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Changing your company year end and Corporation Tax period

Last updated: 12 October 2026

Short version. After a year-end change at Companies House, enter the new accounts period and file the CT600s it needs: one for 12 months or less, two for a longer period up to 18 months. Change a company's accounting reference date through Companies House first. That changes the financial year for annual accounts. It can also change the Corporation Tax period, so check HMRC's dates before filing a CT600.

Go to the steps and entry details

On this page

If your annual accounts dates are correct and only the CT600 dates differ, you may not need a year-end change. Read when tax and accounts dates can differ or how to resolve incorrect HMRC dates first. Enter dates that differ for accounts and Corporation Tax

New accounts periodOne or two CT600sUp to 18 monthsWhere you see it: Company Profile & Period › Accounting period

Before entering figures, the filing setup questions can set the existing destination and CT600 route for an ordinary limited company. They keep the full accounts dates beside the CT600 dates. You can also continue with the controls below without opening the questions.

What this guide covers

After a year-end change at Companies House, enter the new accounts period and file the CT600s it needs: one for 12 months or less, two for a longer period up to 18 months.

  • You choose the changed accounts period under Accounting period, or enter it with Custom dates if it is not listed.
  • A period of 12 months or less files one CT600. A period that ends more than 12 months after it starts, even by one day, files two. How the dates are split
  • Accounts periods up to 18 months are supported, whether it is the first period or a later one.
  • Companies House and HMRC are separate. Changing the date at Companies House does not update HMRC's record, and a date entered in the wizard does not change your year end at Companies House.

Is this the right guide for you?

Use it if all of these are true

  • Companies House has accepted a change to your company's accounting reference date.
  • The changed accounts period has ended and no CT600 has been filed for it yet.
  • HMRC's Corporation Tax dates follow the new accounts period, or you have asked HMRC to update them.

Do not use it if

  • Only HMRC’s Corporation Tax dates differ from your accounts, and the year end is unchanged.Use instead: Set different CT600 dates
  • HMRC’s dates look wrong.Use instead: Wrong HMRC Corporation Tax dates
  • You filed a return for the old dates and need to correct it.Use instead: If a filed return has the wrong dates
  • The changed period is longer than 18 months.Companies House allows this only in limited cases, such as administration. The wizard does not support it.

Common mistake: entering the old year end

Larch Example Ltd lengthened its year to 30 June 2026. Entering the old 12-month dates, 1 April 2025 to 31 March 2026, files one return and leaves out the last three months. The accounts dates then do not match the period Companies House holds.

Old year entered, 1 April 2025 to 31 March 2026

Returns filed
One
Figures
12 months only, profit £24,000
Not filed
1 April 2026 to 30 June 2026
Corporation Tax at 19%
£4,560

New period entered, 1 April 2025 to 30 June 2026

Returns filed
Two
Return 1, 1 April 2025 to 31 March 2026
Profit £24,013
Return 2, 1 April 2026 to 30 June 2026
Profit £5,987
Corporation Tax for both returns at 19%
£5,700

More help: Standard return · GOV.UK: Corporation Tax accounting periods

1. Change the year end first

A company changes its financial year by changing its accounting reference date at Companies House. GOV.UK sets rules on when a date may be changed and how often a period may be extended. A year can be shortened as often as needed. It can be lengthened to at most 18 months, and usually only once every five years. You cannot change the year end if the company's accounts are already overdue.

Start with GOV.UK's change your company year end guidance. Do not enter a guessed new date in a CT600 to try to make the filings match.

2. Corporation Tax periods

The Corporation Tax accounting period is normally the same as the financial year in the annual accounts, but it cannot be longer than 12 months. HMRC may therefore split a longer accounts period into more than one Company Tax Return period.

Changing the Companies House date does not remove the need to make sure the Corporation Tax dates are correct with HMRC. GOV.UK says that after lengthening the year you must update the accounting period dates with HMRC before the original filing date of the Company Tax Return. Check the period shown in your HMRC account or notice to file before starting.

More help: GOV.UK: if your accounts cover more than 12 months

Worked example

Company
Larch Example Ltd
Previous accounts
Ended 31 March 2025
Year end change
Lengthened to 30 June, accepted by Companies House
New accounts period
1 April 2025 to 30 June 2026 (15 months)
HMRC’s Corporation Tax periods
1 April 2025 to 31 March 2026 and 1 April 2026 to 30 June 2026, after the company told HMRC. Neither CT600 filed yet
Figures for the whole period
Turnover £150,000, cost of raw materials £20,000, staff costs £60,000, other charges £40,000

The company and all figures on this page are fictional.

3. Before you file

  • Confirm the new accounting reference date has been accepted by Companies House.
  • Check the Corporation Tax period dates on the HMRC notice or online account.
  • Use those actual dates in your filing.
  • Check the resulting Companies House and HMRC deadlines, as they may have changed. Changing the year end changes the accounts deadline, unless you are lengthening the first financial year.

If you select dates offered from Companies House, they are the official accounts dates we found. If you use custom dates, confirm Companies House has accepted the changed year end and check the matching Corporation Tax periods with HMRC before continuing.

Step by step

  1. Start a new filing

    Dashboard, company card
    Click New filing. If you see Resume filing, click the small arrow beside it and choose Start new filing →
    Confirm the company
    Tick the confirmation box, then click Continue with this company →

  2. Company Profile & Period

    Accounting period
    1 April 2025 to 30 June 2026If it is not listed, choose Custom dates and enter Period start 1 April 2025 and Period end 30 June 2026. A note then confirms that two returns will be prepared. It is headed Extended accounting period.
    Special tax regimes
    No
    What are you filing?
    Companies House & HMRC CT600Or HMRC CT600 if the accounts are already at Companies House.
    Filing purpose
    Standard return
    My HMRC Corporation Tax period is different from my statutory accounts period.
    Leave untickedHMRC’s two periods follow from the accounts dates.
    Change CT600 split date
    Leave as shownThe dates show as 1 April 2025 to 31 March 2026 · 1 April 2026 to 30 June 2026. Open it only if HMRC's first period ends on a different day. Match your two CT600 dates to HMRC
    Trading status
    ActiveThe form starts on Dormant, so make sure you change it.
    How do you want to enter your figures?
    Enter my figures onceChoose Enter figures separately only if you have your own figures for each return. Separate figures
    What the company does
    Trading

    Click Next →.

  3. Financial Data

    Enter the figures for the whole 15-month accounts period. Do not split them yourself.

    Previous year
    Last year’s figures, from your previous accountsRequired before you continue. Figures from Companies House fill in where available. Check them, and enter 0 where a line had no amount.
    Turnover
    150000Box 145
    Cost of raw materials and consumables
    20000
    Staff costs
    60000
    Other charges
    40000
    Company tax type
    Standard company
    Balance sheet
    Your balance sheet at 30 June 2026
    Average number of employees
    Your average number, or 0

    If a dividend, gain or capital purchase belongs to one date, an allocation panel asks which return it belongs to. Click Next →, then Review filing.

    More help: Amounts that belong to one date

  4. Review & file

    The Corporation tax due card shows the total for both returns. For Larch Example Ltd:

    Corporation tax due
    £5,700 .00
    On taxable total profits of £30,000 at 19.0% · small profits
    Optional
    Click Preview your documents (free) to check both returns and the 15-month accounts
    Continue
    Click Unlock Filing if you have not subscribed, then Continue to submission

  5. Filing credentials

    Corporation Tax UTR
    Your 10-digit UTR
    Gateway User ID
    Your Government Gateway user ID
    Gateway Password
    Your Government Gateway password
    Authentication code
    Your Companies House authentication codeWhere to find it
    Select the director submitting this return
    The director’s name
    Accounts approval date
    The date the directors approved the accountsWhat to enter

    Tick the declaration and click Submit to HMRC & Companies House.

  6. After you submit

    Under Gateway submissions, the filing page lists both CT600s. Each shows Accepted once HMRC confirms it. The Companies House accounts filing has its own status on the same page. HMRC usually replies within a few minutes, but it can take longer.

    Each return has its own Corporation Tax payment deadline. The Review & file page shows the dates.

    More help: Deadlines for two returns · Paying your Corporation Tax

Sample outcome

Two new CT600s1 April 2025 to 31 March 2026 and 1 April 2026 to 30 June 2026
£5,700£4,562.47 on return 1 and £1,137.53 on return 2, both at 19%
One set of accounts1 April 2025 to 30 June 2026, filed at Companies House and attached to each CT600

4. If the accounts period is longer than 12 months

One set of annual accounts may cover the full period, but HMRC needs a CT600 for each Corporation Tax period. SimpleCompanyTax supports a Companies House accounts period over 12 months and up to 18 months, whether it is the first period or a later period created by changing the company year end. Enter the full accounts period once. We prepare one set of Companies House accounts and the two Corporation Tax returns HMRC requires for the full period.

With Enter my figures once, recurring income and expenses are shared between the two returns by the number of days in each. Amounts that belong to one date follow the split you confirm. If HMRC's first period ends on a different day from the default, set First CT600 ends to match it.

For the dormant case, read dormant at first, then trading, which explains how to give us the later Corporation Tax start date. That option is not limited to a first year, so it covers a period created by changing your year end.

5. If you shortened the year

A shortened accounts period of 12 months or less files one CT600 for the same dates. GOV.UK says that when you file with software, you enter the new accounting period dates before you file, which is what choosing the shortened period under Accounting period does. The profit limits for a return shorter than 12 months are reduced in proportion to its days, so the same profit can be taxed at a higher rate than in a full year. GOV.UK: Corporation Tax rates

Not the right guide?

Open the guide that matches your situation.

Standard returnFiles every CT600 your accounts period needs.Set different CT600 datesHMRC’s dates differ but the year end is unchanged.Wrong HMRC Corporation Tax datesCorrect HMRC’s record before you file.

Looking for a different field or step? See all filing wizard guides →

Official sources

Whether this route matches your filings is for you to decide, yourself or with your accountant.

Example companies and figures are fictional. Field names match the filing wizard as of 12 October 2026.