How to change your accounting reference date: a UK limited company guide for 2026

Every UK limited company is assigned an accounting reference date (ARD) when it incorporates. That date anchors everything: when your annual accounts are due at Companies House, when your Corporation Tax return lands at HMRC, and when your accountant blocks out two weeks of their calendar. For most founders, the date set at incorporation works fine. For others, it quietly causes problems for years.

Maybe your company incorporated in July but your accountant works on a March year-end cycle and charges a premium for off-cycle work. Maybe you acquired a subsidiary and the parent's year-end doesn't match. Maybe your first trading year was unusually short, and squeezing a full set of accounts into six months makes the numbers look worse than reality. Whatever the reason, Companies House gives you a mechanism to move the date — but the rules have edges that catch people out.

This guide covers exactly how to change your accounting reference date, when you legally can and can't, what it does to your Corporation Tax position, and the mistakes that turn a routine administrative change into a late filing penalty.

What is the accounting reference date?

Your accounting reference date is the last day of your company's financial year. Companies House sets the initial ARD automatically: it falls on the last day of the month in which the anniversary of incorporation falls. If you incorporated on 15 March 2025, your first ARD is 31 March 2026, and every subsequent year-end is 31 March.

This date determines two critical deadlines:

• Annual accounts filing deadline — due 9 months after the ARD (or 21 months from incorporation for your first set of accounts).
• Corporation Tax return (CT600) — due 12 months after the end of the accounting period, with the tax payment itself due 9 months and 1 day after the period ends.

Change the ARD and both deadlines shift. That's why it matters — and why getting it wrong has consequences.

Why would you change your accounting reference date?

Moving your year-end is not something you do on a whim, but there are several legitimate and common reasons:

Aligning with your accountant's cycle

Most UK accountancy firms work to 31 March or 5 April (the tax year-end) and 31 December cycles. If your company's year-end falls in, say, November, your accountant may be preparing your accounts during their quietest period — or, more likely, squeezing you in alongside dozens of other off-cycle clients. Aligning your year-end with a common date can reduce fees, improve turnaround, and make tax planning simpler.

Group company alignment

If your company is part of a group — perhaps you've acquired a subsidiary or been acquired yourself — having mismatched year-ends creates consolidation headaches. Aligning the subsidiary's ARD with the parent's is one of the most common reasons Companies House sees form AA01 filed.

First-year trading adjustments

A company that incorporates in January and starts trading in September has an awkward first accounting period. The accounts cover months of dormancy followed by a few months of activity, making the financial picture misleading. Extending the first year-end gives the accounts a full trading period to work with.

Tax planning

Shifting your year-end can defer or accelerate when Corporation Tax falls due. If your company expects a profitable second half and you shorten the current period, you crystallise a smaller tax bill earlier and push the larger bill into a later period. This is legitimate tax planning, but it needs to be done with professional advice — the interaction between the accounting period and the Corporation Tax period is not always intuitive.

Buying time for accounts preparation

This is the reason nobody admits to, but Companies House sees it regularly. If your accounts are running late and the filing deadline is approaching, extending the current financial year pushes the deadline back. It is a lawful move — but only if you haven't already missed the deadline, and only if you haven't extended in the previous five years. More on those restrictions below.

How to change your accounting reference date: step by step

The process itself is straightforward. The rules around when you can and can't do it are where the complexity lies.

1. Decide whether you are extending or shortening. This determines which rules apply. Shortening has almost no restrictions. Extending has several.
2. Check that your accounts are not overdue. Companies House will reject a change to the accounting reference date if your annual accounts are currently overdue. File the overdue accounts first, then apply.
3. File the change with Companies House. You can do this online through the Companies House online service or by submitting paper form AA01 by post. There is no fee for either method.
4. Notify HMRC. Changing your accounting reference date affects your Corporation Tax accounting period. HMRC needs to know so they can adjust your CT600 filing and payment deadlines. If your accounting period changes, HMRC may split it into two separate Corporation Tax periods (no single CT period can exceed 12 months).
5. Recalculate all your deadlines. Your accounts filing deadline, Corporation Tax return deadline, and Corporation Tax payment deadline all shift. Update your compliance calendar immediately.

The change takes effect as soon as Companies House processes it. Online filings are typically processed within 24 hours. Paper filings can take several working days.

The rules: extending vs shortening your financial year

This is where most founders get caught out. Shortening and extending are not treated equally.

Shortening the financial year

Shortening your financial year is relatively unrestricted:

• You can shorten by as little as one day.
• You can shorten as many times as you like — there is no "once in five years" restriction for shortening.
• You can shorten the current financial year or the immediately previous one (provided accounts for that previous period haven't already been filed).

The main risk with shortening is that it brings your filing deadline forward. If you shorten the current year-end from 31 December to 30 September, your accounts are now due 9 months after 30 September — which may be sooner than you expected. If the new deadline has already passed by the time the change is processed, late filing penalties apply immediately.

Extending the financial year

Extending is more tightly controlled:

• The financial year cannot exceed 18 months in total.
• You can only extend once every five years. If you extended your year-end in 2023, you cannot extend again until 2028.
• You can extend the current financial year or the immediately previous one (again, provided accounts for that previous period haven't been filed).

There are three exceptions to the once-in-five-years rule:

1. The company is in administration. An administrator can extend the year-end to align with the administration timetable.
2. You are aligning with a subsidiary or parent company. Group alignment is a recognised reason to extend more frequently.
3. Companies House grants special permission. This is discretionary and rare. You would need to write to Companies House explaining the exceptional circumstances.

What happens to your Corporation Tax when you change the ARD

This is the part that trips up founders who file the AA01 without thinking about the tax side. Companies House and HMRC operate on different clocks, and changing one doesn't automatically update the other.

HMRC's 12-month rule

A Corporation Tax accounting period can never exceed 12 months. If you extend your Companies House financial year to, say, 15 months, HMRC will split that into two Corporation Tax periods: one of 12 months and one of 3 months. You will need to file two CT600 returns and make two Corporation Tax payments, each with its own deadline.

This catches people by surprise. They extend the year-end thinking they're buying time on everything, then discover HMRC still expects a tax payment 9 months and 1 day after the first 12-month chunk ends.

Shortening and Corporation Tax

If you shorten your financial year, the Corporation Tax accounting period shortens too. A shorter period means a smaller CT bill (assuming consistent monthly trading), but the payment deadline arrives earlier. If you shorten from 12 months to 9 months, you owe the tax 9 months and 1 day after the new, earlier year-end.

Notifying HMRC

You must notify HMRC separately. Filing form AA01 with Companies House does not automatically update your Corporation Tax record. Most accountants handle this as part of the CT600 filing process, but if you're managing your own tax affairs, you need to contact HMRC directly or update the accounting period through your Government Gateway account. If you registered for Corporation Tax when you started business activity, as required within 3 months of starting business activity, HMRC already has your original accounting period on file — it's that record you're updating.

Impact on your accounts filing deadline

Your annual accounts filing deadline at Companies House is calculated from the ARD:

• First accounts: due 21 months from the date of incorporation, or 3 months from the ARD — whichever is longer.
• Subsequent accounts: due 9 months after the ARD.

When you change the ARD, the deadline recalculates based on the new date. But here is the critical point: if the new deadline has already passed, late filing penalties apply from the original deadline. Companies House does not grant a grace period because you moved the goalposts.

Late filing penalties for private limited companies are automatic and escalate with time:

• Up to 1 month late: £150
• 1 to 3 months late: £375
• 3 to 6 months late: £750
• More than 6 months late: £1,500

These penalties double if your accounts were also late in the previous year. For a full breakdown of annual accounts deadlines and penalties, see our guide to annual accounts deadlines.

Which period can you change?

You can change the accounting reference date for:

• The current financial year (the one you're in right now), or
• The immediately previous financial year — but only if accounts for that period have not yet been filed with Companies House.

You cannot change the ARD for any period further back than the immediately previous one. If your 2024 accounts have already been filed, you cannot retrospectively alter the 2024 year-end. The ship has sailed.

This is relevant for founders who realise, after the year-end has passed but before accounts are filed, that a different year-end would have been better. You still have a narrow window to make that change — but only until the accounts are submitted.

Filing the change: online vs paper

Online (recommended)

The quickest route is through the Companies House online service. You'll need your company number, your Companies House authentication code, and the new accounting reference date you want to set. The change is typically processed within 24 hours.

Paper form AA01

If you prefer paper (or if you're a community interest company filing the AA01c variant), download form AA01 from GOV.UK, complete it, and post it to Companies House. Paper filings take longer to process — allow several working days. There is no fee for either online or paper filing.

Common mistakes when changing the accounting reference date

These are the errors we see most often — and each one has real consequences.

1. Forgetting to notify HMRC

Filing form AA01 only updates Companies House. If you don't separately notify HMRC, your Corporation Tax accounting period stays on the old dates. You'll end up filing CT600 returns for the wrong periods, which triggers enquiries, interest charges, and administrative chaos. Your accountant should handle this, but if you're self-managing, don't assume the two systems talk to each other — they don't.

2. Extending when you've already extended in the last five years

Companies House will simply reject the filing. If you're counting on the extension to buy time for late accounts, you now have no safety valve. Check your Companies House filing history before submitting the AA01 to confirm your last extension date.

3. Shortening without recalculating the deadline

If you shorten your year-end from 31 December to 30 June, your accounts are now due by 31 March of the following year (9 months after the new ARD). If you made this change in February, you've just given yourself less than two months to prepare and file accounts. Late filing penalties are automatic — Companies House doesn't care that you shortened the period yourself.

4. Trying to change the ARD while accounts are overdue

This is a hard block. Companies House will reject the change outright. You must file the overdue accounts first, then submit the AA01 for the next period. Founders sometimes try to extend the year-end precisely because their accounts are late — but the system is designed to prevent exactly that tactic.

5. Ignoring the impact on the confirmation statement

Your confirmation statement deadline runs independently of your accounting reference date. Changing your year-end does not change when your confirmation statement is due. Founders who move their year-end sometimes mentally reset all their deadlines and miss the confirmation statement, which runs on its own 12-month cycle from incorporation (or the last statement). The confirmation statement costs £50 online, but missing it can lead to your company being struck off the register.

6. Assuming the extension pushes everything back

Extending your Companies House financial year to 15 months does not extend your Corporation Tax period to 15 months. HMRC caps each CT period at 12 months, so you end up with two periods, two returns, and two payment deadlines. If you only planned for one, you'll miss the earlier one.

Practical scenarios

Scenario 1: Aligning a March incorporation with a December year-end

Your company incorporated on 10 March 2025. The default ARD is 31 March 2026. You want a 31 December year-end to align with your accountant's busiest (and most efficient) cycle. You file form AA01 to shorten the current period, moving the ARD from 31 March 2026 to 31 December 2025. Your first accounts now cover roughly 10 months (March to December 2025) and are due 21 months from incorporation (by 10 December 2026) or 9 months from the new ARD (by 30 September 2026) — whichever is later. Every subsequent year runs January to December.

Scenario 2: Extending the first year to get a full trading picture

Your company incorporated on 1 September 2025 but didn't start trading until February 2026. The default ARD is 30 September 2026, giving you about 7 months of trading in the first accounts. You extend to 31 March 2027, creating an 18-month first period (the maximum). Accounts are due 21 months from incorporation — by 1 June 2027 — or 9 months from the new ARD (31 December 2027), whichever is later. HMRC splits the Corporation Tax into a 12-month period (September 2025 to August 2026) and a 7-month period (September 2026 to March 2027).

Scenario 3: A subsidiary aligning with its new parent

Company B (September year-end) is acquired by Company A (March year-end). Company B files form AA01 to extend its current year-end from 30 September to 31 March. Because this is a group alignment, the once-in-five-years restriction does not apply, even if Company B extended its year-end two years ago. HMRC still splits the Corporation Tax period at the 12-month mark.

How a year-end change fits your wider compliance calendar

Changing your accounting reference date is one piece of a broader compliance picture. Once the change takes effect, review these deadlines:

• Annual accounts: recalculate based on the new ARD (9 months after, or the first-accounts rule).
• CT600: identify whether HMRC will split the period, and note both deadlines.
• Corporation Tax payment: due 9 months and 1 day after each CT period ends.
• Confirmation statement: unchanged — still every 12 months from incorporation or the last statement. The online fee is £50.
• VAT returns: unchanged unless you ask HMRC to realign your VAT periods (only relevant if you're VAT-registered and want your VAT quarters to match your new year-end).

Our annual compliance checklist covers every deadline a UK limited company faces — use it to rebuild your calendar after the change.

Frequently asked questions

Is there a fee to change the accounting reference date?

No. Filing form AA01 with Companies House is free, whether you file online or on paper. There is no HMRC fee either.

Can I change my accounting reference date more than once?

You can shorten your financial year as many times as you like. You can only extend once every five years, unless an exception applies (administration, group alignment, or Companies House permission).

What is the longest a financial year can be?

18 months. Companies House will not allow a financial year to exceed this, regardless of the reason. HMRC will split any period longer than 12 months into two Corporation Tax accounting periods.

Can I change the ARD if my accounts are overdue?

No. Companies House will reject the AA01 if your annual accounts are currently overdue. You must file the outstanding accounts first, then apply for the change.

Does changing the ARD affect my confirmation statement deadline?

No. Your confirmation statement runs on its own 12-month cycle, completely independent of your accounting reference date. Don't assume that moving your year-end resets all your compliance deadlines.

Do I need a board resolution to change the accounting reference date?

The Companies Act 2006 does not require a formal resolution to change the ARD, but good governance practice — especially if you have multiple directors or shareholders — is to record the decision in board minutes. Investor agreements sometimes require shareholder approval for changes to the accounting period, so check your shareholders' agreement and Articles of Association before filing.