The complete guide to filing a CT600 for a dormant company

By Ben Davies · Updated 5 October 2026

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This is the long-form guide to filing a CT600 — the dormant company tax return — for a dormant UK limited company. If you have inherited a CT600 notice from HMRC, are unsure whether you owe one, or you've been quietly ignoring them for several years and the letters are getting more pointed — this guide explains where you stand and what to do about it.

For the basics, you might also want to read what is a CT600. For just the practical steps without the context, see how to file a nil CT600.

What a CT600 actually is

The CT600 is the Company Tax Return that limited companies submit to HMRC. It declares the company's income, expenses, profit (or loss), and the resulting Corporation Tax bill for the accounting period.

A nil CT600 is a CT600 with every income, expense, and tax figure set to zero. It is the dormant version of the same return.

A CT600 is legally due when HMRC sends a Notice to deliver a Company Tax Return, often referred to in HMRC correspondence as a CT603. HMRC sets up a Corporation Tax record for the company shortly after incorporation, and can send a notice every year, even to a dormant company. Once that notice is issued, you have a legal duty to file a CT600 for the accounting period it specifies, even if the company has done nothing all year.

When you need to file a CT600 for a dormant company

You need to file a CT600 for a period if HMRC has issued a notice to deliver a Company Tax Return for it. That is the whole test.

The trap most directors fall into is assuming that because the company is dormant under Companies House rules, the CT600 obligation goes away automatically. It does not. Companies House and HMRC are two separate regulators, and they manage their dormant-company records independently. Filing dormant accounts tells HMRC nothing, and HMRC can keep sending notices for a company that is dormant.

If you have received a notice to deliver a return, you owe a return for the period it names — even if the company was dormant throughout. There is no grace period and there is no automatic exemption.

When you don't need to file a CT600

No notice, no return. If HMRC hasn't sent a notice to deliver a return for a period, there is nothing to file with HMRC for it.

Important: any notice HMRC sends must be answered, even for a period when the company was dormant. File a nil CT600 for the period it names — DormantFile files it.

What "no significant accounting transactions" really means

This phrase appears throughout dormant company law and trips up a surprising number of directors.

A significant accounting transaction is any transaction that should appear in the company's accounts other than:

Payment for shares taken by the subscribers on incorporation
Four payments to Companies House: the confirmation statement fee, a change-of-name fee, a re-registration fee, and the penalty for filing accounts late

That's it. Everything else counts — including the strike-off fee, the fee for registering a charge, Companies House's newer penalties (such as for a late confirmation statement) and bank charges.

This means the following will break dormancy even if no money changes hands:

Receiving an invoice from a supplier (creates a creditor)
Lending money to the company from a director (creates a directors' loan account)
Paying for or renewing a domain name, software licence, or trademark in the company's name
Buying stock or a fixed asset, or paying any cost relating to one the company already holds
An accountant's fee for the company paid from a director's personal account, if the company owes the director for it or pays them back

Note that the test is transaction-based, not asset-based. Merely continuing to hold something the company acquired in an earlier period does not by itself break dormancy, as long as nothing about it changes in the year: nothing paid or collected, no interest added, nothing written off or written down. Paying to acquire, renew, or maintain it does. So does depreciation: an asset that has to be depreciated or written down each year puts an accounting entry through the books, so the only fixed assets that can sit unchanged are land, shares in a subsidiary that haven't lost value, or something already fully written down.

If any of these apply, the company is not dormant under the Companies Act definition. It is still possible to be dormant for HMRC purposes (HMRC uses a different, activity-based test: no trading and no income of any kind), but you should not be filing dormant accounts with Companies House.

The two definitions: Companies House vs HMRC

UK law has two separate definitions of dormant, and they are not identical.

Companies House (Companies Act 2006, s.1169): A company is dormant if it has had no significant accounting transactions during the accounting period. This is a strict, transaction-based test.

HMRC: A company is usually dormant for Corporation Tax purposes if it has stopped trading and has no other income — such as bank interest or investment income. This is an activity-based test, not a transaction-based one. The Companies House test catches more: any recorded transaction ends dormancy for Companies House, while HMRC looks at trading and income. A company that receives income of any kind, buys or sells, or employs someone is not dormant for either.

In practice, most dormant companies satisfy both. But there are edge cases:

A company that pays an annual renewal on a single asset (e.g. a domain name) but does nothing else: dormant for HMRC, not dormant for Companies House.
A company that has incurred some accountant fees but is not trading: dormant for HMRC, not dormant for Companies House.
A company that traded briefly during the period before going dormant: not dormant for either until the next full period.

Use the free dormancy checker to work out which definitions apply to you.

What a nil CT600 actually contains

A nil CT600 is not blank. It is a populated return where every numeric field reads zero or near-zero. You still need to provide:

The company's UTR (Unique Taxpayer Reference)
The accounting period start and end dates
Confirmation of the registered office address
Director details for the signing director
A set of accompanying iXBRL-tagged accounts that match what was filed with Companies House
A CT computation (also iXBRL-tagged) showing how the zero tax figure was arrived at

The iXBRL requirement catches a lot of people out. HMRC will not accept a CT600 without the tagged accounts and computation, even for a nil return. You cannot just attach a PDF of your dormant accounts and call it done.

HMRC's own free filing service used to generate the tagging for you, but it closed on 31 March 2026. DormantFile generates the tagging automatically from your inputs.

How to file a nil CT600: the practical process

Since 31 March 2026, when HMRC closed its free online filing service, a CT600 goes through commercial software.

With DormantFile

DormantFile generates the CT600, iXBRL accounts, and CT computation from a small set of inputs, then submits the package directly via the same HMRC gateway (GovTalk) that accountants use. You don't need to be an accountant to use it.

Compared with an accountant

An accountant prepares and files the return on your behalf using their own software and credentials. They will typically charge £80–£300 a year for a dormant company's filings, the CT600 included. DormantFile files the CT600 and the Companies House accounts from £19 a year.

What happened to HMRC's own free filing service? It closed. Until 31 March 2026 you could sign in to Government Gateway and file your accounts and tax return together through HMRC's free online service, which generated the iXBRL for you. That route no longer accepts returns — a CT600 now has to go through commercial software, whether you use it yourself or an agent does. Paper is only for a reasonable excuse or a return in Welsh.

Either way, the practical steps are:

Confirm your accounting period dates (12 months ending on your accounting reference date for most companies)
Gather your Corporation Tax UTR and Government Gateway login
Prepare or generate a nil CT600 with matching iXBRL accounts
Submit and wait for the acceptance response (usually within minutes)
Save the correlation ID and IRmark as proof of filing

See how to file a nil CT600 for the step-by-step breakdown.

What happens after you submit

HMRC's filing API returns one of three responses:

Accepted. The return has been received, validated, and processed. You receive a correlation ID, an IRmark, and a timestamp. This is your proof of filing — save it.

Rejected. The return has been received but failed validation. The response includes the specific validation errors (most commonly: wrong UTR, mismatched accounting period dates, missing iXBRL tags). You correct the errors and resubmit.

Submitted, awaiting result. Some returns require additional processing. The correlation ID is issued immediately, but the final accept/reject can take a few hours. You poll the HMRC service using the correlation ID until a final result is returned.

For a nil CT600 with clean inputs, entering and submitting takes about two minutes; HMRC's acceptance response usually returns within minutes.

What if you've fallen behind

You are not alone. A significant fraction of dormant-company directors have multiple years of unfiled CT600s when they finally get around to dealing with it.

Here is the practical reality:

HMRC does issue late filing penalties for missed CT600s. The standard penalty is £200 the day after the deadline passes, for returns due on or after 1 April 2026 — £100 where the deadline fell before then, whenever you actually file — and a further £200 if the return is more than three months late. At six months late HMRC estimates your tax bill and adds 10% of the unpaid amount, with a further 10% at twelve months. For a nil CT600 the tax-geared penalties are 10% of zero — nothing — so the real exposure is the fixed £200 + £200 = £400 per missed period.
If you file late three years in a row, the fixed penalties rise from £200 to £1,000 each — or from £100 to £500 each where the return fell due before 1 April 2026, since the escalation moved on the same date as the £200 did. There are no daily penalties for a late CT600 — the £10-a-day regime applies to Self Assessment, not Corporation Tax.
Penalties can be appealed. Where the company was outside the charge to Corporation Tax for the whole period, an appeal can ask HMRC to record the period as dormant, withdraw the notice and cancel the penalty — see appealing an HMRC late filing penalty for a dormant company. DormantFile checks a penalty for that ground and writes the letter, £9.99 a letter. It's a request, and the decision is HMRC's.

The right move if you're behind is to file all outstanding periods as soon as possible, regardless of penalties. Filing won't cancel a penalty already charged, but it stops a return moving into the next penalty band; ignoring the situation makes it worse.

DormantFile files any outstanding period. There is no separate "catch-up" process — you just file each missed CT600 as a normal return.

For the full penalty breakdown, see late filing penalties for dormant companies.

Common pitfalls

A few situations that catch directors out:

Period length is not 12 months. Your first accounting period after incorporation is often longer than 12 months, because Companies House sets your first accounting reference date to the last day of the month of the first anniversary of incorporation. HMRC, however, will only accept a CT600 covering a maximum of 12 months. You file two CT600s for a long first period — one for the first 12 months, one for the remainder. Many filing tools cannot handle this; DormantFile's filing flow walks you through entering both periods.

The Companies House period and HMRC period are different. This is unusual but possible — for example if you have changed your accounting reference date. Your CT600 must cover the HMRC period, which may not match the period covered by your dormant accounts. Make sure the dates on the CT600 match HMRC's records, not Companies House's.

You changed accounting reference date but didn't tell HMRC. Changing the ARD at Companies House does not automatically update HMRC. You may need to file an additional short-period CT600 to align the two systems.

You're dormant for HMRC but received a CT600 notice anyway. It happens. A notice, once sent, must be answered, even for a dormant period. File it as a nil CT600.

You filed dormant accounts with Companies House but no CT600 with HMRC. This is the single most common error among dormant-company directors. Dormant accounts and CT600 are two separate filings to two separate regulators. Filing one does not satisfy the other.

Frequently asked questions

Can I file a CT600 without an accountant? Yes — for a nil CT600, particularly. The iXBRL and CT computation requirements make it impractical to prepare by hand, but DormantFile handles those automatically.

Do I need to register for Corporation Tax if my company is dormant? No. HMRC sets up a Corporation Tax record for every company shortly after incorporation. The question that matters is whether HMRC sends a notice to deliver a return. If it does, a return is due for the period it names, even if the company was dormant.

How long does HMRC keep CT600s on file? HMRC retains CT600s indefinitely. The window for opening an enquiry into a return is normally 12 months from the date you file. After that, HMRC can only raise a discovery assessment — going back up to 4 years in normal cases, 6 years if the error was careless, and 20 years if it was deliberate.

Can I file a nil CT600 if my company has had any income? No. Any income — even a few pounds of bank interest — disqualifies a nil CT600. You need a regular CT600 declaring the income and any associated tax.

What if the company was struck off and then restored? Restored companies usually need to bring all filings up to date back to the strike-off date as a condition of restoration. This includes any missed CT600s.

Key points

A CT600 is the Company Tax Return that limited companies file with HMRC.
A nil CT600 is a CT600 with zero income, zero profit, and zero tax — the version most dormant companies file.
You owe a CT600 for any period HMRC has issued a notice to deliver a return for — even a dormant one.
HMRC and Companies House dormancy are separate things. Filing dormant accounts does not exempt you from the CT600.
A nil CT600 still requires iXBRL-tagged accounts and a CT computation — you cannot file a blank return.
Late filing triggers automatic fixed penalties — £200, then a further £200 at three months, rising to £1,000 each for a third consecutive late return. For a return that fell due before 1 April 2026 the rates are £100, then a further £100, and £500 each on a third consecutive return. Filing late is always better than not filing.
If you're more than one period behind, file everything outstanding as soon as you can — the longer you leave it, the larger the penalty pile.
Long first periods (>12 months) require two CT600s, not one.

If you'd like the practical version of this — a nil CT600 built with matching iXBRL accounts (including dormant accounts that carry figures from a trading past) and submitted directly to HMRC — DormantFile files the return and the Companies House accounts from £19 a year. See how it works or pricing for details.

For everything else CT600-related on this site — guides, plain-English answers, the free CT600 checker, and the penalty-letters reference — start at the CT600 hub.