You've set up your limited company, got your certificate of incorporation, and maybe even started trading. Now what?

Within a few weeks of registering with Companies House, HMRC will write to you about Corporation Tax. Eventually, you'll need to file a CT600 — your company's tax return. Here's what to expect and how to handle it.

Timeline: what happens after you register a company

Week 1–2: Companies House registration

You register your company at Companies House. You get a certificate of incorporation, a company number (CRN), and your company legally exists.

Week 2–4: HMRC writes to you

HMRC finds out about your new company automatically (Companies House tells them). They'll send you:

A letter with your Unique Taxpayer Reference (UTR) — a 10-digit number. Keep this safe. You need it for everything.
Information about when to file and pay — your accounting period dates and deadlines.

If you don't receive your UTR within 4 weeks, call HMRC on 0300 200 3410.

Month 1–12: Your first accounting period

Your first accounting period starts on the date of incorporation and typically runs to your company's financial year end. For most new companies, this is the last day of the month in which you incorporated — or a date you chose when registering.

Example: If you incorporated on 15 March 2025, your first accounting period might run from 15 March 2025 to 31 March 2026 (about 12.5 months).

Your first accounting period can be longer than 12 months

This is unique to your first year. Most accounting periods are exactly 12 months, but your first one can be up to 18 months. If it's longer than 12 months, you'll need to split it into two CT600 returns:

One covering the first 12 months
One covering the remaining period

Taxpipe handles this split automatically if needed.

When is your first CT600 due?

12 months after the end of your accounting period.

For example:

Accounting period ends: 31 March 2026
CT600 filing deadline: 31 March 2027

When is the tax payment due?

9 months and 1 day after the end of your accounting period.

Using the same example:

Accounting period ends: 31 March 2026
Payment deadline: 1 January 2027

Yes, you need to pay before you file. The payment deadline comes first.

What do you need for your first CT600?

Essential information

Company name and CRN — from your certificate of incorporation
UTR — the 10-digit number from HMRC's letter
Accounting period dates — when your financial year started and ended
HMRC login credentials — Government Gateway user ID and password

Financial information

Total turnover (revenue) — all the money your company received from trading
Costs and expenses — everything the company spent on business activities
Any other income — bank interest, investment returns, rental income
Capital expenditure — large purchases like computers, vehicles, equipment

If your company was dormant

If you set up the company but didn't trade during the period:

Turnover: £0
Expenses: £0
Tax due: £0

You still file a CT600, but it's straightforward. See our dormant company filing guide.

How Corporation Tax is calculated

Corporation Tax is charged on your company's taxable profits — which is roughly:

Turnover − Allowable expenses = Taxable profit

The current rates (2025/26):

Taxable profit	Rate
Up to £50,000	19% (small profits rate)
£50,001 – £250,000	26.5% (effective marginal rate)
Over £250,000	25% (main rate)

Most new small companies fall into the 19% bracket.

Example: Your company made £30,000 in revenue and had £12,000 in allowable expenses.

Taxable profit: £30,000 − £12,000 = £18,000
Corporation Tax: £18,000 × 19% = £3,420

Step-by-step: filing your first CT600

Step 1: Get your records together

Pull together:

Bank statements for the accounting period
Sales invoices (income)
Purchase invoices and receipts (expenses)
Any payroll records (if you paid yourself a salary)

Step 2: Calculate your figures

Work out:

Total turnover
Total allowable expenses
Any capital expenditure
Net profit (turnover minus expenses)

If you use accounting software (Xero, FreeAgent, QuickBooks), these figures should be in your profit and loss report.

Step 3: File with Taxpipe

Sign up and add your company (you'll need your CRN and UTR)
Create a new return for your first accounting period
Answer the wizard questions — Taxpipe asks plain-English questions and fills in the CT600 boxes for you
Review and pay — £59 one-time fee
Submit — we file directly with HMRC and you get instant confirmation

Step 4: Pay your Corporation Tax

Pay HMRC directly — not through Taxpipe. Payment methods:

Online banking — pay to HMRC's bank account using your UTR as the reference
Direct Debit — set up through your HMRC online account
CHAPS or Bacs — via your bank

HMRC's bank details for Corporation Tax payments:

Sort code: 08 32 10
Account number: 12001039
Account name: HMRC Cumbernauld
Reference: Your 17-character payment reference (your UTR + the accounting period end date)

Common mistakes new directors make

1. Missing the payment deadline

Remember: you need to pay Corporation Tax 9 months and 1 day after your period ends, but the filing deadline is 12 months after. Many new directors only focus on the filing deadline and miss the payment one.

2. Confusing personal and company tax

Your company's CT600 is separate from your personal self-assessment. If you're a director, you may also need to file a personal tax return — especially if you took dividends.

3. Not keeping receipts

HMRC can ask to see your records for up to 6 years. If you can't evidence an expense, they can disallow it. Start a habit now: photograph every receipt and file it digitally.

4. Forgetting about flat-rate expenses

If you work from home, your company can pay you £6/week (£312/year) tax-free for use of home as office. Many new directors don't know about this.

5. Using the wrong accounting period

Your accounting period for HMRC (Corporation Tax) might differ from your Companies House financial year. Check the dates carefully — HMRC will tell you in their letter.

Do you also need to file with Companies House?

Yes. Your CT600 goes to HMRC, but you also need to file:

Annual accounts with Companies House — within 9 months of your financial year end (for private limited companies)
Confirmation statement — an annual update confirming your company details are correct

These are separate from your CT600. Companies House and HMRC are different organisations with different deadlines.

What if your company made a loss?

If your allowable expenses exceeded your income (i.e. you made a loss), you have options:

Carry the loss forward — offset it against future profits, reducing future tax bills
Carry the loss back — offset it against the previous year's profits (if any) and get a Corporation Tax refund
Surrender the loss — in a group of companies, losses can be transferred between group members

For most new companies making a loss in year one, carrying the loss forward is the simplest option.

Getting help

Free resources

What is a CT600? — our plain English guide
CT600 penalties — what happens if you're late
Allowable expenses — what you can claim
Corporation Tax calculator — estimate your tax bill for free

When to get an accountant

Consider professional help if:

Your company has complex transactions (multiple currencies, investments, R&D)
You're unsure about capital allowances or loss relief
You have employees (PAYE, auto-enrolment pensions)
Your turnover is over £250,000

For straightforward small companies — especially micro-entities and dormant companies — Taxpipe is all you need.

File your first CT600 now →

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Related guides:

Want to pay less tax? 12 Legal Ways to Reduce Corporation Tax →

Corporation Tax for Startups: Complete Guide

Your First Company Tax Return — A New Director's Guide to CT600 Filing