HMRC Corporation Tax penalties can turn a small oversight into a big expense. File your CT600 just one day late, and you're immediately £100 poorer — even if your company owes no tax at all.

Here's exactly what happens when you miss your CT600 deadline, how much it costs, and what you can do about it.

The penalty timeline

HMRC penalties follow a strict timeline. There's no grace period, no warnings, and no "we'll let it slide this once."

Day 1: £100 penalty

Miss your deadline by even one day, and HMRC automatically issues a £100 penalty. This applies to every company, even if:

You have no Corporation Tax to pay
Your company is dormant
You made a loss

3 months late: Another £100 penalty

At exactly 3 months after your deadline, HMRC adds another £100. You're now £200 down.

6 months late: 10% of unpaid tax (minimum £500)

This is where penalties get serious. HMRC estimates your Corporation Tax liability and charges 10% of that estimate as a penalty.

Minimum penalty: £500

Even if your actual tax bill is zero, you'll pay at least £500.

12 months late: Another 10% penalty

HMRC adds another 10% of the estimated tax. Plus everything that came before.

Real penalty examples

Let's see what this means in practice:

Example 1: Dormant company (£0 tax due)

1 day late: £100
3 months late: £100 + £100 = £200
6 months late: £200 + £500 = £700
12 months late: £700 + £500 = £1,200

Total penalties for a company with zero tax liability: £1,200

Example 2: Small trading company (£2,000 tax due)

1 day late: £100
3 months late: £200
6 months late: £200 + £500 (minimum) = £700
12 months late: £700 + £500 = £1,200

Note: 10% of £2,000 is only £200, but the minimum penalty is £500.

Example 3: Profitable company (£15,000 tax due)

1 day late: £100
3 months late: £200
6 months late: £200 + £1,500 (10% of £15,000) = £1,700
12 months late: £1,700 + £1,500 = £3,200

When you can appeal penalties

HMRC accepts appeals based on "reasonable excuse." What counts:

Reasonable excuses HMRC accepts:

Serious illness (you or close family)
Computer/software failure (if you can prove it)
Postal delays (with Royal Mail proof)
Fire, flood, or theft of essential records

What HMRC doesn't accept:

"I forgot the deadline"
"I was too busy"
"My accountant didn't remind me"
"I didn't know about the deadline"
"I couldn't afford to pay"

How to avoid penalties altogether

1. Know your exact deadline

Check your accounting period end date and add 12 months. Set multiple calendar reminders.

2. File early

Don't wait until the deadline. File 2-3 weeks early to allow for any problems.

3. Use filing software

Good software tracks deadlines and reminds you automatically.

At Taxpipe, we automatically track your deadlines and send reminders well before they're due. Our system won't let you file late accidentally, and filing costs just £59 — a fraction of even the smallest penalty.

HMRC penalties are harsh but predictable. Miss your deadline by one day, pay £100. Miss it by a year, pay thousands. The choice is yours, but the calendar doesn't care about your excuses.

File on time, every time. Your bank account will thank you.

Don't risk penalties — file now

Filing late costs at least £100. Filing with Taxpipe takes about 15 minutes and costs £59. Do the maths.

File your CT600 now →

CT600 Penalties: What Happens If You File Late?