Can I change my company year end? AA01 rules explained

Can I change my company year end?

Yes. You can change your company year end, formally its accounting reference date, to make a financial year shorter or longer than 12 months. You can only change the current financial year or the one immediately before it, and you cannot change it once the accounts for that period are overdue.

The year end is the date the company's financial year ends. For a new company, Companies House sets it automatically as the last day of the month in which the first anniversary of incorporation falls.

You may not need a formal change for a small shift. Companies House guidance says you can make up your accounts to the accounting reference date or to a date up to 7 days either side of it.

Source: Change your company's year end

Shortening vs extending: the rules side by side

The limits are very different depending on which way you move the date. Shortening is flexible today. Extending is tightly limited, and Companies House rejects an application that would take a period past 18 months.

Count the 18 months from the start of the accounting period. For a first period, the date of incorporation counts as day 1. Companies House warns against simply adding 6 months to the end of a period, because in some cases that takes it past 18 months.

Source: Change your company's year end

Extending more than once in 5 years

GOV.UK lets you lengthen a financial year more often than once every 5 years only in these cases:

• the company is in administration (an administration can also extend a period beyond 18 months)
• you are aligning the company's dates with a subsidiary or parent undertaking under UK law
• the company has special permission; Companies House guidance describes this as the Secretary of State having approved it

Source: Preparing and filing Companies House accounts (Life of a company part 1)

Only the current or immediately previous period

You can change the current accounting reference date or the one immediately before it. You cannot go back further, for example to re-date accounts from two years ago.

Changing the immediately previous period is only possible while its accounts are not yet overdue. That leads to the most important restriction.

Source: Preparing and filing Companies House accounts (Life of a company part 1)

You cannot change the year end once accounts are overdue

You must change the year end before the filing deadline of the accounts for the period you want to change. If those accounts are overdue, it is too late to change the accounting reference date.

Two further traps apply. If you had an extension to the filing deadline for that period, it no longer applies after the change. And if the new filing deadline has already passed when you make the change, the company gets an automatic late filing penalty. See Companies House late filing penalties for the amounts.

Source: Change your company's year end

The business-reason rule from 1 April 2028

From 1 April 2028, you will need to give a business reason if you want to shorten your company's accounting reference period more than once in 5 years. Companies House says this change is subject to forthcoming regulations, so check the guidance again before you rely on repeated shortening.

It is part of the same package of reforms that makes accounts filing software-only. Read more about software-only accounts filing from 2028.

Source: Preparing and filing Companies House accounts (Life of a company part 1)

How changing the year end affects your filing deadline

Changing the year end also changes your accounts filing deadline, unless you are lengthening the company's first financial year. In that case the deadline for first accounts, 21 months after incorporation, still applies; see the first accounts deadline for a new company.

After shortening, the new deadline for a private company is the later of 9 months after the new accounting reference date, or 3 months after Companies House receives the change. A shorter period usually brings the deadline forward.

These dates are an illustration only. Say a company's current financial year started on 1 January 2026 and ends on 31 December 2026, so its accounts are due by 30 September 2027. On 15 October 2026 it files a change shortening that year to end on 30 September 2026. Nine months after the new year end is 30 June 2027; 3 months after the change is received is 15 January 2027. The later date, 30 June 2027, is the new deadline, 3 months earlier than before.

Source: Preparing and filing Companies House accounts (Life of a company part 1)

How a year end change affects Corporation Tax

Changing the year end with Companies House normally affects the company's accounting period for Corporation Tax. GOV.UK says you must update your accounting period dates with HMRC if you have shortened or lengthened your financial year, and HMRC advises doing this before the original filing date.

A Corporation Tax accounting period cannot be longer than 12 months. If you extend the financial year past 12 months, the company files 2 Company Tax Returns to cover those accounts. The Corporation Tax accounting period guide explains how the periods split.

Source: Accounting periods for Corporation Tax

How to change your company year end

You can make the change in any of these ways:

• online, using the Companies House service
• through software that can file with Companies House
• on paper, using form AA01 sent to the address on the form

Source: Preparing and filing Companies House accounts (Life of a company part 1)

Common year end change mistakes

Watch for these slips, which can lead to a rejected AA01 or an unexpected penalty:

• Adding 6 months to the end of a period instead of counting 18 months from its start.
• Leaving the change until the accounts are already overdue.
• Extending a second time within 5 years without one of the listed exceptions.
• Expecting a longer first financial year to give more time to file. The 21-month deadline still applies.
• Forgetting that an existing filing extension stops applying after the change.
• Shortening close to the deadline without working out the new, earlier deadline.
• Not updating the Corporation Tax accounting period dates with HMRC.

Source: Change your company's year end

Get help changing your year end

SimpleFilings can check the period, dates and current deadline with you and file the change of accounting reference date once you approve it, for a £4.99 SimpleFilings service fee. Companies House's fees list shows no government fee for this change. We do not give tax advice, and updating your Corporation Tax accounting period with HMRC is a separate step. The new year end applies only once Companies House accepts the change, so leave time before the accounts deadline.