Key facts
- A Corporation Tax accounting period can cover a maximum of 12 months, but statutory accounts can run up to 18 months.
- A long period of account is split into two CT600 returns: the first 12 months, then the remainder.
- For most companies GoFile does the split for you: enter the full period once and both returns are prepared, paid for and filed together — whether the period made a profit or a loss.
- Accounts prepared elsewhere are split the same way: upload your accountant’s iXBRL file once and GoFile files both returns with that file attached to each.
- You file one set of statutory accounts — the same full-period accounts are attached to both returns, with box 85 (“accounts made up for a different period”) declared.
- Accounts filed with a CT600 must be in iXBRL format — HMRC does not accept PDF accounts from UK companies.
- Each return has its own filing and payment deadlines, based on its own accounting period.
Why a First Period Is Often Longer Than 12 Months
A new company’s first accounting reference date is set automatically by Companies House: the last day of the month of its first anniversary. A company incorporated on 15 October 2024 gets a first year end of 31 October 2025 — slightly over 12 months. Many companies also extend a year end deliberately (for example, to move to a 31 December or 31 March year end), creating a period of account of up to 18 months.[2]
Statutory accounts can legitimately cover that whole period. Corporation Tax cannot: an accounting period for CT is capped at 12 months, so the long period must be split.[1]
The Split: Two Returns, One Set of Accounts
A long period of account divides into two CT accounting periods — the first 12 months, then whatever remains — and each needs its own CT600:[3]
| Period of Account (the accounts) | CT600 Return 1 | CT600 Return 2 |
|---|---|---|
| 1 Oct 2024 – 31 Dec 2025 (15 months) | 1 Oct 2024 – 30 Sep 2025 | 1 Oct 2025 – 31 Dec 2025 |
| 1 Jan 2025 – 31 Mar 2026 (15 months) | 1 Jan 2025 – 31 Dec 2025 | 1 Jan 2026 – 31 Mar 2026 |
The part that surprises most people: you do not prepare separate accounts for each return. The company has one set of statutory accounts covering the full period, and the same accounts are attached to both CT600s. Each return then declares:
- Box 85 — the attached accounts are made up for a different period than the return
- Multiple returns — more than one return is being delivered for this period of account
The tax computation, by contrast, is specific to each return: each accounting period has its own profits, its own CT calculation, and its own deadlines.
Deadlines still run per return: each CT600 is due 12 months after the end of its own accounting period, and any tax for each period is due 9 months and 1 day after that period ends. See Filing Deadlines.
How GoFile Files a Long Period
For most long periods — profit or loss — GoFile handles the split automatically: enter the full period of account once and both CT600s are prepared together. Profits (or a loss of any kind) are time-apportioned between the two periods, one set of full-period accounts — generated by GoFile, or the iXBRL file you upload under Your own accounts — is attached to both returns, and box 85 and the multiple-returns marker are declared on each. One payment covers both submissions.
Time apportionment is the standard basis for dividing a long period between Corporation Tax accounting periods, even where you could trace each transaction to one period or the other.[3] Whether GoFile prepares the accounts or you upload your accountant’s file, this is the route to use.
Filing the Two Returns Yourself
GoFile asks you to file the two periods separately only where the automatic split cannot represent the return, and it tells you on screen when that applies. The usual reasons are:
- Period-specific claims and reliefs — group relief, tax credits or repayments, supplementary pages other than CT600A, or an amendment
- Apportionment flips the result — time-apportioning gives a loss in one period and a profit in the other
Filing separately means attaching accounts prepared outside GoFile — an iXBRL file from your accountant or accounts software — because GoFile does not produce a stand-alone accounts file for this route. The workflow uses GoFile’s Your own accounts option:
- Start a return for the first 12 months
- On the Accounting step, choose Your own accounts and upload the iXBRL file in the Import your accounts panel beside it (the button reads “Choose iXBRL”)
- GoFile reads the accounts period from the file and apportions the imported profit-and-loss figures to the return’s share of it by days — check them, and replace them with actual figures if you have them
- Submit — GoFile declares box 85 and multiple returns automatically, files your accounts exactly as supplied, and generates only the iXBRL tax computation
- Repeat for the short second period, attaching the same accounts file — it files at no further charge
Nothing is sent to Companies House — your accounts filed there are untouched.
iXBRL, Not PDF
HMRC requires the accounts attached to a UK company’s CT600 to be in iXBRL (machine-readable tagged) format — a signed PDF is not accepted, even if it is the version Companies House holds.[4] If your accountant prepared the accounts, ask for the iXBRL file: accounts-production software creates it as standard. See iXBRL & Digital Filing for the background.
Frequently Asked Questions
Can I attach the same accounts to both CT600 returns?
Yes — that is exactly how HMRC expects a long period of account to be filed. Each CT600 covers its own accounting period with its own figures and tax computation, but both attach the same full-period statutory accounts, with box 85 declared to show the accounts are made up for a different period than the return.
What is CT600 box 85?
Box 85 tells HMRC that the accounts attached to the return are made up for a different period than the return itself covers. It is the standard declaration when a long period of account is filed as two returns, each carrying the same full-period accounts.
Can I file the PDF accounts I sent to Companies House?
No. HMRC requires the accounts attached to a UK company’s CT600 to be in iXBRL format. If an accountant prepared your accounts, ask them for the iXBRL version — accounts software produces it as standard, and it is often the same file Companies House received.
The period made a loss — do I have to file two separate returns?
Usually not. A trading loss is time-apportioned between the two periods in the same way as a profit, and the unrelieved part of the first period carries into the second, so GoFile files a loss-making long period automatically. Property, loan-relationship and management-expense losses are carried between the two periods the same way. Losses brought forward are used by the first return first, with the balance passing to the second, and a carry-back claim (box 45) is placed the same way. GoFile only asks you to file separately where the two periods need different treatment — for example group relief, tax credits or an amendment — and it tells you on screen when that applies.
All my income fell in the first 12 months. Is time apportionment still right?
Yes. Time apportionment is the standard basis for dividing a long period of account between Corporation Tax accounting periods, and it is what the automatic split applies. A different basis is only expected where apportioning by time would give a clearly unreasonable result, which is rare — and in most cases the total tax across the two periods is the same either way.
My accountant prepared the accounts. Can GoFile still split the period for me?
Yes. Choose Your own accounts, upload the iXBRL file once and keep the full accounts period as the return period: GoFile apportions the figures between the two CT600s and attaches your file, exactly as supplied, to both. Filing the two returns separately is only needed in the cases listed under Filing the Two Returns Yourself.
Do I pay twice to file both returns in GoFile?
No. One payment covers the whole period of account: after paying for the first return, the short second return files at no further charge.
Further Reading
- Accounting Periods — how CT periods start, end, and split
- The CT600 Tax Return — what goes into a Company Tax Return
- Trading Losses — loss relief and carry-forward claims
- Filing Deadlines — when each return and payment is due
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