Filing a CT600 isn't difficult — but there are a few traps that catch company directors every year. Here are the 10 most common mistakes and how to avoid them.
1. Using the wrong accounting period dates
The mistake: Entering dates that don't match what HMRC expects. Your accounting period for Corporation Tax may not be the same as your Companies House financial year — especially in your first year.
How to avoid it: Check the dates on HMRC's "notice to deliver" letter. If you're unsure, log in to your HMRC business tax account and verify the periods HMRC has on record.
Why it matters: If the dates are wrong, HMRC will reject the return. You'll need to refile with the correct dates, wasting time and risking late filing penalties.
2. Missing the payment deadline
The mistake: Assuming the payment deadline is the same as the filing deadline. It's not.
- Payment deadline: 9 months and 1 day after accounting period end
- Filing deadline: 12 months after accounting period end
The payment deadline comes first.
How to avoid it: Set a calendar reminder for both dates. Calculate your estimated tax and pay it before the 9-month deadline, even if you haven't filed yet. You can always adjust later.
Why it matters: Late payment triggers interest charges from day one. At current rates, a £10,000 bill paid 3 months late costs roughly £190 in interest.
3. Not filing for a dormant company
The mistake: Thinking that because your company didn't trade, you don't need to file a return.
How to avoid it: If your company is registered with Companies House and HMRC knows about it, you need to file a CT600 — even if every box is zero. The only exception is if HMRC has specifically agreed to treat the company as dormant.
Why it matters: Automatic penalties start at £100 for being one day late. After 3 months, another £100. You could pay £200+ in fines for not filing a return that says "nothing happened."
See our dormant company filing guide for the full walkthrough.
4. Claiming client entertainment as an expense
The mistake: Deducting meals, drinks, or events with clients as a business expense.
How to avoid it: Never include client entertainment in your CT600 deductions. HMRC specifically disallows it — no exceptions. Staff entertainment (like a Christmas party) is allowed up to £150 per person per year, but client entertainment never is.
Why it matters: If HMRC enquires into your return and finds disallowed entertainment expenses, they'll adjust your tax bill upwards and may charge penalties for inaccuracy.
5. Forgetting to include all income
The mistake: Only reporting trading income and forgetting about bank interest, rental income, or one-off asset sales.
How to avoid it: Go through all your bank statements line by line. Look for:
- Interest received (even small amounts)
- Income from property or investments
- Profit from selling assets (capital gains)
- Any other money received by the company
Why it matters: HMRC cross-references data from banks and other sources. Undeclared income can trigger an enquiry, penalties, and interest on underpaid tax.
6. Getting the turnover box wrong
The mistake: Confusing turnover (Box 145) with profit. Turnover is your total sales revenue, not your profit after expenses.
How to avoid it: Turnover = total money in from trading activities. Don't subtract expenses first — that's what the other boxes are for.
For example, if your company invoiced clients £80,000 and spent £30,000 on expenses:
- Turnover (Box 145): £80,000 (not £50,000)
- Trading profit (Box 155): £50,000
Why it matters: Understating turnover can make your return look suspicious. HMRC may ask questions if your turnover seems too low for your industry.
7. Not keeping proper records
The mistake: Throwing away receipts, not recording expenses properly, or mixing personal and business transactions.
How to avoid it:
- Keep all receipts (photograph them immediately — paper fades)
- Use a separate bank account for the company
- Record every transaction with a date, amount, and description
- Keep records for 6 years after the end of the accounting period
Why it matters: If HMRC opens an enquiry, you need to prove every figure on your return. No receipt = no deduction. HMRC can also charge penalties for inadequate record keeping.
8. Filing with the wrong UTR
The mistake: Using your personal UTR instead of the company UTR, or transposing digits.
How to avoid it: Your company's UTR is a 10-digit number, separate from any personal UTR you may have. It's on the letter HMRC sent when you registered for Corporation Tax. Double-check every digit before submitting.
Why it matters: The return will be filed against the wrong entity, meaning your company's record shows as unfiled. You'll get penalty notices while someone else has a mysterious extra return.
9. Missing capital allowances
The mistake: Not claiming capital allowances on equipment, vehicles, or other assets the company purchased.
How to avoid it: Any significant purchase (computer, vehicle, machinery, office furniture) should be claimed through capital allowances, not deducted as a simple expense. The Annual Investment Allowance (AIA) lets you deduct 100% of qualifying purchases up to £1 million per year.
Why it matters: If you bought a £2,000 laptop and didn't claim it, you've overpaid Corporation Tax by up to £500 (at 25% rate). Capital allowances are one of the biggest tax savings available to small companies.
10. DIY-ing a complex return
The mistake: Trying to file a complicated CT600 yourself when your company has R&D credits, group relief, associated companies, foreign income, or complex capital transactions.
How to avoid it: Know your limits. If your company is straightforward — one director, UK trading income, simple expenses — you can absolutely file yourself using software like Taxpipe. But if your tax situation involves any of the following, consider an accountant:
- Research & Development (R&D) tax relief claims
- Group relief or consortium relief
- Transfer pricing adjustments
- Foreign income or overseas branches
- Complex capital gains (goodwill, intellectual property)
- Substantial capital allowances (plant, machinery, vehicles)
Why it matters: Complex returns have higher stakes. Getting R&D relief wrong can mean losing thousands in tax savings or, worse, a penalty for an incorrect claim.
Bonus: the "I'll do it later" trap
This isn't a CT600 mistake per se — it's a human one. Corporation Tax deadlines feel far away (12 months to file!), so it's easy to procrastinate. Then suddenly it's 11 months later and you're scrambling.
The fix: File early. There's no benefit to waiting. File as soon as your accounting period ends and your figures are ready. With Taxpipe, it takes about 15 minutes.
How Taxpipe helps you avoid these mistakes
Taxpipe's guided wizard is designed to prevent the most common errors:
- Date validation — we check your accounting period dates match HMRC's expected format
- Auto-computation — we calculate Corporation Tax, marginal relief, and capital allowances automatically
- Plain English questions — no need to know box numbers; we ask what you earned and spent in normal language
- Built-in checks — we validate your figures before submission to catch inconsistencies
- Direct HMRC submission — no risk of sending the form to the wrong place
All for a one-time fee of £59, with free resubmission if HMRC rejects the return.
Related articles:
- How to Amend a CT600 — Correcting Mistakes on Your Corporation Tax Return
- Directors' Loan Accounts and Your CT600 — What Every Director Needs to Know
- Annual Investment Allowance (AIA) Guide
- Dividends vs Salary: Tax-Efficient Pay
Related guides:
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- HMRC Enquiry Into Your CT600: What to Expect
- Corporation Tax Interest: How to Avoid Charges
- CT600 Supplementary Pages Explained
- HMRC Rejected Your CT600? Common Errors and How to Fix Them
- Accountant vs DIY CT600 Filing: Honest Comparison