Written by Marina Jahan · Reviewed by Shamayun Chowdhury, Senior Accountant · Published: November 2026 · Last reviewed: November 2026 · Applies to the 2026/27 tax year
Pushing your year end back looks like a quick form at Companies House. HMRC sees it differently. When you extend a company’s financial year past 12 months, the Corporation Tax side splits into two separate periods, each with its own return and its own payment date. This guide covers what changes when you extend accounting period dates, with a dated example, the arithmetic and the HMRC step that’s easy to miss.
Companies House allows a company’s financial year to run for up to 18 months, but no Corporation Tax accounting period can exceed 12 months. A longer year is therefore treated as two tax periods, each needing its own CT600 and each with its own payment date. Both returns are normally due 12 months after the whole period ends. Tell HMRC about the change separately.
- However long the accounts run, each Corporation Tax period is 12 months or shorter.
- Tax for each period is due 9 months and 1 day after that period ends, so the first bill isn’t delayed.
- Both CT600s normally share one filing date: 12 months after the end of the full accounts period.
- Trading profit is usually shared out by days, but some items need separate treatment.
- Companies House doesn’t pass the new date to HMRC’s Corporation Tax records. Tell HMRC yourself.
Extend Accounting Period: What Happens to Your Corporation Tax?
To lengthen a year, you send a change accounting reference date request (form AA01) to Companies House, normally online. The usual limits are one extension in any five-year period and a maximum length of 18 months, with a few exceptions. GOV.UK sets out the rules on its change your company’s year end page.
Corporation Tax has a stricter limit: an accounting period can’t run for more than 12 months. A 15-month set of accounts therefore becomes two tax periods, the first covering 12 months and the second the remaining 3. Each needs a Company Tax Return and each produces its own tax bill.
That’s why the extend accounting period question mostly comes down to Corporation Tax. The Companies House form is quick, but the HMRC side needs planning.
What Is the Difference Between a Period of Account and a Corporation Tax Accounting Period?
People often swap these terms, but they mean different things:
| Period of account | Corporation Tax accounting period | |
|---|---|---|
| Meaning | The span covered by your statutory accounts | The span HMRC taxes profits for |
| Relevant body | Companies House | HMRC |
| Longest allowed | 18 months | 12 months |
| Filing | One set of accounts | One CT600 for each tax period |
| A 15-month year becomes | One 15-month set of accounts | Two periods: 12 months, then 3 |
Why your accounts and tax return dates can differ
When a year lasts exactly 12 months, both periods line up and nobody notices they’re separate. They split apart only when the accounts run longer, typically after an extension or in a company’s first year. Either way, the first tax period stops at 12 months and the next one begins the following day.
Extend Accounting Period Example: 15 Months and Two Tax Returns
Northfield Joinery Ltd has always used 31 December as its year end. Its new parent company reports to 31 March, so in 2027 the directors move the year ending 31 December 2026 to 31 March 2027. The accounts now span 15 months.
Assumed: a private limited company, not filing its first accounts, not large enough to pay by instalments, and with no associated companies.
| Item | How it’s worked out | Date |
|---|---|---|
| Tax period 1 | First 12 months | 1 Jan 2026 – 31 Dec 2026 |
| Period 1 tax payable | 9 months + 1 day after 31 Dec 2026 | 1 Oct 2027 |
| Tax period 2 | Remaining 3 months | 1 Jan 2027 – 31 Mar 2027 |
| Period 2 tax payable | 9 months + 1 day after 31 Mar 2027 | 1 Jan 2028 |
| Both CT600 returns due | 12 months after the whole period ends | 31 Mar 2028 |
| Tell HMRC about the change | Before the original return date of 31 Dec 2027 | Aim for about 30 Sep 2027 |
| Accounts due at Companies House | 9 months after the new year end | 31 Dec 2027 |
These are the Corporation Tax payment deadlines to watch. Tax for 2026 falls due on 1 October 2027, almost six months before either return is due. So the company needs a dependable 2026 profit figure well before the full accounts are finished. Interest starts the day after a missed payment date.
How Do You Split Profits Between the Two Periods?
For an extended accounting period, the adjusted trading profit for the whole set of accounts is normally shared out by time:
For Northfield Joinery, 1 January 2026 to 31 March 2027 is 455 days: 365 fall in period 1 and 90 in period 2. With £90,000 of adjusted trading profit:
Count days, not months, and keep your calculation with the records.
“Each return is due 12 months after its own tax period.” Not so. Where the period of account is no longer than 18 months, both returns are due 12 months after the whole period ends. In GOV.UK’s example, a year end moves from 31 December 2024 to 31 March 2025, and every return is due by 31 March 2026.
What isn’t split by days?
- Capital allowances are calculated for each tax period, according to when the money was spent. The type of allowance also matters. The Annual Investment Allowance and writing-down allowances are scaled down for a short period, but first-year allowances work differently. Check each purchase separately.
- Chargeable gains usually fall in the tax period of the sale. For example, if the company sold an investment on 20 February 2027, the gain would normally sit in period 2.
- Non-trading income, such as bank interest, may need to follow when it was earned, not a straight split by days. These are illustrations, not universal rules.
Tax rates and limits in a short Corporation Tax accounting period
For 2026/27, Corporation Tax is 19% on small profits and 25% at the main rate, with marginal relief in between. This short second period is an often-missed side effect of an extend accounting period decision. A short Corporation Tax accounting period gets proportionally lower £50,000 and £250,000 limits, and associated companies cut them further.
Assumptions for these figures: no associated companies, profits taxed at the normal 2026/27 rates, and no other reliefs, losses or adjustments. They are illustrations only, and a real company’s bill may be different.
| Step | Period 1 (365 days) | Period 2 (90 days) |
|---|---|---|
| Taxable profit | £72,197.80 | £17,802.20 |
| Lower limit | £50,000 | £12,328.77 |
| Upper limit | £250,000 | £61,643.84 |
| Tax at 25% | £18,049.45 | £4,450.55 |
| Less marginal relief: 3/200 × (upper limit − profit) | £2,667.03 | £657.62 |
| Corporation Tax | £15,382.42 | £3,792.93 |
Both periods’ profits sit between their limits, so marginal relief applies to both. The figures are rounded to the nearest penny. Check the current rates before relying on them.
How Do You Tell HMRC You’ve Extended Your Accounting Period?
GOV.UK says that after lengthening a financial year you should contact HMRC before the original filing date of the Company Tax Return. If you don’t, a late filing penalty could follow.
In practice, HMRC’s records don’t always show a longer period straight away. ICAS has reported cases where HMRC’s systems kept an earlier filing date, which could produce a late-filing notice in error. Its practical pointers for agents are:
- Contact HMRC early, ideally about three months before the first anniversary of the end of the first tax period.
- Once HMRC says the record is updated, allow roughly 48 hours before filing online so the new deadline shows.
- An amendment made after the second anniversary of a period’s end may have to go by post.
Accepting your AA01 doesn’t update HMRC’s Corporation Tax records. Contact HMRC as a separate step. Explain the new accounting reference date, then confirm the filing deadline for each of the two tax periods. Keep a note of when you got in touch and what HMRC said.
Contact HMRC as soon as Companies House accepts the change, not when the returns are nearly ready. A dated note of the call makes any penalty appeal far easier.
Filing Two CT600 Returns: Step by Step
- Check the new year end appears on the Companies House register.
- Update HMRC so it holds both tax periods, then confirm both return dates.
- Create two return periods in your software, matching HMRC’s dates exactly.
- Allocate the profit by days, then handle capital allowances and any separate items for each period.
- Attach the accounts and separate computations. Send the full statutory accounts in iXBRL with both returns, as required, and prepare a separate Corporation Tax computation for each period. Each one should show the profits, adjustments, reliefs and tax.
- Check both returns’ dates, then submit them. You can file them together.
- Pay each bill by its own deadline, using the right payment reference for each period.
GOV.UK’s Company Tax Returns guidance covers the general CT600 rules, and its annual accounts guide explains the company accounts filing deadlines.
Common Problems When You Extend Your Accounting Period
Most extend accounting period problems come from HMRC’s records not matching the new dates:
| Problem | Likely cause | What to do |
|---|---|---|
| HMRC still shows the old dates | HMRC wasn’t told, or the update hasn’t gone through | Contact HMRC again with the new dates and record the call |
| Late filing notice despite filing on time | HMRC’s records held an earlier deadline | Appeal, quoting the statutory deadline and your notification |
| Software won’t create a second period | Dates entered as one long period | Set up 12 months, then the remainder |
| One return rejected | Dates don’t match HMRC’s records | Confirm dates with HMRC, correct and resubmit |
| AA01 rejected | Extended within the last 5 years, or accounts overdue | Check the Companies House rules first |
| First payment missed | Waiting for the accounts | Estimate and pay on time; interest runs after the due date |
Should You Extend Your Accounting Period?
It can make sense, for example to match a parent company, to avoid a busy trading season, or to give a new company longer before its first accounts. It won’t delay tax, so weigh both sides before you extend accounting period dates:
| Possible benefit | Trade-off |
|---|---|
| Year end matches a group or season | Two Corporation Tax returns for one set of accounts |
| One set of accounts covers more months | The first tax payment stays on its 12-month timetable |
| Fewer accounts in the short term | A larger year-end job; lenders may ask questions |
Extending works best for lining up with a group or trading cycle, not for putting off tax. Our annual accounts service for limited companies can help.
Extending Accounting Period: Your Questions Answered
How many CT600s do I need for an 18-month accounting period?
Two. One return covers the first 12 months and the other covers the last 6, because a Corporation Tax period can’t be longer than 12 months.
When is Corporation Tax due if I extend my accounting period?
Usually 9 months and 1 day after each tax period ends. Companies with profits above the large-company threshold pay by instalments instead. The standard £1.5 million threshold is reduced for short periods and associated companies, and exceptions apply, including where the tax bill is £10,000 or less. Companies with very large profits follow separate instalment rules, so check HMRC’s guidance.
Does extending my accounting period delay my Corporation Tax payment?
Not necessarily. Tax for the first 12-month period is normally still due 9 months and 1 day after that period ends. A longer financial year doesn’t automatically give you more time to pay it.
Do both CT600 returns for a long accounting period have the same filing deadline?
Generally, yes. If the period of account is 18 months or shorter, both returns are due 12 months after it ends. Each return still covers its own tax period and has its own payment date.
Do I need to tell HMRC if I extend my year end?
Yes. GOV.UK says to contact HMRC before the original Company Tax Return deadline. Companies House doesn’t update HMRC’s Corporation Tax records for you.
Can I file both Company Tax Returns at the same time?
Yes. Once the accounts are ready you can submit both together, as long as they’re in by the shared filing deadline.
Does changing my company’s year end change its VAT return dates?
No. VAT return periods are separate from Corporation Tax periods, so a new accounting reference date doesn’t move them. You can ask HMRC to change your VAT stagger separately if needed.
Can I extend my company’s first financial year?
Yes. A first financial year can generally run for up to 18 months from incorporation. A Corporation Tax period still can’t exceed 12 months, so you may need two CT600s. Companies House and HMRC deadlines are worked out separately, so check both.
For larger companies, see GOV.UK’s guide to paying Corporation Tax in instalments.
Final Verdict: Extend Accounting Period Plans Need an HMRC Step
At Companies House, a longer year is a single form. For Corporation Tax it means two tax periods, two returns and two payment dates, and the first payment doesn’t move. Tell HMRC early and check that its records have changed. Split profits carefully, treat special items separately and diary both payment dates. Do that and the change should go smoothly.
- Work out both tax periods and both payment dates before you file the AA01.
- After Companies House accepts the change, contact HMRC to update your accounting period dates.
- Estimate period 1’s profit early so the first payment isn’t late.
- Read GOV.UK’s accounting periods guidance and our guide to the end of the financial year.
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Why trust this guide
- Technically reviewed by a chartered management accountant (ACMA, MAAT) with more than 15 years’ UK practice experience.
- Every deadline and limit is taken from GOV.UK guidance on accounting periods, Corporation Tax payments and marginal relief for 2026/27.
- The worked example’s day counts, apportionment and reduced limits were calculated and checked line by line.
- Last reviewed: November 2026.
- GOV.UK – Accounting periods for Corporation Tax
- GOV.UK – Change your company’s year end
- GOV.UK – Pay your Corporation Tax bill
- GOV.UK – Company Tax Returns
- GOV.UK – Corporation Tax rates and allowances
- GOV.UK – Marginal Relief for Corporation Tax
- GOV.UK – Claim capital allowances
- ICAS – HMRC gives update on issues with corporation tax extended accounting periods (Nov 2024)