Can I File CT600 Myself?
Yes — as a company director, you can file your own company tax return without using an accountant. HMRC does not require professional involvement to prepare or submit the CT600 form, and many directors of small limited companies handle their own filing each year.
Understanding what's involved before you start matters. The CT600 covers your company's profits, tax calculations, capital allowances, and any reliefs you're claiming. Getting it right means knowing what information you need, which software to use, and how to interpret your accounts correctly.
What Is the CT600?
The CT600 is the official name for the company tax return — also called the corporation tax return. Every UK limited company must submit one to HMRC after the end of each accounting period, regardless of whether the company made a profit, broke even, or operated at a loss. Even dormant companies often need to file.
The form reports your company's taxable profit, calculates the corporation tax due, and records any reliefs or credits you're claiming. For a breakdown of what the form contains and how each section works, see CT600 boxes explained.
The deadline for filing is 12 months after the end of your accounting period.
What Do You Need to File?
Before starting your CT600, gather the following:
- Your company's UTR (Unique Taxpayer Reference) — issued by HMRC when you registered for corporation tax
- HMRC online account credentials — or access via your filing software's gateway
- Company accounts — a profit and loss account and balance sheet covering the accounting period
- Accounting period dates — the exact start and end dates you're reporting on
- Tax calculation — the amount of corporation tax owed based on your taxable profit
- Details of any reliefs — such as capital allowances, R&D credits, or losses brought forward from previous years
Can I Use Free Software?
Yes — free options exist for straightforward cases. Some software providers offer free filing for simple companies with basic profit and loss accounts and no complex reliefs. See our guide to free CT600 filing options for what's currently available.
More complex situations — such as companies with capital allowances on multiple asset pools, R&D claims, or marginal relief calculations — typically require paid software.
When Does Self-Filing Make Sense?
Handling your own CT600 works well when:
- Your company has simple finances — straightforward trading income and basic allowable expenses
- You already use accounting software (such as Xero or QuickBooks) that can export data in a format compatible with CT600 filing tools
- You're comfortable reading a profit and loss account and understanding what is and isn't deductible for tax
- Your company is dormant — dormant company returns are the simplest type and well-suited to self-filing
- You're willing to invest some time in understanding the process
When Should You Use a Professional?
Some situations benefit from specialist input:
Marginal relief — if your company's taxable profit falls between £50,000 and £250,000, a marginal relief fraction applies that gradually raises the effective rate from 19% to 25%. Getting this calculation wrong means overpaying or underpaying tax.
Associated companies — if your company has associated companies (broadly, those under common ownership or control), the profit thresholds are divided between them. Two associated companies each have thresholds of £25,000 and £125,000, not £50,000 and £250,000. Applying the wrong thresholds changes your tax rate.
R&D tax credits — these are valuable but require careful identification of qualifying expenditure. Overclaiming risks an HMRC enquiry; underclaiming means leaving money behind.
Extended accounting periods — if your accounting period is longer than 12 months (common for newly incorporated companies), HMRC requires two separate CT600 submissions with profit apportioned between them.
Complex capital allowances — plant and machinery, vehicles, and commercial property each have specific rules around annual investment allowance (AIA) and writing down allowances (WDA).
Corporation Tax Rates
Current rates from GOV.UK, effective from 1 April 2023:
| Taxable profit | Rate |
|---|---|
| Up to £50,000 | 19% (small profits rate) |
| £50,001 – £250,000 | Marginal relief — effective rate between 19% and 25% |
| Over £250,000 | 25% (main rate) |
Deadlines You Must Know
Two separate deadlines apply:
| Deadline | When |
|---|---|
| Corporation tax payment | 9 months and 1 day after the accounting period ends |
| CT600 filing | 12 months after the accounting period ends |
Late filing penalties (from GOV.UK):
| How late | Penalty |
|---|---|
| 1 day | £200 |
| 3 months | Additional £200 |
| 6 months | 10% of unpaid tax |
| 12 months | Further 10% of unpaid tax |
How to File the CT600 Yourself
- Register for corporation tax — if you haven't already, register online via HMRC. You'll receive a UTR within a few weeks.
- Prepare your accounts — produce a profit and loss account and balance sheet for the accounting period.
- Calculate your taxable profit — adjust your accounting profit for any disallowable expenses (client entertaining, fines) and add back depreciation. You then deduct capital allowances in its place.
- Claim your allowances and reliefs — capital allowances on equipment, trading losses from prior years, and any other applicable credits.
- Calculate the tax — apply the appropriate rate or marginal relief calculation.
- Choose filing software — use HMRC-recognised software that supports online CT600 submission. Enter your company details, complete each box, and attach your accounts as an iXBRL file.
- Submit the CT600 — online, using your company's UTR and HMRC gateway credentials.
- Pay your corporation tax — on or before the payment deadline, via bank transfer to HMRC.
Common Mistakes Directors Make When Self-Filing
- Including non-deductible expenses — client entertaining, penalties, and certain legal costs are not deductible for corporation tax even if they appear in your accounts
- Confusing payment and filing deadlines — many directors don't realise the tax must be paid three months before the CT600 is due; interest starts from the payment deadline
- Incorrect accounting period dates — a newly incorporated company's first accounting period can be shorter or longer than expected, which affects deadlines and thresholds
- Not filing during a loss year — even if your company owes no tax, you must still file the return
- Forgetting to attach iXBRL accounts — the CT600 must be submitted with your company accounts in iXBRL format; omitting them causes the submission to be rejected
Summary
You can file the CT600 yourself — HMRC has no requirement for professional involvement. For simple companies with straightforward finances, self-filing is a manageable task, particularly if you're already using accounting software. For situations involving marginal relief, associated companies, R&D credits, or extended accounting periods, professional help is usually worthwhile. Start with the checklist, use recognised software, and keep an eye on both the payment and filing deadlines.