Key takeaways

  • You change a company's year end by filing form AA01 with Companies House, online, through software or by post.
  • You cannot change the year end once the accounts for that period are overdue.
  • You can currently shorten a financial year as often as you like, by as little as one day.
  • You can extend a financial year to a maximum of 18 months, and normally only once every 5 years.
  • From 1 April 2028, Companies House says you will need a business reason to shorten more than once in 5 years, subject to regulations.
  • After shortening, a private company's deadline is the later of 9 months from the new year end or 3 months from the date Companies House receives the AA01.
  • A Corporation Tax accounting period cannot be longer than 12 months, so an 18-month set of accounts needs two Company Tax Returns.

To change your company year end, file form AA01 with Companies House to set a new accounting reference date (ARD). You can do it online, through software or by post, but only before the accounts for that period are overdue. You can shorten the year as often as you like, or extend it to a maximum of 18 months, normally once every 5 years.

What is an accounting reference date?

The accounting reference date is the last day of your company’s financial year. It sets the period your annual accounts cover and the date your accounts fall due.

A new company’s first ARD is the last day of the month in which the first anniversary of incorporation falls. A company formed on 10 March 2026 has a first ARD of 31 March 2027. Our guide to choosing a financial year end covers why companies pick a different date, and fiscal year vs tax year explains how the dates differ from the tax year.

How to change your year end

You change the year end by telling Companies House the new date on form AA01. Follow these steps.

  1. Check the accounts are not overdue. Companies House says that if your accounts are overdue, it is too late to change the ARD.
  2. Decide which period to change. You can change the current period or the one immediately before it, as long as its filing deadline has not passed.
  3. Choose shorten or extend. Pick the new date and check it against the limits below.
  4. File AA01. Use the Companies House online service, your accounts software, or post the paper form to the address on it. Online filing needs the company authentication code.
  5. Tell HMRC. Update your Corporation Tax accounting period dates before you file the return.

The Companies House guidance we checked does not list a fee for filing AA01.

Shortening vs extending

The two directions have different limits, set by section 392 of the Companies Act 2006.

ShorteningExtending
How oftenAs often as you like (until April 2028)Once every 5 years
Limit on lengthMinimum of one day shorterMaximum 18 months
ExceptionsNone neededAdministration, parent or subsidiary alignment, Secretary of State approval

Companies House warns against simply adding 6 months to a period. If that takes the period past 18 months from its start date, the form is rejected. When you extend a first period to the full 18 months, count the incorporation date as day one.

How does changing the year end affect your filing deadline?

Your filing deadline moves with the new ARD. A private company normally files accounts 9 months after its year end.

If you shorten the period, section 442 of the Companies Act 2006 gives you the later of:

  • 9 months from the new ARD (6 months for a public company), or
  • 3 months from the date Companies House receives the AA01.

Two traps catch people out. Changing the year end cancels any extension to your filing deadline for that period. And if the new deadline has already passed when you change the date, Companies House charges an automatic late filing penalty. See Companies House late filing penalties for the amounts, and use the filing deadline calculator to check your new date.

What changes from April 2028?

Companies House announced on 23 October 2026 that it will limit how many times a company can shorten its ARD from April 2028. Its updated Life of a company guidance says that from 1 April 2028 you will need a business reason to shorten more than once in 5 years.

The guidance says this is subject to forthcoming regulations. We have not seen those regulations, so the exact test for a “business reason” is not yet known. Today’s rules apply until then.

What about Corporation Tax?

Changing your year end at Companies House usually changes your Corporation Tax accounting period too. HMRC says an accounting period cannot be longer than 12 months.

  • If you extend beyond 12 months: you file two Company Tax Returns to cover the accounts. Update the dates with HMRC before the original filing date, or you may get a late filing penalty.
  • If you shorten: you get a short accounting period. Contact HMRC to update the dates if you file through HMRC’s service, or enter the new dates in your software.

A change of year end can also change when Corporation Tax is due, because payment is normally due 9 months and 1 day after the end of each accounting period. Use our Corporation Tax calculator to estimate each period’s bill.

What advisers should do

  1. Check the current and previous periods’ deadlines before agreeing any change.
  2. Keep a record of each extension so you can show the 5-year rule is met.
  3. Note any client that shortens repeatedly; from April 2028 they may need a business reason.
  4. Update HMRC and your practice software at the same time as filing AA01.

Frequently asked questions

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

No. Companies House says that if the accounts are overdue, it is too late to change the accounting reference date for that period.

How many times can I extend my company’s financial year?

Normally once every 5 years, up to a maximum of 18 months. The exceptions are companies in administration, alignment with a UK parent or subsidiary, and Secretary of State approval.

Can I shorten my year end to get more time to file?

Shortening can give you up to 3 months from the date Companies House receives the AA01, if that is later than the normal deadline. From 1 April 2028 shortening more than once in 5 years will need a business reason.

Does changing the year end cancel an extension?

Yes. Any extension to the filing deadline for the affected period stops applying once you change the date.

Do I need two tax returns if I extend to 18 months?

Yes. HMRC says a Corporation Tax accounting period cannot exceed 12 months, so you file two Company Tax Returns to cover an 18-month set of accounts.

Sources

Rates, thresholds and rules in this guide were checked against HMRC and GOV.UK guidance on 6 November 2026. Tax rules change, so check the current position on GOV.UK or with a qualified adviser before acting. This guide is general information, not personal advice.