UK Company Tax Return: The Complete Beginner's Guide (2025)
Just incorporated a limited company? You'll need to file a Company Tax Return (form CT600) with HMRC every year. Here's everything you need to know, in plain English.
What Is a Company Tax Return?
The CT600 is the form your limited company uses to tell HMRC:
- How much profit you made
- How much Corporation Tax you owe
- What reliefs you're claiming
Every UK limited company must file one, even if you made no profit or didn't trade.
When Do I Need to File?
- First return: Due 12 months after the end of your first accounting period
- Subsequent returns: Due 12 months after each accounting period end
Your first accounting period usually runs from incorporation to your accounting reference date (ARD) — which Companies House sets as the last day of the month you incorporated.
Watch out for long first periods. A CT600 can only cover a maximum of 12 months. If you incorporate part-way through a month, your first accounting period will be slightly longer than 12 months — and HMRC requires you to file two separate CT600s (one for the first 12 months, one for the remainder), apportioning your figures between them.
Example: Incorporated 15 June 2025 → ARD is 30 June → First accounting period runs 15 June 2025 to 30 June 2026 (12.5 months). Because this exceeds 12 months, you must file two CT600s:
- Period 1: 15 June 2025 to 14 June 2026 → CT600 due by 14 June 2027
- Period 2: 15 June 2026 to 30 June 2026 → CT600 due by 30 June 2027
What Information Do I Need?
Before you start, gather:
| Document | Why |
|---|---|
| Company accounts | Your profit & loss and balance sheet |
| Bank statements | To verify income and expenses |
| Receipts and invoices | Evidence of business expenses |
| PAYE records | If you have employees (including yourself) |
| UTR number | Your Unique Taxpayer Reference (10 digits) |
| Company registration number | Your Companies House number |
Step-by-Step Process
1. Register for Corporation Tax
You should have done this within 3 months of starting to trade. If not, do it now via HMRC's service.
2. Prepare Your Accounts
Your annual accounts show your company's financial position. For most small companies, this means:
- Profit & Loss: Income minus expenses = profit (or loss)
- Balance Sheet: Assets minus liabilities = net assets
3. Calculate Corporation Tax
Apply the correct rate to your taxable profits:
- Under £50,000: 19%
- £50,000-£250,000: 19%-25% (marginal relief)
- Over £250,000: 25%
4. File the CT600
HMRC's own free CT600 filing service closed on 31 March 2026. You can now file:
- Through commercial software like Taxpipe
- Through your accountant
5. Pay the Tax
Payment is due 9 months and 1 day after your accounting period ends — 3 months before the filing deadline.
What Are Allowable Expenses?
You can deduct business expenses from your income before calculating tax:
- ✅ Office costs (rent, utilities, internet)
- ✅ Staff costs (salaries, employer NIC, pensions)
- ✅ Travel costs (business travel only)
- ✅ Professional fees (accountant, solicitor)
- ✅ Marketing costs
- ✅ Software and subscriptions
- ✅ Insurance
- ❌ Entertaining clients (not deductible)
- ❌ Clothing (unless uniforms/PPE)
- ❌ Fines and penalties
Common First-Timer Mistakes
- Missing the deadline — for returns due on or after 1 April 2026, the initial late-filing penalty is £200, rising by a further £200 if still late at 3 months. Tax-geared penalties (10% of unpaid tax) apply at 6 months and again at 12 months.
- Not filing because you didn't trade — you must file even if dormant
- Claiming personal expenses — only genuine business costs are deductible
- Forgetting to pay — the payment deadline is BEFORE the filing deadline
- Not keeping records — HMRC requires 6 years of records
Do I Need an Accountant?
For a simple company (one director, few transactions), you can absolutely file yourself using software like Taxpipe. Consider an accountant if:
- You have complex tax affairs (R&D credits, capital gains, group relief)
- You have employees beyond just yourself
- You want tax planning advice
- You simply don't have time
First time filing? Taxpipe makes it easy — we guide you through every step. £59, no jargon, no subscription.