SimpleReturns

Is it safe to file my own Corporation Tax?

Updated 29 June 2026
The short answer

Yes. You are allowed to file your own Corporation Tax, and nothing in the law says you must pay an accountant to do it. A director can file directly. The parts that go wrong on their own are the tax sums, the special HMRC file format, and the dates. Good software does those three parts for you and shows you every figure before anything is sent, which is what makes doing it yourself safe.

Am I actually allowed to file it myself?

Yes. HMRC lets you choose: get an accountant to prepare and file your return, or do it yourself. There is no rule that forces you to use an accountant. Plenty of small companies file their own. So the real question is not "am I allowed", it's "how do I do it without making a mistake". That part has a clear answer too.

What does filing it yourself actually involve?

It sounds like one job. It's really four small ones, and they happen in order:

  • Prepare the accounts. A short, standard set of figures showing what your company earned and spent over the year. For a small company this is the simple "micro" version.
  • Work out the tax adjustments. Your profit in the accounts is not the same as the profit you pay tax on. A few costs get added back, a few reliefs come off. The result is called the computation.
  • Tag both in HMRC's file format. HMRC won't accept a plain document. The accounts and the computation have to be turned into a special tagged format called iXBRL, which is just a way of labelling each figure so HMRC's computers can read it.
  • Send it in. The tax return (the CT600 form, plus the computation and accounts) goes to HMRC, and a copy of your accounts goes to Companies House. That's two filings to two places, not one.

One more thing changed recently: HMRC's old free online filing tool closed at the end of March 2026, so the tax return now goes to HMRC through approved software rather than that old website.

So what's actually risky about doing it raw?

Filing yourself goes wrong in three predictable places, and they're the same three every time:

  1. The tax adjustments. Getting the computation wrong means you either overpay or file a figure HMRC later questions. This is the bit with the rules a first-timer doesn't know.
  2. The iXBRL tagging. Doing this by hand is fiddly and easy to get wrong, and HMRC rejects a file that isn't tagged properly.
  3. The deadlines. There are two of them, and people mix them up. You pay the tax 9 months and 1 day after your year ends, but you file the return up to 12 months after. Miss the filing date and HMRC charges a penalty that grows the longer you leave it.

None of these is about being clever. They're about knowing the rules and not slipping up. That's exactly the kind of work a computer is good at and a tired human is bad at.

How does software make it safe?

This is the whole point. You don't have to learn the three hard parts. The software does them:

  • It builds your accounts and works out the tax adjustments for you, so the computation is right.
  • It produces the iXBRL file automatically, so there's nothing to tag by hand.
  • It tracks both deadlines and files the return to HMRC and the accounts to Companies House for you.

And it shows you every figure before anything is sent. You read it in plain English, check it looks right, and approve it. Filing yourself is safe when you can see exactly what's going off and nothing is hidden.

A plain example

Say your company made £40,000 profit in its accounts. During the year it spent £500 taking clients to lunch. The tax rules don't let that £500 reduce your tax, so it gets added back:

For example

£40,000 profit + £500 added back = £40,500 to be taxed.
Corporation Tax at 19% = £7,695.

That "add the £500 back" step is a tax adjustment. Miss it and your figure is wrong; do it by hand and it's easy to forget. The software spots it, makes the change, and shows you the £7,695 so you can see how it got there before you approve it. (Real returns can have several of these little steps. You don't work them out, you check them.)

When is an accountant the better call?

Honestly, sometimes they are, and it's worth saying. If your company is part of a group, owns a trading business plus property, has complicated director's loans, or your own salary and dividend mix needs planning, an accountant can be the right fit. We'd rather tell you that than sell you something that doesn't suit you. For a straightforward small company filing a normal year, doing it yourself with software is safe and a lot cheaper.

How SimpleReturns handles it

Connect your bank or upload a statement. We build your accounts, work out the tax adjustments, create the iXBRL file, and file to both HMRC and Companies House for you. You see every figure first and approve it. The hard, risky parts are the parts we do.

Common questions

Do I legally have to use an accountant to file Corporation Tax?

No. You can get an accountant to do it or file it yourself. There's no rule that says you must pay one.

Can a company director file the Corporation Tax themselves?

Yes. You can file your own company tax return directly with HMRC, on your own or using software.

What do I actually have to send, and to whom?

A tax return (the CT600 form, the computation and your accounts) goes to HMRC, and a copy of the accounts goes to Companies House. Two filings, two places.

Why can't I just upload a normal document to HMRC?

HMRC needs the figures in a special tagged format called iXBRL so its computers can read them. Software makes that file for you, so you never tag anything by hand.

What happens if I get a deadline wrong?

You pay the tax 9 months and 1 day after your year ends, and file the return up to 12 months after. Miss the filing date and HMRC charges a penalty that grows the longer it is late, so the safe move is software that tracks both dates for you.

Is it risky to do it myself?

Only in three places: the tax sums, the iXBRL format, and the dates. Software does all three and shows you the result to check, which takes the risk out.

Ready to do it the safe way?

You don't need to understand any of the above to file. We do the accounts, the tax adjustments and the iXBRL tagging, file to HMRC and Companies House, and show you every figure before anything is sent, for £99, once, no subscription.

Start your return →

Or, if your company is more complex, like a group or with tricky director's loans, an accountant may be the better fit, and that's an honest call 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.