Filing a Long 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]

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.

How GoFile Files a Long Period

For a standard profit-making long period, GoFile handles the split automatically: enter the full period of account and both CT600s are prepared together — profits time-apportioned between the two periods, one set of full-period accounts generated and attached to both returns, box 85 and the multiple-returns marker declared on each. One payment covers both submissions.

Filing the Two Returns Yourself

GoFile asks you to file the two periods separately — rather than splitting automatically — in two situations:

• The period made a loss. How a loss divides between the periods, and how much carries forward into the second, is a claim only the company can make — pro-rating it automatically could misstate both returns.
• Your accounts were prepared elsewhere. If an accountant has already prepared (and perhaps filed at Companies House) the statutory accounts, those signed accounts — not regenerated ones — should go to HMRC.

In both cases the workflow is the same, using GoFile’s Your own accounts option:

• Start a return for the first 12 months and enter that period’s own figures
• On the Accounting step, choose Your own accounts
• Set “Period the accounts cover” to the full accounts period (e.g. 1 Oct 2024 – 31 Dec 2025)
• Attach your accounts iXBRL file under Additional Information
• 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.

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