Your first Company Tax Return: why HMRC expects two CT600s

By Max Szolcek, founder of Dormant Return. Last updated 2 October 2026

Many directors sign in to file their company's first Company Tax Return and find HMRC expecting two of them: one for twelve months and one for a few days or weeks. Nothing has gone wrong. It is the result of two rules, set by two different bodies, that do not quite fit together.

Rule one: Companies House gives you a long first year

Companies House sets a new company's accounting reference date automatically. In its guidance: for all new companies, their first accounting reference date will be the last day of the month in which the anniversary of their incorporation falls. Its own example is a company incorporated on 6 April 2025, whose first accounting reference date is 30 April 2026 and then 30 April every year after.

So the first set of accounts runs from the day of incorporation to the end of the month a year later. Unless you happened to incorporate on the first day of a month, that is more than twelve months: in the example, twelve months and 24 days. The deadline for those first accounts is also unusual: 21 months from the date of incorporation for a private company, rather than the nine months after year end that applies afterwards.

Rule two: HMRC will not accept a period over twelve months

Gov.uk's page on Corporation Tax accounting periods is blunt: the accounting period cannot be longer than 12 months and is normally the same as the financial year covered by the company's annual accounts. It then spells out the consequence for accounts that run longer: you must file 2 returns to cover the period of your accounts because your accounting period cannot be longer than 12 months.

Put the two rules together and the first year of almost every company produces one set of accounts and two Corporation Tax accounting periods. In the 6 April example, HMRC's periods are 6 April 2025 to 5 April 2026 (twelve months) and 6 April 2026 to 30 April 2026 (25 days). Each period gets its own CT600, each with its own filing deadline twelve months after that period ends, and both returns carry the same set of accounts.

When HMRC's first period actually starts

There is a subtlety for companies that did nothing for a while after incorporation. Companies House's period starts on the day of incorporation regardless. HMRC's does not: gov.uk explains that the Corporation Tax accounting period begins when the company starts business activities, and that the dates may be different in the year you set up your company. HMRC confirms the dates by letter after you add Corporation Tax to the company's business tax account, and they are visible in that account afterwards.

For a company that has never traded and has told HMRC it is dormant, there may be no Corporation Tax period at all yet, and therefore no return due, however long the Companies House accounts run. The rules for that situation are in how to tell HMRC your company is dormant. For a company that HMRC does treat as active, the split described above is what to expect, and the notice HMRC sends will name the periods it wants; see the CT603 notice to deliver a Company Tax Return.

What to do in practice

Find the periods HMRC holds. Sign in to the company's HMRC business tax account, or read the notices to deliver you have received. Do not assume the dates from your Companies House accounts; check what HMRC actually expects.
File a CT600 for each period. The first covers twelve months, the second the short remainder. Both attach the same accounts, the ones for the whole long period. For a company that traded, the profit or loss for the whole period is shared between the two returns by the number of days in each; for a dormant company it is nil for both.
Watch both deadlines. They fall a few weeks apart, twelve months after each period ends. Missing either one starts the penalty clock for that return on its own. The CT600 deadline calculator will show both dates and flag a first period that needs splitting.
Keep the two regulators separate in your head. The long first year is fine for Companies House; it simply cannot be squeezed into one HMRC return.

Two periods, two returns, still five minutes each. Dormant Return files a long first year as two separate returns, one for each period HMRC expects, and both carry the accounts for the whole year. If the company traded, you enter the figures from those accounts once and we work out each return's share by days. Each return is £5 for a dormant company or £15 for one that traded, filed under your own Government Gateway sign-in and refunded automatically if HMRC rejects it.

Start your return

Two common mistakes

Filing one return for the whole long period. Software that accepts a period over twelve months will be rejected by HMRC's validation, and a return for the wrong period does not satisfy the notice for the right one. If you filed for twelve months and forgot the stub, the stub is still outstanding and its own penalties apply.

Assuming the short period is too small to matter. A 25-day period with nothing in it still needs its own CT600 if HMRC has asked for one. Gov.uk also warns that if you lengthen your company's financial year later on, you need to update your accounting period dates with HMRC, and may get a late filing penalty if you do not.

After the first year

Once the accounting reference date settles down, the company's financial year is exactly twelve months and HMRC's accounting period matches it. From the second year onwards there is one set of accounts and one CT600 a year, unless the company changes its year end or, for HMRC's purposes, stops and restarts business activities. How the filing itself goes, from the details you need to HMRC's acceptance receipt, is in how to file a dormant company CT600 online.