You've just incorporated a limited company at Companies House. Congratulations — but your responsibilities don't end there. Within three months, you must register for Corporation Tax with HMRC or face penalties. This guide walks you through the entire process, from formation to your first CT600 filing.

What happens when you form a company?

When you register a company with Companies House, two things happen automatically:

Companies House creates your company record and issues a certificate of incorporation with your company number
HMRC is notified — Companies House shares your details with HMRC, who will send you a CT41G form (or the equivalent letter) asking you to register for Corporation Tax

However, automatic notification doesn't mean automatic registration. You still need to actively complete the registration process with HMRC.

The CT41G form

The CT41G (or "New company details" form) is HMRC's way of collecting the information they need to set up your Corporation Tax record. You'll typically receive this by post within a few weeks of incorporation.

The form asks for:

Company name and registration number
Registered office address
Date you started trading (or intend to start)
Your accounting period end date
Nature of business (SIC code)
Details of directors and company secretary
Whether the company has taken over another business
Expected annual turnover

What if you don't receive the CT41G?

Don't wait. If the form hasn't arrived within 2–3 weeks of incorporation, register online. You have a legal obligation to notify HMRC within 3 months of starting any activity that's liable to Corporation Tax — whether or not you've received their letter.

How to register for Corporation Tax

You have two options:

Option 1: Register online (recommended)

Go to GOV.UK and use the "Register for Corporation Tax" service. You'll need:

Your company's 10-digit UTR (Unique Taxpayer Reference) — but if you don't have this yet, you can still register and HMRC will issue one
Your company number from Companies House
The date your company became active
Your company's accounting year-end date
The nature of your business

Online registration is usually processed within a few days.

Option 2: Complete the CT41G by post

Fill in the paper form and return it to HMRC. This takes longer — allow 2–3 weeks for processing.

Your Unique Taxpayer Reference (UTR)

After registration, HMRC will issue your company a 10-digit UTR number. This is different from any personal UTR you might have as a self-employed individual.

Your company UTR is essential for:

Filing your CT600
Paying Corporation Tax
Corresponding with HMRC about your company's tax affairs
Setting up your HMRC online account

Keep this number safe. You'll need it for every interaction with HMRC regarding your company's Corporation Tax.

Setting up your HMRC online account

Once you have your UTR, set up your company's HMRC online services account:

Go to the HMRC Gateway
Create an organisation account (if you don't already have one)
Enrol for Corporation Tax using your UTR and the activation code HMRC sends by post
The activation code typically arrives within 7–10 working days

Once activated, you can:

File your CT600 online
View your Corporation Tax account balance
Set up payment arrangements
Manage your company's tax affairs

When does Corporation Tax start?

Your company becomes liable to Corporation Tax from the date it starts any of the following:

Trading — buying and selling goods or services
Carrying on business activity — even pre-trading activities like purchasing stock, setting up a website, or signing contracts
Receiving income — bank interest, rental income, or any other taxable income

For most new companies, this is the date of incorporation or very shortly after.

Dormant companies

If your company is dormant (not trading and receiving no income), you can tell HMRC and they won't require a CT600 until you start trading. However, even dormant companies need to file annual accounts with Companies House.

Choosing your accounting year-end

When registering, you'll need to specify your company's accounting period end date. This is one of the most important decisions for a new company because it affects:

When your CT600 is due
When your Corporation Tax payment is due
How your first accounting period works

Popular year-end dates

Date	Advantages
31 March	Aligns with the tax year; simplest for rate calculations
31 December	Calendar year; easy to remember
5 April	Aligns with the personal tax year
Your incorporation date anniversary	Maximises your first period length

First accounting period — the long period trap

Your first Companies House accounting period can be up to 18 months from incorporation. However, HMRC's accounting periods can never exceed 12 months.

If your first Companies House period is longer than 12 months, HMRC splits it into two Corporation Tax accounting periods, each requiring a separate CT600.

Example: Company incorporated 1 July 2025, year-end 31 March 2027 (21 months):

CT600 #1: 1 July 2025 – 30 June 2026 (12 months)
CT600 #2: 1 July 2026 – 31 March 2027 (9 months)

This catches many new company directors off guard. See our detailed guide on first-year filing for more.

Key deadlines for new companies

Once registered, you're on the clock. Here are the critical deadlines:

Corporation Tax registration

Deadline: Within 3 months of starting to be active
Penalty for late registration: HMRC may charge a penalty, and you'll receive estimated tax assessments

CT600 filing

Deadline: 12 months after the end of the accounting period
Penalty: £100 for 1 day late, £200 after 3 months, then 10% of the tax after 6 months, and a further 10% after 12 months

See our full CT600 deadline guide for more details.

Corporation Tax payment

Deadline: 9 months and 1 day after the end of the accounting period
Interest: HMRC charges interest from the day after the deadline

Note that the payment deadline comes before the filing deadline. You need to calculate and pay your tax before you've filed the return.

Companies House annual accounts

First accounts: 21 months from incorporation
Subsequent years: 9 months after year-end
Penalty: Escalating fines from £150 to £1,500

PAYE registration

If you're going to pay yourself a director's salary, you also need to register for PAYE. This is separate from Corporation Tax registration.

Company details
Director information
Expected first payment date

Most directors set up a salary at or just below the NIC threshold to maintain their State Pension record while keeping the tax charge minimal. The remainder is taken as dividends.

VAT registration

VAT is separate from Corporation Tax, but worth considering at formation:

Mandatory registration: Required when your taxable turnover exceeds £90,000 in any 12-month period (2025/26 threshold)
Voluntary registration: You can register voluntarily below the threshold, which lets you reclaim input VAT on purchases

For some businesses, voluntary registration makes sense from day one — especially if you sell to VAT-registered businesses (B2B) and have significant input VAT to reclaim.

What records to keep from day one

Start good habits immediately. Your company must keep records to support its tax returns:

Bank statements — open a dedicated business bank account immediately
Invoices — all sales and purchase invoices
Receipts — for all business expenses
Contracts — employment contracts, leases, supplier agreements
Board minutes — documenting key decisions (dividend declarations, salary changes)
Share records — share certificates, register of members

Keep records for at least 6 years after the end of the relevant accounting period. See our record-keeping guide for full details.

Step-by-step checklist

Here's everything you need to do after forming your company, in order:

Step	Action	Deadline
1	Open a business bank account	ASAP
2	Register for Corporation Tax	Within 3 months of becoming active
3	Set up HMRC online account	Once UTR received
4	Register for PAYE (if paying salary)	Before first payroll
5	Consider VAT registration	When approaching £90,000 turnover
6	Set up accounting software	ASAP
7	Start recording all income and expenses	From day one
8	Pay Corporation Tax	9 months + 1 day after period end
9	File CT600	12 months after period end
10	File Companies House accounts	21 months from incorporation (first year)

Common mistakes new companies make

1. Not registering for Corporation Tax at all

Some directors assume Companies House handles everything. It doesn't. You must register with HMRC separately, and failure to do so can result in estimated assessments and penalties.

2. Missing the first payment deadline

Because the payment deadline (9 months + 1 day) comes before the filing deadline (12 months), many new directors focus on the filing and forget to pay on time. Late payment interest starts accruing immediately.

3. Not understanding the two-return trap

If your first Companies House period exceeds 12 months, you'll need two CT600s, not one. This means two tax computations, two payments, and two sets of deadlines.

4. Mixing personal and business finances

Using a personal bank account for business transactions creates a mess at year-end and can cause problems with directors' loan accounts. Open a dedicated business account from day one.

5. Not seeking advice early

The decisions you make at formation (year-end date, salary level, dividend strategy) have long-lasting tax implications. Getting advice before you start is much cheaper than correcting mistakes later.

Filing your first CT600

When it's time to file your first return, you'll need:

Your company's UTR
Annual accounts (prepared to your chosen year-end)
iXBRL-tagged accounts
A Corporation Tax computation
Details of all income, expenses, and tax adjustments

You can file through Taxpipe, which handles the iXBRL tagging, CT600 form completion, and HMRC submission in one integrated process. It's designed for directors filing without an accountant — no accounting jargon, just clear questions and automatic calculations.

Frequently asked questions

How long does Corporation Tax registration take?

Online registration is typically processed within a few days. You'll receive your UTR by post within 2–3 weeks. The HMRC Gateway activation code takes a further 7–10 working days after you enrol.

Do I need to register if my company is dormant?

You should still notify HMRC that the company exists but is dormant. They'll mark it as dormant and won't expect CT600 filings until you start trading. Once you begin any activity, you must tell HMRC.

Can I register for Corporation Tax before the company is incorporated?

No. You need your Companies House company number to register with HMRC. The company must exist legally first.

What's the difference between my company UTR and my personal UTR?

They're completely separate 10-digit numbers. Your personal UTR is for Self Assessment (if you file a personal tax return). Your company UTR is for Corporation Tax. Don't mix them up when filing or making payments.

Do I need an accountant to register?

No. You can register yourself online in about 15 minutes. However, many directors choose to appoint an accountant early so they can act as the company's tax agent and handle HMRC communications.

What happens if I register late?

HMRC may issue a penalty and could raise an estimated Corporation Tax assessment based on what they think you owe. You'd then need to file the actual return to correct this. It's always better to register early — there's no penalty for registering too soon.