Can I file my own CT600?

Yes. You can file your own CT600 corporation tax return. You do not need to be an accountant, and you do not need to hire one for a straightforward limited company return. Most single-director companies have finances simple enough to manage this themselves.

The CT600 is HMRC's corporation tax return form. Every UK limited company must file one for each accounting period, along with statutory accounts and a computation of how the tax was calculated. Since HMRC's free CATO filing tool closed on 31 March 2026, you need commercial software to do it.

What does filing a CT600 involve?

Filing a CT600 has four parts:

1. Categorise your transactions. Every payment in and out of your business needs to be categorised as income, allowable expense, disallowable expense, or capital expenditure.
2. Compute your taxable profit. Start with your accounting profit, add back disallowable expenses, subtract capital allowances, and apply the correct tax rate.
3. Prepare statutory accounts. A simple profit and loss account and balance sheet in iXBRL format.
4. Submit to HMRC and Companies House. The CT600 goes to HMRC via their API. Your accounts go to Companies House separately.

What is a CT600 actually asking?

The CT600 asks you to report your company's income, allowable deductions, capital allowances, and the resulting tax. The main boxes you need are:

Software like NVC Comply calculates all of these from your categorised transactions. You review the numbers and confirm before filing. You do not need to know which box maps to what.

What are allowable expenses?

Allowable expenses reduce your taxable profit. The main categories for a single-director company are:

• Director salary (PAYE)
• Employer National Insurance contributions
• Pension contributions to a company pension scheme
• Office rent, utilities, and business rates
• Software subscriptions used for business
• Professional fees (accountant, solicitor)
• Business travel and accommodation (not commuting)
• Marketing and advertising
• Business insurance
• Bank charges on business accounts

Disallowable expenses are costs that go through your P&L but cannot be deducted for tax purposes. Common examples are client entertainment, fines and penalties, and the depreciation charge (you claim capital allowances instead).

What about capital allowances?

When you buy equipment, computers, or vehicles for business use, you cannot deduct the full cost as an expense. Instead, you claim capital allowances. The main one for small companies is the Annual Investment Allowance (AIA), which lets you deduct 100% of qualifying capital expenditure up to £1 million per year.

So if you bought a £3,000 laptop for the business, you claim £3,000 in capital allowances (not a depreciation charge). This reduces your taxable profit by £3,000.

What corporation tax rate do I pay?

From 1 April 2023, corporation tax rates are:

• 19% if your taxable profit is £50,000 or below (small profits rate)
• 25% if your taxable profit is £250,000 or above (main rate)
• Between 19% and 25% if your profit is between £50,000 and £250,000 (marginal relief applies)

Most single-director service companies have taxable profits well below £50,000 and pay the flat 19% rate. NVC Comply calculates marginal relief automatically if you fall in the middle band.

When do I need to pay?

• CT600 filing deadline: 12 months after your accounting period end
• Corporation tax payment deadline: 9 months and 1 day after your accounting period end

Note that the payment deadline is earlier than the filing deadline. If your year ends 31 March 2026, tax is due by 1 January 2027, but the CT600 is due by 31 March 2027. Budget for the payment before the filing.

When should I use an accountant instead?

DIY filing is suitable for most straightforward limited companies. Consider an accountant if your company has any of the following:

• R&D tax credits (requires specialist preparation)
• Controlled foreign companies or overseas income
• Complex group structures or group relief
• A VAT dispute or HMRC enquiry in progress
• Property income alongside trading income
• Large capital transactions (land, buildings)

If none of these apply, and your company earns money from selling services or products and has normal running costs, you can almost certainly file yourself.