United Kingdom / Guides / The UK company year: ARD, periods and the tax year
United Kingdom · guideThe UK company year: ARD, periods and the tax year
The short answer
A UK company's financial year is fixed by its accounting reference date (ARD) — a date Companies House sets automatically on incorporation, not one directors choose. The ARD can be shortened at any time but, with limited exceptions, can only be extended once in any five-year period and by no more than 18 months in total. It has no connection to the 6 April personal tax year, which stays fixed for every individual regardless of when any company they run or hold shares in ends its own financial year, so a company's accounting calendar and the tax calendar its directors and shareholders file under can sit many months apart.
Key facts — verified dates on each
Two calendars, set by two different rules
A UK limited company runs on its own financial year, separate from the personal tax year that applies to every individual in the UK. The two are easy to conflate because both eventually produce a tax return, but they are fixed by different mechanisms, administered by different bodies, with no requirement for them to line up.
The company's financial year is anchored to its accounting reference date (ARD), a concept defined in the Companies Act 2006 and administered by Companies House. The personal tax year is fixed by statute at 6 April to 5 April and does not move for any individual, whatever their company's ARD happens to be. Understanding the ARD on its own terms — how it is set, how it can change, and what it drives — is the part that trips people up.
How the accounting reference date is set
A newly incorporated company does not choose an ARD at registration. Companies House sets it automatically, and the default rule is fixed by the Companies Act 2006: the ARD is the last day of the month in which the first anniversary of incorporation falls.
For example, a company incorporated on 14 June 2025 has its first anniversary on 14 June 2026, so its ARD defaults to 30 June — and 30 June every year after that, unless the company later changes it. Nothing needs to be filed to get this default date; it applies automatically from incorporation, and it only changes if the company actively files a change with Companies House.
The first accounting period is deliberately longer than the rest
The first accounting period is not the same length as every period after it. Under the Companies Act 2006, a company's first accounting period must be more than six months and no more than 18 months, running from the date of incorporation to the ARD. Every accounting period after the first runs exactly 12 months, from the day after the previous one ended to the next ARD.
In the example above, the first period runs from 14 June 2025 to 30 June 2026 — just over 12 months — because the default ARD calculation always produces a first period slightly longer than a year unless the company shortens it. This is a different concept from the deadline to file the resulting accounts, and the two get conflated often enough to be worth separating: the period length is how much trading activity the accounts cover; the filing deadline is how long the company then has to prepare and deliver those accounts once the period has ended — 21 months from incorporation for a first set of accounts, 9 months from the ARD for every set after that.
Changing the ARD: what the rules allow
A company can shorten its financial year at any time, as often as it wants, by as little as one day. Lengthening is far more restricted: a company can generally extend its financial year only once in any five-year period, and the extension cannot take the accounting period beyond 18 months measured from the start of that period.
Exceptions to the once-per-five-years limit exist for a company in administration, a company aligning its ARD with a parent or subsidiary undertaking, and cases where Companies House grants permission. Outside those exceptions, a second extension inside the same five-year window is not accepted.
A company cannot change its ARD while its accounts are already overdue, and changing the ARD moves the filing deadline with it — except when lengthening the very first financial year, where the original 21-month first-accounts deadline is unaffected. The change is filed on form AA01, and the resulting deadline needs to be checked directly rather than assumed from the old schedule.
Why the company year and the 6 April tax year never had to match
The personal tax year HM Revenue and Customs uses for Self Assessment runs 6 April to 5 April every year, fixed for every individual taxpayer, with no relationship to any company's ARD — a director or shareholder's Self Assessment obligations sit on the 6 April cycle regardless of when a company they are involved with ends its own financial year. The date is unusual among major economies and is commonly traced to the historical shift from the Julian to the Gregorian calendar in 1752, but the mechanism matters more than the history: it is a fixed statutory date for individuals, untouched by a company's own accounting choices.
A one-person company is the clearest illustration. The director can set or change the company's ARD to whatever date suits the business, and separately, as an individual, still files Self Assessment — if they need to — against the fixed 6 April to 5 April year, reporting salary, dividends, or other personal income drawn from the company during that calendar window, independent of where the company's own financial year happens to fall.
What each date actually drives
The ARD is the anchor for the company's Companies House obligations: it sets the accounting period the annual accounts must cover, and it sets the 9-month filing deadline for every set of accounts after the first (21 months from incorporation applies to the first set specifically). The ARD also normally sets the company's Corporation Tax accounting period, because HMRC's accounting period is usually the same as the period covered by the statutory accounts — with one structural limit: a Corporation Tax accounting period cannot exceed 12 months, so a financial year that runs longer than 12 months (as a first period can) is automatically split into two Corporation Tax accounting periods, each requiring its own Company Tax Return.
From there, HMRC's own deadlines run off the end of the Corporation Tax accounting period, not the ARD directly: the Company Tax Return (CT600) is due 12 months after the period ends, and any Corporation Tax owed is due 9 months and 1 day after the period ends for companies with taxable profits up to £1.5 million — a shorter window than the return itself, so payment usually falls due before the return is filed. Companies House and HMRC are separate regulators running related but independently calculated dates, so a change to the ARD needs checking against both filing calendars, not just one.
The 6 April personal tax year sits outside all of this. It governs an individual's own Self Assessment return and any personal tax payment on income from the company, and it does not move, split, or reset because of anything the company does with its ARD.
CapEasy serves accounting and compliance clients in the United States and Australia today. UK company services, including Companies House and Corporation Tax filings, are under consideration but not currently offered; this guide is provided for orientation only.
The figures, and when we checked them
These numbers change by year or by notification. Each one shows the date we last verified it against the source — if that date looks old, check the source before relying on it.
Questions on this
What is a company's accounting reference date?
The accounting reference date (ARD) is the date Companies House treats as the end of a UK company's financial year. It is set automatically on incorporation and can later be changed by the company within Companies House rules.
How is the ARD set when a company is first incorporated?
Companies House sets it automatically as the last day of the month in which the first anniversary of incorporation falls. No filing is needed to get this default date — it applies from incorporation unless the company later changes it.
Why is a company's first accounting period often longer than 12 months?
Because the default ARD is tied to the month of the incorporation anniversary rather than the exact incorporation date, the first accounting period usually runs slightly over 12 months. The Companies Act sets the outer limits for a first period at more than 6 months and no more than 18 months.
Can a first accounting period be shortened instead?
Yes. A company can shorten its financial year, including its first one, at any time and by any amount down to a single day. There is no limit on how often a financial year can be shortened.
How often can a company extend its financial year?
Generally only once in any five-year period, and the extension cannot take the accounting period beyond 18 months measured from its start. Exceptions apply for companies in administration, companies aligning their ARD with a parent or subsidiary, or where Companies House grants specific permission.
Can the ARD be changed while the company's accounts are already overdue?
No. Companies House does not allow a change to the accounting reference date while the current accounts are overdue.
Does changing the ARD change the accounts filing deadline?
Yes, in almost all cases — the filing deadline is calculated from the ARD, so shortening or extending the financial year moves the deadline with it. The one exception is lengthening a company's very first financial year, where the original 21-month first-accounts deadline still applies.
Why doesn't the UK company financial year follow the 6 April personal tax year?
They are governed by entirely separate rules. The personal tax year is a fixed statutory date — 6 April to 5 April — that applies to every individual and does not move. A company's ARD is set independently on incorporation and can be changed by the company; there is no requirement for the two to align.
Does the ARD also set the Corporation Tax deadline?
It usually sets the Corporation Tax accounting period, since that period normally matches the period covered by the statutory accounts. But the Corporation Tax filing and payment deadlines are then calculated from the end of that accounting period, not from the ARD directly, and a period longer than 12 months is split into two Corporation Tax accounting periods.
Who works out what a company owes HMRC and when?
Corporation Tax accounting periods, payment amounts, and filing deadlines are calculated and confirmed with HMRC by the company's appointed accountant or tax adviser, working from the company's actual financial records — this guide describes the mechanism, not a specific company's figures.
Primary sources
- Companies House — Change your company's year end
- Companies Act 2006, section 391 (accounting reference periods and dates)
- Companies House — Prepare annual accounts for a private limited company
- HMRC — Corporation Tax accounting periods
- HMRC — Self Assessment tax returns: deadlines
- HMRC — Pay your Corporation Tax bill
- HMRC — Company Tax Returns
- Companies House — Change your company accounting reference date (AA01)
Last reviewed 2026-08-14. Statutes and schedules change — the sources above are authoritative, this page is orientation.
This page is information, not an offer of services. CapEasy serves the US and Australia today; UK services are under consideration.