Filing a CT600 When Your HMRC Period Differs From Companies House | TinyTax Support

Filing a CT600 When Your HMRC Period Differs From Companies House

TinyTax shows filing periods on the dashboard based on your company's accounting reference date (ARD) at Companies House. In some cases, your HMRC CT600 period may have different start or end dates — for example, if HMRC's Notice to Deliver (CT603) shows different dates than your Companies House accounts period. This guide explains how to file for the correct HMRC period.

Why Your HMRC Period Might Differ

The most common reasons your CT600 dates differ from your Companies House accounts dates:

  • Trading start date — Your company became active or notified HMRC of trading activity part-way through a Companies House accounts period. HMRC issues the CT603 from the date trading started, not the Companies House accounts start date.
  • Dormancy transition — You filed dormant accounts with Companies House while your HMRC period starts from when trading began. See Your First CT600 After Dormancy.
  • Changed accounting reference date — You changed your ARD at Companies House, creating a period mismatch.
  • First accounting period — Your Companies House period starts from incorporation, but HMRC's period may start from the date HMRC first issued a Notice to Deliver.

How to File for Your HMRC Period

The dashboard shows the Companies House period — use the CT600 with custom dates option to file for your HMRC period instead.

  1. Go to your Dashboard and click on your company to open the filing panel
  2. Find the CT600 row for the relevant period
  3. Click File — a dropdown menu appears
  4. Choose CT600 with custom dates
  5. Confirm the pop-up that appears
  6. On the filing form, you will see editable date fields — enter your HMRC period start and end dates
  7. Complete and submit the form as normal
ℹ Your HMRC period dates are on your Notice to Deliver (form CT603). Use those dates exactly. If you have not received a CT603, check your Government Gateway account under Corporation Tax or contact HMRC.

Where to Find Your HMRC Period Dates

HMRC sends a Notice to Deliver a Company Tax Return (form CT603) when they expect a CT600 filing. This letter specifies:

  • The exact period start date
  • The exact period end date
  • The filing deadline
If you have not received a CT603, you can check your expected filing period in your HMRC business Government Gateway account under Corporation Tax.

Common Questions

The "CT600 with custom dates" option is not showing

This option appears in the dropdown when clicking File on a CT600 row that has not been filed yet. If it is not there, you can access editable date fields directly using this link instead:

File CT600 with custom dates

This bypasses the dropdown entirely and takes you straight to the filing form with editable date fields.

If you cannot see a CT600 filing row at all, your filing window may not have opened yet — Companies House periods must have ended before TinyTax can show them. Contact support if you need to file urgently.

My HMRC period is longer than 12 months

If your HMRC period covers more than 12 months, HMRC requires two separate CT600 returns. TinyTax handles this automatically when you enter the full extended period. See Extended Accounting Periods.

Will this affect my Companies House accounts submission?

No — the CT600 and Companies House accounts are separate submissions. Filing a CT600 for a custom HMRC period does not change your Companies House accounts period or dates.

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