How to Register for Corporation Tax in the UK — Complete Guide for New Companies

When Do You Need to Register?

• Under HMRC rules, a UK company must notify HMRC that it has started trading within 3 months of:
• Starting to trade (selling, buying, or doing business)
• Receiving income from any business activities
• Starting to charge for services

Note: forming the company and starting to trade are different events. You can form a company in January and not start trading until June. The 3-month clock starts from June.

HMRC usually sends an automatic "New Company" notification to your registered address within weeks of incorporation (via Companies House data sharing). But you must not rely on this — register proactively.

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Step-by-Step: How to Register for Corporation Tax

Step 1: Receive your UTR (Unique Taxpayer Reference) HMRC automatically issues a UTR (10-digit number) to your registered address within 2–4 weeks of Companies House registration. This arrives by post (HMRC does not email UTRs). The envelope typically says "Important — Tax Office notification."

If your registered address is a service address provider, ensure they forward post promptly. UTR letters are time-sensitive.

• Step 2: Create an HMRC Online Services account
• Go to hmrc.gov.uk → Sign in → Create sign-in details (Government Gateway). You need:
• Email address
• Password
• Government Gateway ID (created during setup)

Step 3: Enrol for Corporation Tax Within your HMRC Online account → Add a tax → Corporation Tax → Enter your UTR → Enter your company registration number (from Companies House).

HMRC then sends an activation code by post (2–7 days) to your registered address. Enter this code to activate CT access.

Step 4: Notify HMRC of start date and accounting period Once enrolled, log in → Corporation Tax → Tell HMRC your company's start date, accounting period start date, and registered address.

Step 5: Set a reminder for your Corporation Tax return deadline CT return (CT600) is due 12 months after the end of your accounting period. CT payment is due 9 months and 1 day after the end of your accounting period (for companies paying less than £1.5M CT). Set calendar reminders.

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CT Rates 2025/26

Associated companies affect the thresholds: if your company has one or more "associated companies" (broadly, companies controlled by the same person or group), the £50,000 and £250,000 thresholds are divided by (1 + number of associated companies).

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Annual CT Compliance Timeline

• For a company with a December 31 year-end:
• January 1: New accounting period starts
• December 31: Accounting period ends
• September 30 (9 months + 1 day): Corporation Tax payment due
• December 31 (12 months): Corporation Tax return (CT600) due

Penalties for late CT600 filing: £100 (1 day late), £200 (3 months late), 10% of unpaid CT (6 months late), additional 10% (12 months late).

Penalties for late CT payment: interest accrues from day after payment due date. Late payment penalties also apply for persistent late payers.

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FAQs

What if my company trades but makes no profit? You still must file a CT600 return, even if showing a loss. The loss can be carried forward to offset future profits. Filing is mandatory regardless of profitability.

My company is dormant — do I need to register for CT? A dormant company (no income, no trading) does not need to register for CT initially. You must notify HMRC when you start trading. However, you must still file annual accounts at Companies House and file a Confirmation Statement.

Can I change my company's accounting period? Yes, by notifying HMRC and Companies House. The first accounting period is fixed from incorporation to the anniversary (approximately 12 months). You can change subsequent periods — useful if you want your year-end aligned with the calendar year or your personal tax year.

What is the CT600? The Corporation Tax return form filed annually with HMRC. It calculates your taxable profit, applies allowances, and determines the CT liability. It must be filed online via HMRC's software or third-party accounting software (Xero, QuickBooks, FreeAgent, etc.).

Does HMRC ever estimate CT if you don't file? HMRC can and does issue determinations (estimated assessments) if returns are not filed. These determinations are often significantly higher than actual liability and must be displaced by filing the actual return. Interest and penalties continue to accrue regardless. File on time.