• Matching a group. Subsidiaries usually share a year end with their parent so group accounts can be prepared.
• Avoiding a busy season. A retailer may not want its year end in the middle of December stock-taking.
• Buying time. Extending a period pushes back the accounts deadline, which can help if records aren't ready, though it also means a longer period to report on.
• Preparing for a sale, investment or closure. Buyers and investors may want accounts made up to a particular date.

AA01 can change either the current accounting reference period or the immediately previous one, as long as the filing deadline for that period hasn't already passed.

Companies House will reject an AA01 if:

• The accounts for the period are already overdue. You can't move the end of a period once its filing deadline has passed.
• The extension would take the period past 18 months, unless the company is in administration.
• You've already extended within the last 5 years and none of the exceptions apply.
• Accounts for the period have already been filed and you're trying to change the previous period after the event.

If your accounts are already late, file them for the existing period as soon as possible. Late filing penalties are automatic and start at £150. See Companies House late filing penalties.

AA01 (Change of accounting reference date) is free to file online with Companies House. You'll need the company's authentication code or a Companies House account linked to the company.

1. Sign in to Companies House's online filing service and choose the company.
2. Select Change of accounting reference date.
3. Enter the end date of the period you're changing, as currently shown on the register.
4. Enter the new end date and confirm whether the period is being shortened or extended.
5. If you're extending within 5 years of a previous extension, state which exception applies.
6. Submit. Online changes are usually accepted within a day, and the register will show the new next accounts made up to date and due date.

After the change, the accounts deadline is worked out on the new period:

• First accounts: 21 months after incorporation (or 3 months after the ARD, if later).
• Later accounts: 9 months after the end of the period.
• After shortening: the later of the normal deadline above, or 3 months from the date you filed the AA01.

The 3-month protection stops a company being pushed into instant default by a shortening, for example where a period is shortened to a date that's already more than 9 months in the past.

This is where most problems start. Companies House and HMRC use different ideas of a period:

• A period of account is what your statutory accounts cover. It can be up to 18 months long.
• A Corporation Tax accounting period can never be longer than 12 months.

If your period of account is longer than 12 months, HMRC splits it into two accounting periods: the first 12 months, and the rest. You file two CT600 returns, both with the same set of accounts attached, and each has its own payment date (9 months and 1 day after the end of each accounting period).

Changing your ARD at Companies House doesn't automatically update HMRC's records. HMRC sets up your expected accounting periods when the company is registered for Corporation Tax, based on the original dates. When you change your year end, tell HMRC. You can do this through your business tax account or by writing to the Corporation Tax office with your UTR, the old and new dates, and the date the AA01 was filed.

A company is incorporated on 26 January 2026. Companies House sets an automatic ARD of 31 January, so the first period of account runs from 26 January 2026 to 31 January 2027, which is just over 12 months.

Before the change, HMRC would expect:

That's two returns for the first set of accounts, one of them covering just 6 days.

After filing AA01 to shorten the period to 25 January 2027:

One period of account and one CT600 each year. The 6-day period and the old 31 January periods no longer exist, but HMRC needs to be told so it can remove them from its records.

It's common to change the ARD, tell HMRC, and find that HMRC updates one period but your online account still lists others based on the old year end. If that happens:

1. Contact HMRC again with your UTR. Explain that the ARD was changed by AA01 on a given date and list the periods that should be replaced, with the correct dates.
2. Don't file a CT600 for a period that doesn't exist, even if HMRC sends a notice to file for it. Reply explaining the period isn't a genuine accounting period.
3. File your return on the correct dates that match your accounts. HMRC usually aligns its records when it receives a return showing the true accounting period.
4. If HMRC rejects the return or writes asking you to refile on its dates, check which side is correct before you change anything. Sometimes HMRC's records reflect an earlier period end that's a few days different from the Companies House register.

For HMRC rejection codes related to period mismatches, see HMRC submission errors explained.

Company law allows accounts to be made up to a date up to 7 days either side of the accounting reference date. Many businesses use this to end their year on the last Friday of a month, or on a fixed weekday each year.

Because of this tolerance, a small difference between your accounts date and the ARD doesn't always need an AA01. But be careful:

• The Companies House register will still show the ARD, and your accounts must fall within the 7-day window.
• HMRC's accounting period follows the actual date your accounts are made up to, so the start of the next Corporation Tax period moves with it.
• Small drifts from year to year can leave HMRC's and Companies House's start dates a few days apart, which is a common cause of rejections.

If you want a different year end permanently, filing an AA01 is cleaner than relying on the 7-day rule.

Once your AA01 is accepted, filing with WeFile works as normal:

• Create the filing on your new dates. Enter the period start and the new period end exactly as shown on the Companies House register.
• Long periods are split automatically. If your period of account is longer than 12 months, WeFile prepares the two CT600 returns HMRC needs from one set of accounts and calculates the tax and payment date for each.
• Accounts and CT600 together. Your iXBRL accounts go to Companies House and your CT600, computations and accounts go to HMRC in one filing.
• Period corrections. If HMRC later rejects a return because its records hold different dates, the filing's period can be updated and resubmitted without starting again.

Read more about WeFile's online CT600 software, or see our guide to Corporation Tax deadlines and penalties.

Does it cost anything to change my company's year end?

No. Filing AA01 with Companies House is free, online or on paper.

Can I change my year end after the accounts are overdue?

No. Companies House won't accept an AA01 for a period whose filing deadline has already passed. File the overdue accounts on the existing dates.

Does Companies House tell HMRC about the change?

Don't rely on it. HMRC's Corporation Tax records are separate, so tell HMRC yourself, quoting your UTR and the new dates.

Can I extend my first accounting period?

Yes, up to a maximum of 18 months from incorporation. Remember that a period over 12 months means two CT600 returns, and the first one is due based on the end of the first 12 months.

Will shortening my year end bring my tax payment forward?

It can. Corporation Tax is due 9 months and 1 day after the end of each accounting period, so an earlier period end means an earlier payment date. See how to pay Corporation Tax.

How many times can I shorten my accounting period?

There's no limit on shortening. The 5-year restriction only applies to extensions.