The complete guide to filing a CT600 for a dormant company

By DormantFile · Updated 27 May 2026

This is the long-form guide to filing a CT600 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.

HMRC issues a CT600 obligation when it sets up a Corporation Tax record for the company — typically shortly after incorporation. The obligation comes in the form of a Notice to deliver a Company Tax Return, often referred to in HMRC correspondence as a CT603. 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 if all of the following are true:

HMRC has issued you a notice to deliver a Company Tax Return for the period
The company existed for some or all of the period
HMRC has not marked the company as dormant for Corporation Tax in its own systems

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. You are dormant for one and not the other by default — until you actively notify HMRC.

If you have received a CT600 notice and not yet had HMRC confirm your dormant Corporation Tax status, you owe a return. There is no grace period and there is no automatic exemption.

When you don't need to file a CT600

You can stop filing CT600 returns if HMRC has formally acknowledged that your company is dormant for Corporation Tax purposes. This is sometimes called being dormant for CT in HMRC shorthand.

To get there, you need to:

Notify HMRC that the company has ceased trading and is now dormant
Wait for HMRC to update your company's Corporation Tax record
Receive written confirmation that no further CT600s are required

Once that's done, HMRC will normally stop issuing CT603 notices. Some directors receive a one-off confirmation letter; others see the next year's CT603 simply never arrive.

Important: if HMRC has not formally confirmed your dormant CT status, you must keep filing CT600 returns — even if the company has been dormant for years. The act of telling HMRC the company is dormant is not the same as HMRC accepting it. Until you have written confirmation, the obligation continues.

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
Companies House late filing penalties
Companies House annual confirmation statement filing fees

That's it. Everything else counts.

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)
Owning a domain name, software licence, or trademark registered to the company
Holding any stock or fixed asset on the balance sheet
Paying for accountant fees from a personal account "on behalf of" the company

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 slightly looser test based on actual trading), 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 dormant for Corporation Tax purposes if it is not actively trading. This is a looser, activity-based test that focuses on whether the company is carrying on business.

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

A company that holds a single asset (e.g. a domain name) and 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.

If you are filing directly through HMRC's own software, the iXBRL tagging is generated for you. If you are filing through commercial software (including DormantFile), the tagging is generated automatically based on your inputs.

How to file a nil CT600: the practical process

There are three routes:

1. HMRC's own filing service

Sign in to your company's Government Gateway account, navigate to Corporation Tax, and use HMRC's online return service. This is free but the interface is built for accountants — it expects you to know what every box means.

2. Commercial filing software

Tools like DormantFile generate the CT600, iXBRL accounts, and CT computation from a small set of inputs, then submit the package directly via the same HMRC API (GovTalk) that accountants use. This is the path of least resistance for a non-accountant.

3. Through an accountant

An accountant prepares and files the return on your behalf using their own software and credentials. They will typically charge £150–£300 a year for a dormant CT600.

Whichever route you choose, 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, the typical end-to-end time from clicking submit to acceptance is under five 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 £100 for being one day late, £200 if you are six months late, plus an extra 10% of the unpaid tax if you are more than twelve months late. For a nil CT600 the "10% of unpaid tax" is zero, but the £100 + £200 fixed penalties still apply per missed period.
HMRC can charge a daily penalty of £10 per day after three months, capped at £900 per period, on top of the fixed penalties.
For a fully nil position, HMRC has historically been willing to discuss reducing or waiving penalties if you bring everything up to date promptly and provide a reasonable explanation. Whether they do so in your case depends on the circumstances and the inspector.

The right move if you're behind is to file all outstanding periods as soon as possible, regardless of penalties. Filing late stops the penalties accruing further; ignoring the situation makes it worse.

You can file any outstanding period through commercial filing software or with an accountant. 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 one year after the 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 splits the period automatically.

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. Until HMRC formally updates its records, you owe the return. File it as a nil CT600 and then chase HMRC to confirm dormant CT status going forward.

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 commercial filing software handles those automatically.

Do I need to register for Corporation Tax if my company is dormant? Companies are automatically registered for Corporation Tax shortly after incorporation. The question is not whether you are registered but whether HMRC considers you dormant for CT purposes. See above on how to get there.

How long does HMRC keep CT600s on file? HMRC retains CT600s indefinitely. They can raise enquiries up to six years after filing in normal cases, twelve years if the omission was careless, and twenty 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, unless HMRC has formally confirmed your dormant CT status.
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 penalties (£100 + £200 fixed, plus daily penalties after three months). 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 — generate compliant nil CT600 returns with iXBRL accounts and submit directly to HMRC — DormantFile does both filings 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.