UK Directors: Change Accounting Reference Date, Avoid 2 Tax Returns
Yes, you can change your company’s accounting reference date for the current or immediately previous financial year by filing form AA01 with Companies House. Shortening a period is straightforward and can happen at almost any time; extending one is restricted, generally to once every five years, and cannot push a period past 18 months except in specific cases. The moment you file, your Companies House filing deadline shifts, but HMRC’s records do not update automatically, so you still need to notify them separately.
TL;DR:
Shortening an accounting period can be done at any time with no restrictions and triggers an immediate update of the Companies House deadline.
Extending a period is limited to once every five years and can never surpass 18 months, except in specific cases like group alignment or administration.
Changing the ARD resets Companies House filing deadlines but does not automatically update HMRC, requiring separate notification to avoid tax period mismatches.
Filing online through WebFiling provides instant confirmation and reduces risk of missing a deadline, especially for urgent or last-minute changes.
Directors should carefully check account status, previous extensions, and group arrangements before submitting AA01, as mistakes can lead to complex tax and compliance issues.
Table of Contents
How does changing your ARD affect Companies House and Corporation Tax deadlines?
How do you complete and file AA01 without missing a deadline?
What mistakes do directors make when changing their year end?
When should you handle this yourself, and when should you call in an accountant?
When you can, and can’t, change an accounting reference date
How do you change your accounting reference date?
The AA01 form is the only mechanism for changing your company’s accounting reference date (ARD), and it only covers the current accounting period or the one immediately before it. You cannot use it to rewrite history further back than that.
You’ve got three ways to file it:
Companies House WebFiling — the fastest route, with confirmation typically arriving within hours.
Third-party filing software — many accounting packages submit AA01 directly, useful if your accountant already manages your compliance.
Paper form by post — still accepted, but Companies House has been steadily narrowing paper options for small companies, and postal delays can eat into a deadline you’re trying to protect.
On the form you’ll state your new ARD, confirm whether you’re shortening or extending, and declare whether you’ve extended within the last five years. Extensions involving group alignment or administration need supporting detail. You can download the AA01 PDF directly from Companies House, and WebFiling gives you an on-screen and emailed confirmation of receipt, which you should keep.
What are the legal limits on shortening or extending?
Shortening and extending are treated very differently under company law, and mixing up the rules is where most directors trip up.
Shortening has almost no restriction. You can bring your year end forward as many times as you like, and you can do it with immediate effect, provided your accounts for that period aren’t already overdue.
Extending is deliberately constrained:
You generally cannot extend more than once in five years, unless the company is in administration, has Secretary of State approval, or is aligning its ARD with a parent or subsidiary undertaking.
An accounting reference period can never exceed 18 months from its start date, regardless of the reason for the extension.
If you’re aligning with a group company, the previous filing period must not already have expired, or the notice becomes ineffective under the statutory rules.
These limits trace back to sections 224 and 225 of the Companies Act, which set out how accounting reference periods are calculated and when a change notice takes effect. The registrar’s own AA01 guidance mirrors this closely, with tick boxes on the form itself for each exception.
How does changing your ARD affect Companies House and Corporation Tax deadlines?
Moving your year end doesn’t just shift a date on a form. It resets the clock on every filing obligation tied to that date, and the two authorities involved don’t talk to each other automatically.
Companies House deadline moves immediately. Private companies have nine months from the new ARD to file accounts. If you’ve shortened the period, the actual deadline becomes whichever is later: nine months from the new ARD, or three months from the date Companies House receives your AA01. That second rule catches people out when they shorten a year-end that’s close to expiring.
HMRC’s Corporation Tax accounting period does not change on its own. You must notify HMRC directly before your original filing deadline, or your Corporation Tax period will keep running against the old dates.
A change can split your tax period in two. Corporation Tax accounting periods can’t exceed 12 months, so lengthening your year-end past that mark forces two separate Company Tax Returns covering the split periods.
Any extension you were granted previously is wiped out. If your company had an earlier extension on record, changing the ARD again voids it, sometimes leaving you with far less time to prepare accounts than you expected.
How do you complete and file AA01 without missing a deadline?
Getting the sequence right matters more than getting the form filled in quickly.
Check your current ARD and account status first. Look up your incorporation anniversary and confirm your last set of accounts isn’t overdue. Companies House will not let you change an ARD for a period where accounts are already late.
Decide shorten or extend, then run the checks. Work out whether you’re within the five-year extension limit and whether the new period stays under 18 months. Calculate your resulting Companies House deadline before you file anything.
Complete AA01 and file online where possible. WebFiling or filing software processes fastest and gives you an instant confirmation, which matters if you’re working close to a deadline.
Update your accounting software and tell HMRC. Change the period dates in your bookkeeping system and your HMRC business tax account before you submit any Company Tax Return covering the new period. This dedicated software setup guide walks through getting HMRC-ready records aligned properly.
Gather evidence for exceptions before you file. If you’re extending under an exception, get the supporting paperwork, whether that’s Secretary of State approval or group accounts showing alignment, ready before you submit.
Pro Tip: File the AA01 online even if your accountant normally handles paper work for you. The instant confirmation receipt is the single easiest way to prove you met a deadline if Companies House ever queries the timing.
What mistakes do directors make when changing their year end?
The same handful of errors show up again and again in Companies House’s own guidance on accounting reference periods, and every one of them is avoidable with a five-minute check beforehand.
Filing AA01 while accounts for the current period are already overdue, which the registrar simply won’t process.
Assuming the change synchronises HMRC automatically. It doesn’t, and HMRC will keep chasing the old dates until you tell them otherwise.
Not checking whether a previous extension exists. Changing the ARD again cancels it, and you may end up with far less runway than you planned for.
Filing by post close to a deadline, when Companies House’s move toward digital-only filing means online really is the safer default now.
Pro Tip: Before you file, pull your accounting software’s period settings and your HMRC business tax account side by side. If the dates don’t match after you submit AA01, that’s your sign to update HMRC before you go anywhere near your next Company Tax Return.
When should you handle this yourself, and when should you call in an accountant?
A straightforward shortening, done with plenty of lead time and no prior extension on record, is well within reach for most directors to file themselves. Where it gets genuinely risky is anywhere involving an extension, group alignment, or a split Corporation Tax period, because one wrong date can trigger two tax returns instead of one or void an exception you were relying on. That’s the point where an accountant earns their fee: checking the AA01 fields against the five-year and 18-month rules, liaising directly with HMRC on the split-period returns, and making sure your accounts are iXBRL-ready before either deadline lands.
Change your year end without the guesswork of chasing HMRC
There are practical alternatives to muddling through AA01 yourself or paying agency-scale fees for a job that often takes one focused conversation. We handle accounts preparation, Companies House filings, Corporation Tax reconciliation, and the HMRC liaison that catches split-period tax returns before they become a penalty problem.
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You can bring us in for a one-off AA01 filing and deadline check, or fold it into an ongoing engagement. Our Flexible Accounting Services That Grow With You start from moderate monthly fees, scaling with your business rather than locking you into a fixed package you’ll outgrow. If you’re mid-way through a shortened period and need your statutory accounts prepared to a new tighter deadline, that’s exactly the kind of work we take on daily. Visit the KeystoneFA site to book a consultation and get a fixed-fee quote for your AA01 filing.
Sources
For the rules in full, go straight to the source rather than a summary of it. Companies House publishes the AA01 guidance and downloadable form, alongside its change your company’s year end page. HMRC covers the tax side in its accounting periods for Corporation Tax guidance, and the statutory basis sits in sections 224 to 225 of the Companies Act. These pages are the definitive references if you’re ever unsure how a rule applies to your specific filing.
This article is general information, not a substitute for advice from a qualified financial advisor. Consult a qualified financial professional about your own circumstances before acting on anything here.
FAQ
What is the seven day rule for accounting reference dates?
There isn’t a formal “seven day rule” in the Companies Act itself. What directors usually mean is the short window Companies House allows around processing an AA01 near a deadline, so file well before your existing deadline rather than relying on same day turnaround.
How can I change my company’s accounting year end date?
File form AA01 with Companies House, online through WebFiling for the fastest result, stating your new date and whether you’re shortening or extending. You can only do this for the current accounting period or the one immediately before it, and not if accounts for that period are already overdue.
How do I notify HMRC of a change of accounting period?
Filing AA01 with Companies House does not update HMRC automatically. You need to tell HMRC directly about the new period dates, ideally before your original Corporation Tax filing deadline, so your return isn’t assessed against the wrong dates.
Can I shorten my accounting period by one day?
Yes, there’s no minimum length for shortening an accounting period, though it must still result in a period of at least one day. Bear in mind your new filing deadline becomes whichever is later: nine months from the new date, or three months from the date Companies House receives your AA01, so an extremely short shortening can leave very little preparation time.
Can I revert to my company’s original accounting reference date later?
Yes, provided the change still falls within the statutory limits. You’d file another AA01 to move the ARD back, but remember that doing so counts as a fresh extension or shortening in its own right, and any prior extension on record will still be voided by the new filing.
When you can, and can’t, change an accounting reference date
Public companies face the same core AA01 mechanism as private ones, but the practical stakes differ. A private company’s nine month filing deadline gives more breathing room to absorb a shortened period than a public company’s six-month deadline does, so the same ARD change can be routine for one and genuinely tight for the other.
Consider two directors filing on the same day. One shortens a year end that’s still four months from its deadline, no prior extension exists, and the AA01 goes through WebFiling with confirmation in an hour. The other tries to extend a year end that was already stretched once eighteen months ago, discovers they’re inside the five year limit, and has to secure group alignment evidence before Companies House will accept the notice. Same form, wildly different outcomes, because one director checked the account status and extension history first and the other didn’t.
Why the “just file the form” advice misses the real risk
Most guidance on changing an accounting reference date treats it as a form-filling exercise: fill in AA01, submit, done. That undersells where the actual damage happens, which is almost never on the Companies House side. It’s the silent gap between filing AA01 and HMRC’s Corporation Tax period, because the two systems genuinely don’t talk to each other, and a director who assumes they do can end up filing a Company Tax Return against dates that no longer match their statutory accounts.
The other underrated risk is timing arithmetic. Directors fixate on the 18 month cap and the five year extension limit, both real constraints, while missing the more mundane trap: shortening a period so close to its existing deadline that the three month receipt rule bites harder than the nine month rule ever would. That’s not a legal grey area, it’s just easy to miss under deadline pressure.
Where this genuinely calls for professional judgement is any ARD change touching a group structure, a prior extension, or a period that will straddle two Corporation Tax years. Get those wrong and you’re not filing a Company Tax Return late, you’re potentially filing two incorrect ones. That’s where paying for a proper reconciliation earns its cost back many times over.
— Shoaib
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