SimpleReturns

How do I register my company for Corporation Tax?

Updated 27 June 2026
The short answer

When you set your company up at Companies House, you're usually registered for Corporation Tax at the same time. But there's one more step that's yours to do: once your company starts doing business, you have to tell HMRC it's active within 3 months. You sign in to your business tax account, give your company's tax reference and a few dates, and that's it. A company that hasn't started trading yet tells HMRC it's dormant instead.

So am I already registered, or not?

A bit of both, and this is where new directors get tangled. When you registered your company at Companies House, you were probably set up for Corporation Tax at the same time. That part may already be done, but it is not the whole job.

The part that's still on you is telling HMRC your company is active, that it has actually started doing business. Being set up for the tax is not the same as telling HMRC you've started, and only you can do that second part. Even if Companies House set the tax up for you, you still have to tell HMRC you're active, and there's a clock on it.

So don't assume "Companies House handled it" means there's nothing left to do. Read on for the one step that's still yours.

When do I have to tell HMRC?

You have 3 months from when your company starts doing business to tell HMRC it's active.

"Starts doing business" means your company has begun trading, anything like selling, buying to sell, providing a service, earning interest, or otherwise bringing money in to make a profit. The day that starts is the day your 3-month clock starts.

The simplest approach: do it as soon as you've started, rather than leaving it. There's nothing to gain by waiting, and it's a short job once you've got your details together.

What if my company hasn't started trading yet?

If you've set the company up but it hasn't done any business at all, no sales, no buying, nothing coming in, then your company is usually dormant for Corporation Tax. A dormant company doesn't register as active, and it doesn't pay Corporation Tax or file a tax return yet.

You still have to tell HMRC it's dormant, though, so they know not to expect a return. When the company later starts trading, that's when the 3-month "tell HMRC you're active" clock begins. If you're not sure which side of the line you're on, that's exactly the kind of thing we'll check with you.

How do I actually tell HMRC my company is active?

You do it online, through your company's business tax account. Here's what's involved:

  1. Sign in to your business tax account. You use a Government Gateway user ID and password. If you don't have one yet, you create one as part of signing in.
  2. Add the Corporation Tax service to that account.
  3. Give HMRC a few details about your company (the next section lists them).

Once that's done, HMRC knows your company is active and trading, and knows when your first set of accounts will cover. You're registered.

What details do I need to hand?

Get these together before you start and it's a five-minute job:

  • Your company registration number. The number Companies House gave you when you set the company up.
  • Your company's Unique Taxpayer Reference (UTR). A 10-digit number HMRC posts to your company's registered office, usually within a few days of you setting the company up. It's on any letter from HMRC. If it hasn't turned up within 15 working days of registering the company, you can ask HMRC for it online.
  • The date your company started doing business. The day trading began.
  • The date your first accounts are made up to. In plain terms, the end date of your company's first year.
  • The nature of your business and where you run it from. A short description of what the company does, and its main address.

That's the lot. You're giving HMRC the basics so they can set your company up for the tax correctly.

Your quick checklist

Before you tell HMRC your company is active, make sure you have:

  • Your company registration number (from Companies House)
  • Your 10-digit company UTR (posted to your registered office; ask HMRC online if it hasn't arrived)
  • The date your company started doing business
  • The date your first accounts run to (your first year-end)
  • A short line on what your company does and its main address
  • A Government Gateway user ID and password (create one if you don't have it)

Do it within 3 months of starting to trade. If the company hasn't started trading, tell HMRC it's dormant instead.

How SimpleReturns fits in

Registering for Corporation Tax is a one-off admin step you do directly with HMRC. It's free, and it's quick, so there's no need to pay anyone for that bit, and we don't do it for you.

What we do is everything that comes after. Once your company is registered and the year is done, you connect your bank or upload a statement, we work out your figures, prepare your accounts and tax return, and file them, for £99, once, no subscription. You never have to work out the tax yourself.

Common questions

Isn't my company automatically registered for Corporation Tax when I set it up?

Usually you're set up for the tax at the same time as registering at Companies House. But you still have to tell HMRC separately once your company starts doing business, within 3 months. The two aren't the same step.

How long do I have to register?

Three months from the day your company starts doing business. The sensible move is to do it as soon as you've started trading.

What if my company hasn't started trading yet?

Then it's usually dormant for Corporation Tax. You don't register as active; you tell HMRC it's dormant instead, and the 3-month clock only starts when you begin trading.

Where do I find my company UTR?

HMRC posts it to your company's registered office, usually within a few days of setting the company up, and it's on any letter from HMRC. If it hasn't arrived within 15 working days, you can ask HMRC for it online.

Do I need a Government Gateway account?

Yes. You sign in to (or create) a business tax account with a Government Gateway user ID and password, then add the Corporation Tax service to it.

Does SimpleReturns register my company for me?

No, that one step is yours to do with HMRC, and it's free. Once you're registered, we do the figures and file your accounts and tax return for £99.

Once you're registered, leave the rest to us

Telling HMRC your company is active is a quick, free job you do directly. After that, you never have to work out the tax. We read your year's money in and out, prepare your accounts and tax return, show you every figure, and file them, for £99, once, no subscription.

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SimpleReturns handles straightforward small companies. If you're an LLP, a charity, a community interest company, or you're claiming a specialist relief like R&D, an accountant is the right call, and that's an honest one to make.

General guidance, not advice. This guide explains how the rules generally work for small UK limited companies. It isn't tax advice for your specific situation, if you're unsure, check with us or an accountant.