Filing Your First CT600: A Step-by-Step Guide for New Companies
You've just set up a UK limited company. Congratulations. Now comes the part nobody tells you about: Corporation Tax.
Every UK limited company must file a CT600 return with HMRC — even if you made no profit, had no income, or did nothing at all. Here's how to get it right the first time.
When Do You Need to File?
Registration
HMRC should automatically register your company for Corporation Tax when Companies House notifies them of your incorporation. You'll receive a letter with your Unique Taxpayer Reference (UTR) — a 10-digit number you'll need for filing.
If you haven't received your UTR within 2-3 weeks of incorporation, contact HMRC or register manually via the Government Gateway.
Your First Accounting Period
Your first accounting period starts on the date of incorporation and typically ends on the date you choose as your year-end. For most companies, this is:
- 31 March (aligns with the tax year)
- 31 December (calendar year)
- The last day of the month you incorporated (Companies House default)
Your first accounting period can be up to 18 months long. If it's longer than 12 months, you'll need to file two CT600 returns (HMRC doesn't accept returns for periods longer than 12 months).
Filing Deadline
Your CT600 is due 12 months after the end of your accounting period. Your Corporation Tax payment is due earlier — 9 months and 1 day after your accounting period ends.
| What | When |
|---|---|
| First accounting period ends | Your chosen year-end |
| Tax payment due | 9 months + 1 day after period end |
| CT600 filing due | 12 months after period end |
What You'll Need
Before you start, gather:
- UTR — your 10-digit Unique Taxpayer Reference
- Company Registration Number (CRN) — from Companies House
- Government Gateway credentials — for online filing
- Accounts — your company's financial records for the period
- Bank statements — to verify income and expenses
- Receipts — for any business expenses you're claiming
If You've Done Nothing (Dormant Company)
If your company hasn't traded, received income, or had any financial activity, you're classed as dormant. You still need to file a CT600, but it's straightforward — most boxes will be zero.
Step-by-Step: Filing Your First CT600
Step 1: Prepare Your Accounts
You need to know your:
- Turnover (total sales/revenue)
- Expenses (costs of running the business)
- Profit or loss (turnover minus expenses)
- Capital allowances (if you bought equipment, vehicles, or other assets)
For a simple company, this might just be your bank transactions categorised into income and expenses.
Step 2: Calculate Your Corporation Tax
The current rates (from April 2023):
| Profit | Rate |
|---|---|
| Up to £50,000 | 19% (small profits rate) |
| £50,001 – £250,000 | Marginal relief (effective rate between 19-25%) |
| Over £250,000 | 25% (main rate) |
If you have associated companies (companies controlled by the same people), the thresholds are divided by the number of associated companies.
Example: £30,000 profit → £5,700 Corporation Tax (19%)
Step 3: Choose Your Filing Method
Since April 2011, all CT600 returns must be filed electronically. From April 2026, HMRC's own free filing service is closing, so you'll need commercial software.
Taxpipe is designed specifically for directors filing their own CT600:
- Guided wizard walks you through every box
- Automatic tax calculation including marginal relief
- iXBRL accounts generated automatically
- Direct submission to HMRC
- £59 per return
Step 4: Complete the CT600
The CT600 has over 200 boxes, but most companies only need to fill in about 20-30. The key sections:
| Section | What It Covers |
|---|---|
| Company info | Name, CRN, UTR, accounting period |
| Income | Turnover, trading profits |
| Deductions | Expenses, capital allowances |
| Tax calculation | Corporation Tax due |
| Declaration | Director signs off |
Software like Taxpipe shows you only the relevant boxes and explains each one in plain English.
Step 5: Attach Your Accounts
Your CT600 must include iXBRL accounts — a machine-readable version of your company's financial statements. This sounds technical, but filing software generates these automatically.
For micro-entities (most small companies), the accounts are a simplified balance sheet and profit & loss statement.
Step 6: Submit to HMRC
Electronic submission takes seconds. You'll receive:
- An acknowledgement with a correlation ID
- HMRC processes the return (usually within hours)
- A success or rejection response
If rejected, HMRC provides error codes explaining what needs fixing.
Step 7: Pay Your Corporation Tax
Remember, payment is due 9 months and 1 day after your accounting period ends. Payment methods:
- Direct Debit (set up through Government Gateway)
- Online banking (using your payment reference)
- CHAPS or Bacs transfer
Your payment reference is your UTR followed by the relevant accounting period end date.
Common First-Time Mistakes
1. Missing the Deadline
Your first deadline might be sooner than you think, especially if your first accounting period is short. Set a calendar reminder.
2. Not Filing Because "Nothing Happened"
Dormant companies must file. Zero-profit companies must file. The only companies that don't file CT600s are those that have been struck off or are in the process of being dissolved.
3. Forgetting Capital Allowances
If you bought a laptop, phone, desk, or any other business asset, you can claim capital allowances. The Annual Investment Allowance lets you deduct the full cost of qualifying assets (up to £1 million per year) from your profits.
4. Using Personal Expenses
Only genuine business expenses can be deducted. HMRC is strict about this — if an expense has a personal element, you may need to apportion it.
5. Not Keeping Records
HMRC requires you to keep records for at least 6 years after the end of the accounting period. Digital records are fine — just make sure they're backed up.
Frequently Asked Questions
Do I need an accountant for my first CT600?
No. Many directors with simple companies file their own CT600 using commercial software. If your company has complex affairs (multiple trades, international income, R&D claims), an accountant may be worthwhile.
What if my company made a loss?
You still file a CT600, but you'll owe no Corporation Tax. You can carry the loss forward to offset against future profits, potentially reducing your tax bill in later years.
Can I change my accounting year-end?
Yes. You can shorten your accounting period at any time or extend it once every 5 years (with some exceptions). Your first period can be any length up to 18 months.
What's the difference between CT600 and annual accounts?
The CT600 is your Corporation Tax return — it goes to HMRC. Annual accounts go to Companies House. They're related but separate filings with different deadlines.
Ready to file your first CT600? Sign up for Taxpipe — we'll guide you through every step. £59, no subscription, no accountant needed.