CT600 Filing for First-Time Directors — Complete Beginner Guide

What Is a CT600?

A CT600 is the corporation tax return that every UK limited company must file with HMRC. It tells HMRC how much profit your company made and how much corporation tax you owe.

If you've just set up a limited company — whether for contracting, freelancing, or running a small business — this guide explains everything you need to know about your first CT600 filing.

Do I Need to File a CT600?

Yes, if you have a UK limited company. This includes:

• Active trading companies
• Dormant companies that have received a notice to deliver from HMRC
• Property investment companies
• Holding companies

The exception is if your company is dormant and you have notified HMRC of this. Once HMRC confirms your company is dormant for corporation tax purposes, no CT600 is due unless HMRC later sends you a formal notice to deliver one. If you haven't told HMRC your company is dormant, HMRC may issue a notice — and you must file.

When Do I Need to File?

Two deadlines matter:

1. Filing deadline: 12 months after the end of your accounting period
2. Payment deadline: 9 months and 1 day after the end of your accounting period

Example: If your accounting period ends 31 March 2026:

• File your CT600 by 31 March 2027
• Pay your corporation tax by 1 January 2027

Notice the payment deadline comes first. Many first-time directors get caught out by this.

What Do I Need to Prepare?

Company Details

• Company Registration Number (CRN): 8-digit number from Companies House (e.g., 12345678)
• Unique Taxpayer Reference (UTR): 10-digit number from HMRC (e.g., 1234567890)
• Accounting period: Usually matches your company's financial year

Financial Information

You need to know your company's:

• Total turnover (all sales/income before deductions)
• Cost of sales (direct costs of delivering your services/products)
• Administrative expenses (rent, insurance, software, phone, travel etc.)
• Director's salary and employer's NI (if applicable)
• Bank interest received
• Capital purchases (computers, equipment, vehicles)

Where to Find Your Numbers

• Bank statements: Total up income and categorise expenses
• Invoices: Both issued (income) and received (expenses)
• Payroll records: If you run payroll for yourself or employees
• Previous CT600: If this isn't your first filing

Understanding Corporation Tax Rates (2024-25)

The rate you pay depends on your taxable profits:

Marginal relief means companies with profits between £50,000 and £250,000 pay an effective rate between 19% and 25%. The exact amount is calculated using a formula — Taxpipe's calculator does this for you automatically.

If your company has associated companies (other companies controlled by you or your family), the thresholds are divided by (the number of associated companies + 1). For example, with one associated company both thresholds halve: the small profits limit falls to £25,000 and the main rate limit to £125,000.

The Filing Process Step by Step

1. Calculate Your Taxable Profit

Start with your turnover, subtract allowable expenses:

Turnover (Box 145)                    £80,000
Less: Cost of sales                   -£5,000
Less: Administrative expenses        -£30,000
Less: Director's salary + NI         -£12,570
= Trading profit (Box 155)            £32,430

Box 155 is your tax-adjusted trading profit — not simply your accounting profit. You add back any expenses HMRC disallows (such as depreciation) and deduct capital allowances to arrive at the taxable figure. Your tax computation must show these adjustments.

2. Claim Capital Allowances

If you bought equipment, computers, or vehicles for business use, you can claim capital allowances:

• Annual Investment Allowance (AIA): 100% deduction on the first £1,000,000 of qualifying purchases
• Writing Down Allowance: 18% (main pool) or 6% (special rate) per year for items not fully relieved

Most small companies can claim 100% of their equipment costs through AIA.

3. Calculate Corporation Tax

For a company with £32,430 profit (well under the £50,000 threshold):

Taxable profit: £32,430
Rate: 19% (small profits rate)
Tax due: £6,161.70

4. Prepare Accounts in iXBRL

Your CT600 submission must include statutory accounts and tax computations in iXBRL format. This is a digital tagging standard that HMRC uses to process returns automatically.

You don't need to create iXBRL files yourself — filing software like Taxpipe generates them from the information you enter.

5. Submit to HMRC

File electronically through HMRC-recognised software. You'll receive a confirmation with a timestamp and correlation ID. Keep this for your records.

6. Pay Your Tax

Pay by the deadline (9 months and 1 day after your accounting period ends). Use the payment reference shown on your HMRC payslip or Business Tax Account — do not try to construct it yourself, as an incorrect reference can delay allocation of your payment.

Common First-Timer Mistakes

Confusing Filing and Payment Deadlines

Payment is due before the filing deadline. Set calendar reminders for both.

Forgetting to Claim Expenses

Every legitimate business expense reduces your tax. Common ones people miss:

• Working from home allowance
• Professional subscriptions
• Training courses
• Accountancy software
• Business insurance
• Travel to client sites

Assuming a Dormant Company Has Nothing to Do

If HMRC has issued a notice to deliver a return, you must file — even if the company earned nothing and the return shows zero. If you haven't formally notified HMRC of your company's dormancy, do so now. Once HMRC confirms dormancy, no return is required unless they later issue a notice. Penalties apply for ignoring a notice to deliver regardless of whether any tax is owed.

Missing the First Filing Date

Your first accounting period might not be 12 months — it depends on when you incorporated. Check your confirmation letter from Companies House.

Do I Need an Accountant?

For a straightforward company — single director, simple income, standard expenses — you don't need an accountant. Modern filing software handles the calculations, iXBRL generation, and HMRC submission.

Consider an accountant if:

• You have complex tax affairs (R&D claims, international income, multiple companies)
• You want tax planning advice beyond basic filing
• You don't have time to prepare the return yourself

For basic filing, Taxpipe charges £59 per return — significantly less than most accountants (£300-£800 for a simple CT600).

Next Steps

1. Check your deadline — when does your accounting period end?
2. Gather your numbers — bank statements, invoices, payroll records
3. Try the calculator — Taxpipe's free calculator estimates your tax instantly
4. File your return — enter your data, review, and submit to HMRC

Your first CT600 feels daunting, but it's manageable. The form is structured logically, and with the right software, most of the complexity is handled for you.