How to file a CT600 yourself (without an accountant)

In short: You can file your own Company Tax Return (the CT600) without an accountant. You need your company's UTR, a Government Gateway login enrolled for Corporation Tax, and your accounts and tax computations in iXBRL format. File within 12 months of your accounting period end, and pay any tax within 9 months and 1 day (GOV.UK). For a straightforward company this is very doable; the hard part is producing the iXBRL accounts, which is what filing software handles for you.

Filing it yourself is realistic for a simple company — a small trading business, a property-holding company, or a dormant one. If your affairs are complex (capital allowances on large assets, group relief, chargeable gains), get an accountant to at least review the figures first.

Can I file a CT600 myself?

Yes. There is no legal requirement to use an accountant. HMRC lets companies file their own Company Tax Return, and thousands of directors do. What you cannot do is file a CT600 as a plain PDF or paper form for most companies — HMRC requires the return, accounts and computations to be submitted online in a tagged iXBRL format. So "filing it yourself" really means using either HMRC's own online service (only suitable for the simplest companies) or commercial filing software that produces the iXBRL for you.

What do I need to file a CT600?

Before you start, gather these:

• Company UTR — the 10-digit Unique Taxpayer Reference HMRC sent when the company registered for Corporation Tax (on the "Notice to deliver a Company Tax Return", form CT603). Not sure where to find it? See what a UTR is and where to find it.
• Government Gateway user ID and password, enrolled for the Corporation Tax online service.
• Company registration number (CRN) from Companies House.
• Your accounting period dates (usually your financial year).
• Your figures: a profit & loss account and a balance sheet for the period. See what a CT600 is and how to file one.

How to file your CT600, step by step

Step 1: Confirm your accounting period and deadlines

Your Corporation Tax accounting period is normally your company's 12-month financial year. Note the two deadlines that flow from its end date: pay within 9 months and 1 day, and file the CT600 within 12 months. The payment deadline comes first — see Corporation Tax deadlines explained. A long first period over 12 months is split into two returns.

Step 2: Prepare your statutory accounts as iXBRL

Your accounts must be tagged to the FRC taxonomy in inline XBRL. Micro-entities use FRS 105; slightly larger small companies use FRS 102 §1A — see FRS 105 vs FRS 102 §1A. This tagging is the step you cannot realistically do by hand; software generates it from your figures.

Step 3: Work out the Corporation Tax

Apply the rate to your taxable profit: 19% up to £50,000, 25% above £250,000, with marginal relief in between (adjusted for associated companies and short periods). Our rates and marginal relief guide shows the maths, or use the free Corporation Tax calculator to check the figure.

Step 4: Complete the CT600 form

Enter your company details, the accounting period, turnover and the computed profit and tax into the CT600 boxes. Attach the iXBRL accounts and computations. Make the director's declaration (CT600 boxes 975/980/985) that the return is correct and complete — that responsibility is yours.

Step 5: Submit to HMRC through the Government Gateway

Sign in with your Government Gateway credentials and submit the CT600 plus accounts and computations. HMRC's transaction engine validates the submission and returns an acceptance (or a rejection with a reason). If it is rejected, fix the flagged issue and resubmit.

Step 6: Pay the tax and keep your records

Pay any Corporation Tax due by the payment deadline (this is separate from filing). Keep copies of everything you filed and your supporting records for at least six years.

Verify it worked

You have filed successfully when HMRC returns an acceptance for the submission (filing software shows this, and it appears in your HMRC online account). Filing is not complete until you see that acceptance — a saved draft is not a filed return.

Should I file a CT600 myself or use an accountant?

File it yourself if your company is straightforward: one trade or a property let, no complex reliefs, figures you understand. Use — or at least involve — an accountant if you have significant capital allowances, group companies, chargeable gains on asset sales, R&D claims, or you're simply not confident in the numbers. Even when filing yourself, a one-off review is cheap insurance, because the declaration and the accuracy of the figures are your legal responsibility.

Filing your CT600 with Taxley

Taxley is built for exactly this: you enter your figures in plain English, and it produces the CT600, the FRS 105/102 iXBRL accounts and the computations, works out the tax (rate, marginal relief and adjustments), and files to HMRC — you review everything and pay only when you file. Start your return and see your Corporation Tax before you pay.

Frequently asked questions

Do I need an accountant to file a CT600?

No. There is no legal requirement to use an accountant — you can file your own Company Tax Return. The figures and the declaration are your responsibility, so for anything beyond a simple case a review is wise.

Can I file a CT600 for free?

HMRC's online service is free but only suitable for the simplest companies, and it still requires your accounts as iXBRL. Most people use low-cost filing software that generates the iXBRL and computations for them.

What happens if I file my CT600 late?

HMRC charges an automatic £100 penalty as soon as the return is late, with further penalties the longer it stays outstanding. Interest also runs on any tax paid late. See Corporation Tax deadlines.

Can I file a dormant company's return myself?

Yes — a dormant company files a nil CT600 and dormant accounts. It's one of the simplest returns to do yourself; see dormant company tax return.