Changing a UK subsidiary's year end to match the parent

Companies House sets a UK subsidiary's first year end automatically, on the day it is incorporated: the last day of the month in which the first anniversary of incorporation falls. A company incorporated on 15 March 2026 gets 31 March 2027 as its first year end, whatever the parent's own financial calendar runs to. For a subsidiary of a group with a 31 December or 30 June year end, that default needs changing, and it is a filing most new subsidiaries need to make inside the first year.

Why group reporting wants it aligned

A parent consolidating a subsidiary's results into its own group accounts needs figures drawn up to the same date, or close to it. Where the dates differ, the parent's auditors have to work with management accounts for the gap period, or restate the subsidiary's figures onto the group's date, every single reporting cycle, for as long as the mismatch lasts. The management accounts and group reporting service exists partly because of this gap, and the cheaper fix is usually to close it at the source: change the subsidiary's accounting reference date once, early, and the recurring reconciliation stops.

The rule for shortening

A company can shorten its accounting reference period as many times as it likes, by any amount, down to a minimum of one day. There is no five-year limit on shortening and no cap on how often it is done. A subsidiary with a parent whose year end falls earlier in the calendar than the Companies House default can shorten its first period to land on the parent's date, with no exception to claim and no restriction to check.

The rule for lengthening

Lengthening is more restricted, because repeatedly extending a year end is also a way of delaying when accounts become due. A company can lengthen its accounting reference period, but not past 18 months in total, and not more often than once every five years, counted from the end of the last accounting reference period that was itself extended.

The exception for aligning with a parent or subsidiary

Three situations are exceptions to the five-year limit on lengthening: the company is in administration; the Secretary of State directs otherwise; or the company is a subsidiary or parent undertaking aligning its accounting reference date with that of another undertaking in the same group. That third exception is written into company law for exactly this situation: a subsidiary brought into a group after its own year end was already set, needing to move onto the parent's date even though five years have not passed since any previous change. The 18-month ceiling still applies regardless of which exception is used.

Corporation tax splits into two returns for a long period

A Corporation Tax accounting period cannot exceed 12 months, whatever the statutory accounts cover. Where lengthening the accounting reference period produces a set of accounts covering, for example, 15 months, HMRC still needs two separate Corporation Tax returns for that stretch: one covering the first 12 months, and a second covering the remaining 3. Each has its own profit calculation, its own payment date nine months and one day after it ends, and its own filing deadline twelve months after it ends. The accounts are prepared once, for the full period; the tax return is filed twice. A group planning a lengthening to align year ends should expect this and budget the extra compliance work before the first filing deadline arrives.

What stops the change going through

Companies House will not accept a change to the accounting reference date while the company's accounts are overdue. A subsidiary that has fallen behind on filing needs to catch up before it can realign its year end, which is one more reason to make the change early, while the company is still filing on time.

How to make the change

The change is filed with Companies House, either through its online service or on paper using form AA01, and it can be made for the current accounting reference period or the one immediately before it. The notice has to say plainly whether the period is being shortened or lengthened, and to what new date. Once accepted, the new date becomes the ongoing year end for every future period, so get the target date right first time: the parent's own exact year end.

A worked example

A Canadian manufacturing group, financial year ending 31 December, incorporates a UK subsidiary on 10 June 2026. Companies House sets the first accounting reference date at 30 June 2027, a full year away and six months out of step with the parent. The group wants the UK company reporting on the same calendar as every other subsidiary in the consolidation.

Shortening the first period to 31 December 2026 would give the UK company a first accounting period of under seven months, which is allowed with no exception needed, and the company reports on the parent's date from its very first set of accounts. The alternative, waiting and lengthening the following period instead, would require invoking the group-alignment exception and would still leave the company reporting on the wrong date for a further eighteen months in the meantime. Filed early, the shortening route is both simpler and faster. The subsidiary files its first set of accounts for a six-and-a-half-month period, and every year after that runs 1 January to 31 December alongside the parent.

What to do this week

1. Find the parent's financial year end, and compare it against the UK subsidiary's Companies House-assigned accounting reference date.
2. If the subsidiary's default date falls after the parent's, work out whether shortening the current period reaches the parent's date. If it does, file the shortening notice: no exception to claim, no five-year count to check.
3. If the subsidiary's default date falls before the parent's and a lengthening is needed, confirm the move stays within 18 months total, and record that the group-alignment exception applies, since it is what permits the change outside the usual five-year gap.
4. Check the accounts are not overdue before filing the change. If they are, file the accounts first.
5. Once the new date is set, tell whoever runs the company's bookkeeping and tell HMRC, since a lengthened period splits into two separate Corporation Tax returns.

Making this change inside the first year, before the first confirmation statement and the first set of accounts are due, avoids doing the group's consolidation workarounds even once. The step-by-step incorporation guide covers where this sits against the other post-incorporation filings, and the accounts and audit guide covers what happens to the filing deadlines once the date is set.