Does anything remind you about a CT600 deadline?

By Ben Davies · Updated 17 September 2026

Yes — twice. HMRC is not silent about the CT600 (the Company Tax Return it asks limited companies to file). What it will not do is reach you anywhere except the registered office letterbox, and for a dormant company that is very often the weak link in the whole arrangement.

The two letters, and when each one comes

The notice to deliver (form CT603) goes out about one month after the accounting period it names has ended. It is not a nudge — it is the thing that makes the return legally due, whether or not the company traded a penny. It names the period, and the filing date follows from that.

A reminder (form CT208) follows much later: HMRC's systems issue one 42 days — six weeks — before the filing date, where no payment has been made and the return is still outstanding. That is a dormant company exactly: nothing to pay, nothing filed yet. So you do get a second letter, and it is the one that actually says "this is nearly due".

Between the two is about nine and a half months in which nothing arrives at all. A year ending 31 March 2026 gets its notice around the end of April 2026, its reminder around mid-February 2027, and its deadline on 31 March 2027.

The filing date itself is normally 12 months after the end of the period, and never less than three months after the notice was served — which matters if your notice arrives late, because a late notice buys you three months from its date, not a fresh year.

Both letters are paper, and they go where the record says

This is the part that catches dormant companies. Neither letter is an email. Neither appears in a calendar. Both are posted to the registered office, or to whatever communication address HMRC holds on the Corporation Tax record for the company.

If you run the company yourself, with nobody acting for it — which is the ordinary dormant-company case, and the one this page is about — that post is coming to you and to nowhere else. And for a company that does nothing, the address on the record is very often a formation agent, a virtual office, an accountant you stopped using, or a house you moved out of three years ago.

Post to an address you do not check is not a reminder. It is a reminder someone else receives — and it still counts as given. A letter that was sent and never read pauses nothing: the filing date stands and the penalty is automatic.

One case genuinely is different, and it is worth knowing which. Where HMRC's record shows the address as undeliverable, it does not issue the notice to deliver at all. It holds it, and issues it once a valid address goes on the record — provided the accounting period ended less than four years earlier. With no notice served there is no filing date yet, because the date can never fall earlier than three months after service, so the flat-rate penalty has nothing to attach to. That is not a let-off. It is a suspended obligation that restarts, with three months' warning, the moment the address is fixed — and the Companies House accounts deadline runs all the way through it regardless.

While we are on post that is not about this: Companies House does write to you, and its reminders cover annual accounts, the confirmation statement, overdue filings and strike-off. Corporation Tax belongs to HMRC and never appears in that post. Being completely up to date with one register tells you nothing about what the other is waiting for.

Six weeks is shorter than it looks

Say the CT208 does reach you. You now have 42 days to establish that the company was genuinely dormant for the period, find its 10-digit UTR, get into the Government Gateway account (your HMRC login) enrolled for that company's Corporation Tax, and produce a return HMRC will accept — which means iXBRL-tagged accounts and a computation attached to it, not a form you can fill in by hand.

None of that is hard. All of it takes longer than you think when the authentication details were posted to an address you no longer control.

Several companies means several envelopes

Hold three dormant companies and the post does not add up to a system. Three notices and three reminders arrive on six unrelated dates, keyed to three different year ends, and nothing cross-references them — so being on top of one company's letters feels like being on top of all three. It is the failure described in I have three dormant companies — what do I file?, and it is why portfolios get caught more often than single companies.

What relying on the post costs

HMRC's late filing penalties for a Company Tax Return:

Those flat rates doubled for returns due on or after 1 April 2026, and it is the deadline that decides which rate applies, not when you get round to filing. On a nil return the tax-geared half is 10% of nothing, so a dormant company's real exposure is £400 per period — and if it is the third consecutive late return, each fixed penalty rises to £1,000, so £2,000. A return that fell due before 1 April 2026 carries the previous rates all the way along: £100 and £100, or £500 each on a third consecutive return. Our guide to HMRC's letters and penalties takes each letter in turn.

Companies House runs a separate clock for the accounts, at £150 to £1,500 depending on lateness, doubled if you are late two years running. The penalty calculator gives you that figure exactly. Miss both deadlines for one year and you pay both.

Put the date somewhere you actually look

This is the gap DormantFile closes, and it is worth being precise about how.

We never work out a Corporation Tax period for you. Nothing on the public register holds one, and a guessed tax deadline would be worse than none — HMRC's own notice is always definitive. So you take the period off the letter and add it once: start date, end date, done.

After that we remind you about any Corporation Tax or partnership return deadline you add, on the same ladder as everything else — 90, 30, 14, 7, 3 and 1 day before, then 1, 7, 30 and 90 days after if it is still unfiled. Ten emails rather than one letter, to an address you chose, consolidated into one message a day however many companies you hold. The date also lands in the calendar feed next to your accounts dates.

The reminders cost nothing. They run for subscribers and on the free Watchdog tier alike — one company, no card — and the only account they pause for is one whose card payment is currently failing, where the billing conversation takes over. Filing the return is the part that needs a subscription: £19 a year, covering every attempt that year, never a fee per return.

If it is an LLP, the letter is a different one

An LLP never files a CT600. The return it can owe is the SA800 partnership return, and the principle is the same: HMRC's notice to file is what creates the obligation, and a dormant year is no defence once one has been issued. The sting is worse, because late-filing penalties on a partnership return land on every member individually, £100 each. Add the year the notice asks for and it joins the same ladder, on the same free terms. See does a dormant LLP need to file a tax return?

What to do with the envelope in your hand

1. Check who sent it. HMRC letters carry a form code in the top-right corner — CT603 for the notice, CT208 for the reminder. Companies House writes about accounts, the confirmation statement or strike-off. Our guide to HMRC's letters and penalties lists the codes and what each one means.
2. Find the accounting period. The notice always states one. Your filing date is normally the end of that period plus 12 months — and never earlier than three months after the date on the notice.
3. Check whether a return is genuinely owed, if you are not sure — the free CT600 checker answers it in seven questions.
4. Move the date off paper. Put it wherever you will actually see it, because the next thing HMRC posts to that address is either the six-week reminder or a penalty notice.

Read the full guide: The complete guide to filing a CT600 for a dormant company