Changing your company year end (AA01): what happens to tax

In short: You change a company's year end, its accounting reference date, by filing form AA01 with Companies House before the accounts deadline for that period (GOV.UK: Change your company's year end). You can shorten as often as you like, or extend up to 18 months once every 5 years. A period over 12 months needs two Company Tax Returns (CT600s).

Check the new dates with the Corporation Tax deadline calculator, or take the 30-second check to see whether Taxley fits your company.

Key facts (checked on 28 September 2026)

How do you change your company year end at Companies House?

You change a company's year end by giving Companies House notice on form AA01, either through its online service or on the paper form, choosing the new accounting reference date and saying whether the current or previous period is shortened or extended (GOV.UK: Change your company's year end). Only those two periods can be changed.

Section 392 of the Companies Act 2006 sets the limits on an AA01 notice (legislation.gov.uk: CA 2006 s.392). A company can shorten its current or previous accounting reference period as often as it likes. It can extend a period to no more than 18 months, unless it is in administration. An extension has no effect if it comes less than 5 years after the end of an earlier extended period, except where the company is aligning its date with a UK parent or subsidiary undertaking, is in administration, or has a direction from the Secretary of State. A notice for the previous period cannot be given once that period's accounts deadline has passed, and GOV.UK adds that a company cannot change its year end while its accounts are overdue (GOV.UK: Change your company's year end).

Changing a company's year end also changes its Companies House accounts deadline, unless the change lengthens the company's first financial year, and any extra time Companies House granted for that year no longer applies (GOV.UK: Change your company's year end). If the new deadline has already passed when the company changes its year end, the company pays a late filing penalty. The rules will tighten for shortening too. From 1 April 2028, a company that shortens its accounting reference period more than once in 5 years will need to give a business reason, subject to forthcoming regulations (Companies House guidance).

Does changing your year end change Corporation Tax?

Yes. Changing the year end normally changes the company's Corporation Tax accounting period, and GOV.UK says you must update the dates with HMRC (GOV.UK: Change your company's year end). An accounting period cannot exceed 12 months, so accounts longer than 12 months need two CT600s: one for the first 12 months and one for the rest.

The 12-month cap on a Corporation Tax accounting period comes from section 10 of the Corporation Tax Act 2009, which ends an accounting period at the earliest of 12 months from its start, the company's accounting date and events such as ceasing to trade (legislation.gov.uk: CTA 2009 s.10). HMRC's manual says that when a company makes up accounts for longer than 12 months, the first 12 months will normally be one accounting period (CTM01510). The remainder forms a second accounting period ending on the new year end. Each accounting period gets its own return, computation, payment reference and payment date, while one set of statutory accounts covers both. The guide to first company accounts and two CT600 returns shows the same two-return pattern for a first period.

Profits of a long period of account are split between the two Corporation Tax accounting periods item by item, not with one blanket percentage. Trading profit, after tax adjustments but before capital allowances, is apportioned by the number of days in each accounting period under section 52 of the Corporation Tax Act 2009 (legislation.gov.uk: CTA 2009 s.52). HMRC's manual allows a transactions basis only where it gives a more accurate result, typically a lumpy trade such as property development (CTM01405). An accountant is worth paying for when a company wants to argue for that basis, or when large sales or purchases sit close to the boundary between the periods, because the choice moves profit between two tax bills.

Capital allowances and chargeable gains are not split by days when a long period of account becomes two Corporation Tax accounting periods. For a company, the chargeable period for capital allowances is the accounting period, so allowances are worked out separately for each accounting period, using only the spending and disposals in that period (CA11510). A chargeable gain falls wholly in the accounting period in which the disposal happens, and for a sale under an unconditional contract that is the date the contract is made, not the completion date; a conditional contract counts when the condition is met (TCGA 1992 s28). An unconditional property sale exchanged in March and completed in May therefore belongs to the period containing March. The chargeable gains guide covers working out the gain itself.

Payment dates and filing dates work differently when one set of accounts produces two CT600s. Each accounting period has its own payment date, 9 months and 1 day after that period ends, for a company with taxable profits of up to £1.5 million (GOV.UK: Pay your Corporation Tax bill: Overview). Filing is shared: paragraph 14 of Schedule 18 to the Finance Act 1998 makes a return due on the latest of 12 months after its own period ends and, where the accounts cover no more than 18 months, 12 months after the accounts period ends (legislation.gov.uk: FA 1998 Sch 18 para 14). Both CT600s for a period of account longer than 12 months but no longer than 18 therefore fall due on the same day, 12 months after the new year end, while the first period's tax is payable months earlier.

What are the 2026 dates when a 31 March year end is extended to 30 June?

Taxley's worked example: a company extends its 31 March 2026 year end to 30 June 2026, giving 15-month accounts. It files two CT600s, for 1 April 2025 to 31 March 2026 and 1 April to 30 June 2026, pays their tax by 1 January and 1 April 2027, and files both by 30 June 2027 (GOV.UK: Accounting periods for Corporation Tax).

In Taxley's first example, the extension moves the Companies House accounts deadline from 31 December 2026 to 31 March 2027, 9 months after the new 30 June accounting reference date, because the period is not the company's first (GOV.UK: Change your company's year end). The first Corporation Tax payment does not move: the first 12 months still end on 31 March 2026, so their tax is due on 1 January 2027. That date is a bank holiday, so the payment has to reach HMRC by Thursday 31 December 2026 unless it goes by Faster Payments or card (GOV.UK: Pay your Corporation Tax bill: Overview). The company therefore pays the first period's tax three months before the 15-month accounts are due, and needs a reliable estimate of the split by December 2026.

The profit split in Taxley's first example uses round numbers to show the method. Suppose the 456-day accounts show an adjusted trading profit, before capital allowances, of £91,200. Split by days, the 365-day first period takes £73,000 and the 91-day second period takes £18,200 (CTM01405). A van bought in May 2026 would go only into the second period's capital allowances. The second period is short, so its marginal relief limits fall in proportion: by days, the £50,000 lower limit becomes about £12,466 and the £250,000 upper limit about £62,329 (GOV.UK: Marginal Relief for Corporation Tax). With no allowances to deduct, taxable profits of £18,200 in those 91 days sit inside the marginal relief band, where the same profits over a full year would be taxed at the 19% small profits rate. The first period, a full 12 months, keeps the full limits.

What happens when you shorten a 31 December year end to 30 September?

Taxley's second worked example: a company shortens its period starting 1 January 2026 to end on 30 September 2026. The single 9-month return is due by 30 September 2027, the tax by 1 July 2027 and the accounts at Companies House by 30 June 2027, three months earlier than under the old 31 December year end (GOV.UK: Accounting periods for Corporation Tax).

After a company shortens its year end, the Companies House deadline is the later of 9 months after the new accounting reference date and 3 months from the date of the AA01 notice, under section 442(4) of the Companies Act 2006 (legislation.gov.uk: CA 2006 s.442). In Taxley's second example the company files AA01 in October 2026, so 3 months from the notice ends in January 2027 and the 9-month date, 30 June 2027, applies. The next accounting period runs from 1 October 2026 to 30 September 2027. GOV.UK says a shortened Corporation Tax accounting period normally ends on the same day as the accounts, and a company filing with software enters the new dates before it files (GOV.UK: Accounting periods for Corporation Tax).

Shortening a year end shrinks the Corporation Tax thresholds in proportion. Section 18D of the Corporation Tax Act 2010 reduces the £50,000 lower limit and the £250,000 upper limit proportionately for an accounting period of less than 12 months (legislation.gov.uk: CTA 2010 s.18D). HMRC's manual uses the same dates, 1 January to 30 September, in 2024, and shows £37,500 and £187,500 on a 9/12 basis, noting that the strict calculation uses days (CTM03930). By days, 273 of 365, the limits are about £37,397 and £186,986. A 9-month taxable profit of £45,000 is above the reduced lower limit, so it pays the 25% main rate less marginal relief, where £45,000 over a full year would pay 19%. The Corporation Tax rates and marginal relief guide shows the formula.

Capital allowances shrink with a short accounting period too. GOV.UK's own example is a 9-month accounting period, whose maximum annual investment allowance is 9/12 × £1,000,000 = £750,000, claimable only in the period the item is bought (GOV.UK: Claim capital allowances: Annual investment allowance). Writing down allowances are reduced in proportion as well. The 1 January to 30 September 2026 period spans 1 April 2026, when the main pool rate for Corporation Tax fell from 18% to 14%, so the company works out a hybrid rate for that period (CA23220). A large purchase made in October 2026 instead of September 2026 moves its allowance into the next accounting period, which starts on 1 October 2026.

How do you change your year end and file the tax returns, step by step?

Seven steps take a company from choosing a new year end to filed returns: check the change is allowed, file AA01, tell HMRC, map the accounting periods, split the figures, file and pay, then check both records (GOV.UK: Change your company's year end). Allow about 2 hours of your own time, not counting preparing the accounts.

Step 1: Check the change is allowed

Confirm which period you are changing: only the current accounting reference period or the one immediately before it can move, and not once its accounts are overdue. For an extension, check that the new period lasts no more than 18 months and that no earlier period has been extended in the last 5 years, unless an exception applies. For a first period, count the date of incorporation as day one, because Companies House warns that simply adding months can overshoot 18 months (Companies House guidance).

Step 2: File form AA01 before the accounts deadline

Sign in to Companies House WebFiling, which needs GOV.UK One Login, and enter the company's authentication code, or complete paper form AA01, and give the new accounting reference date. The online service may also ask you to verify your identity through GOV.UK One Login first (GOV.UK: Tell Companies House about changes to your limited company). File before the accounts deadline for the period you are changing; in Taxley's first example, before 31 December 2026 ends. Then note the new accounts deadline shown on the company's record.

Step 3: Tell HMRC the new accounting period dates

GOV.UK says you must update your accounting period dates with HMRC after shortening or lengthening the financial year. For an extension, contact HMRC before the original filing date of the Company Tax Return; for the period to 31 March 2026 in Taxley's first example, that means before 31 March 2027. For a shortened period filed with software, enter the new dates in the software before filing (GOV.UK: Accounting periods for Corporation Tax).

Step 4: Map the Corporation Tax accounting periods and dates

Cut the accounts period into Corporation Tax accounting periods of no more than 12 months: the first 12 months, then the remainder. Against each period, note the payment date, 9 months and 1 day after that period ends (GOV.UK: Pay your Corporation Tax bill: Overview), and the filing date, 12 months after the accounts period ends when the accounts cover 18 months or less. The Corporation Tax deadline calculator lists each period with its dates when you enter the start and end of the accounts.

Step 5: Split the figures between the returns

Split the adjusted trading profit before capital allowances by days between the accounting periods, unless a transactions basis clearly gives a more accurate result (CTM01405). Work out capital allowances separately for each period, using only that period's spending and disposals and the reduced limits of a short period. Put each chargeable gain in the period in which the disposal contract was made. Reconcile both returns back to the one set of accounts.

Step 6: File the accounts, both returns and both payments

File the one set of accounts at Companies House by the new deadline. File each CT600 and tick box 50, which HMRC's guide says to mark when you make more than one return for the company at the same time (GOV.UK: Completing your Company Tax Return). Pay each period's tax with that period's own 17-character payment reference, and pay the first period's tax on its original date even while the accounts are unfinished.

Step 7: Check the new dates at Companies House and HMRC

Check that the Companies House record shows the new accounting reference date and next accounts due date, and that the company's HMRC online account lists each accounting period with the dates you expect. Confirm each CT600 has an HMRC acknowledgement and each payment sits against its own period. Keep the AA01 confirmation, the split workings and both submission receipts with the company's records.

Can Taxley file the returns after a year-end change?

Partly. Taxley (taxley.co.uk), UK online software that prepares and files the Company Tax Return (CT600) with HMRC and the annual accounts with Companies House, splits accounts longer than 12 months into two accounting periods and prepares both CT600s. It does not file form AA01.

In Taxley, a period of account longer than 12 months becomes two Corporation Tax accounting periods automatically, each with its own payment date. Taxley splits income and expenses by days and has a grid for figures that belong to one period, such as capital allowances, chargeable gains and interest. It sends both returns to HMRC together with box 50 ticked, and applies the reduced marginal relief limits of a short period (GOV.UK: Marginal Relief for Corporation Tax). Once the return is paid and the accounts are finished, Taxley files micro-entity, small, abridged or dormant accounts at Companies House at no extra cost. Change the year end on the Companies House service first, then enter the new period in Taxley. Take the 30-second check to see whether Taxley fits your company, or read what Taxley covers.

Frequently asked questions

Can you change a company year end after the accounts deadline?

No. GOV.UK says a company cannot change its year end while its accounts are overdue, and a notice for the previous period cannot be given after its filing deadline has passed (GOV.UK: Change your company's year end). File the overdue accounts first, then change the current period if you still need to.

Do you need two sets of accounts for a 15-month period?

No. One set of statutory accounts covers the whole 15 months and goes to Companies House once. HMRC gets two CT600s, one for the first 12 months and one for the last 3 months, both based on the same accounts (GOV.UK: Accounting periods for Corporation Tax).

Does shortening the year end mean paying Corporation Tax sooner?

Yes. Corporation Tax is due 9 months and 1 day after the accounting period ends, so an earlier year end brings the payment forward. In Taxley's second example, moving from 31 December to 30 September 2026 moves the payment from 1 October 2027 to 1 July 2027 (GOV.UK: Pay your Corporation Tax bill: Overview).

Can a company extend its year end twice in five years?

Only in three cases: the company is in administration, it is aligning its date with a UK parent or subsidiary undertaking, or the Secretary of State directs that the 5-year rule does not apply (CA 2006 s392). Otherwise a second extension within 5 years has no effect.

Will shortening a year end be restricted from 2028?

Yes. From 1 April 2028, a company will need a business reason to shorten its accounting reference period more than once in 5 years, subject to forthcoming regulations (Companies House guidance). Until then, a company can shorten as often, and by as many months, as it likes.

General information, not personalised tax or accounting advice.