You've just set up a limited company. Maybe you're a freelancer who incorporated, a startup founder, or someone who bought a ready-made company. Whatever brought you here, you've now got a legal obligation that won't go away: filing a CT600 corporation tax return with HMRC.

If the phrase "CT600" means nothing to you, don't worry. By the end of this guide, you'll know exactly what it is, when to file, what information you need, and how to do it yourself — without an accountant if you choose.

This guide is written specifically for first-time directors who've never filed a CT600 before. No jargon. No assumptions. Just a clear path from "I have no idea what I'm doing" to "my CT600 is filed and I'm done."

What Is a CT600?

A CT600 is your company's corporation tax return. It's an official form that tells HMRC:

How much money your company made (income/revenue)
How much your company spent (allowable expenses)
How much profit your company made
How much corporation tax your company owes

Every UK limited company must file a CT600 — even if the company made no profit, didn't trade, or owes zero tax. There are no exceptions.

The CT600 is filed annually, covering one accounting period (always 12 months or less). It must be submitted electronically — you can't post a paper form.

When Do You Need to File?

Your filing deadline is 12 months after the end of your company's accounting period.

What's Your Accounting Period?

When you incorporated your company, Companies House set an accounting reference date (ARD). This is usually the last day of the month in which your company was incorporated, at the end of the following year.

Example (exactly 12-month first period):

Company incorporated: 31 July 2024
First accounting reference date: 31 July 2025
First accounting period: 31 July 2024 – 31 July 2025
CT600 filing deadline: 31 July 2026

Important — long first periods: A CT accounting period cannot exceed 12 months. If your company incorporated mid-month (for example, 15 July 2024 with an ARD of 31 July 2025), your first period is slightly longer than 12 months — and you must file two CT600 returns: one for the first 12 months (15 July 2024 – 14 July 2025) and a short stub return for the remainder (15 July 2025 – 31 July 2025). You apportion your profits between the two returns by the number of days in each. See our guide to accounting periods longer than 12 months for a full walkthrough.

You can check your accounting reference date on your Companies House profile.

Important: Filing vs Payment Deadlines Are Different

What	When
File your CT600	Within 12 months of accounting period end
Pay your corporation tax	Within 9 months and 1 day of accounting period end

Yes, you need to pay before you file. This catches many first-time directors off guard. Where you have two CT600 periods, each period has its own payment deadline.

Using the 31 July example above:

Tax payment due: 1 May 2026 (9 months + 1 day after 31 July 2025)
CT600 filing due: 31 July 2026 (12 months after 31 July 2025)

For a full breakdown of deadlines, see our CT600 deadline guide.

What Information Do You Need to File?

Before you sit down to complete your CT600, gather these documents:

1. Profit and Loss Account (Income Statement)

This shows your company's income minus expenses for the accounting period. If you use bookkeeping software (Xero, QuickBooks, FreeAgent), you can generate this as a report.

If you don't use software, you'll need to create one manually from your bank statements and records.

2. Balance Sheet

This is a snapshot of what your company owns (assets) and owes (liabilities) at the end of the accounting period. Again, accounting software generates this automatically.

3. Bank Statements

You'll need these to verify your figures. HMRC may ask for them if they open a compliance check.

4. Records of Income

Invoices you sent to clients/customers
Other income: bank interest, grants, etc.

5. Records of Expenses

Allowable business expenses reduce your taxable profit. Common ones include:

Office costs (rent, utilities, internet)
Travel and subsistence
Professional subscriptions
Software and tools
Marketing and advertising
Insurance
Bank charges
Accountancy fees (yes, you can deduct these)
Phone bills (business portion)

6. Capital Expenditure Records

If you bought equipment, a computer, vehicle, or other business assets, you may be able to claim capital allowances to reduce your tax bill.

7. Director's Loan Account

If you've put personal money into the company or taken money out beyond your salary/dividends, you need to track this. A director's loan account balance appears on your balance sheet.

8. Dividend Records

If you paid yourself dividends, you need records of:

Date each dividend was declared
Amount paid
Minutes from the board meeting (even if it's just you)

For guidance on the most tax-efficient way to pay yourself, see our dividend vs salary guide.

How Corporation Tax Is Calculated

Here's the simplified version:

Start with your total income (revenue/turnover)
Subtract allowable expenses = Trading profit
Add any other income (bank interest, etc.)
Subtract capital allowances (if applicable)
= Taxable profit
Apply the corporation tax rate

Corporation Tax Rates (2025/26)

Taxable Profit	Rate
Up to £50,000	19% (small profits rate)
£50,001 – £250,000	19–25% effective (marginal relief applies)
Over £250,000	25% (main rate)

Most first-time directors running small companies will pay the 19% small profits rate.

In the marginal relief band, your effective rate rises gradually from 19% towards 25% as profits increase — it never exceeds 25%. (The figure "26.5%" you may see elsewhere is the marginal rate on each additional pound within the band, not the effective rate on your total profits.)

Example:

Revenue: £80,000
Allowable expenses: £30,000
Taxable profit: £50,000
Corporation tax: £50,000 × 19% = £9,500

For a more detailed explanation of rates, read our corporation tax rates guide.

Want a quick estimate? Use our free corporation tax calculator — enter your numbers and see your liability instantly.

The CT600 Form: Key Sections Explained

The CT600 has many boxes, but for a typical small company, you'll only need to fill in a handful of sections. Here's what they mean:

Company Information (Boxes 1–30)

Basic details: company name, UTR number, accounting period dates, type of company. Most of this is pre-populated or straightforward.

Income and Expenses

Box 145 — Turnover: Your total revenue for the period
Box 155 — Trading profits: Your tax-adjusted profit from trading activities — starting from accounting profit and adjusted for items HMRC treats differently (e.g. depreciation is added back, capital allowances are deducted). See our Box 155 guide
Box 170 — Bank interest: Your company's non-trading interest income — for most small companies this is simply the interest earned on your business bank account
Box 235 — Total profits: Combines all income sources before deductions and reliefs

Tax Calculation

Box 440 — Tax chargeable: The corporation tax amount after applying the rate(s) and any marginal relief
Box 475 — Net Corporation Tax liability: Box 440 minus any tax credits such as Double Taxation Relief — your core CT bill before any Section 455 charges on director loans
Box 525 / 528 — Tax payable: Your final self-assessed amount; box 528 is the overall total including any additional charges

Supplementary Pages

Depending on your company's activities, you may need supplementary pages. Most simple companies don't. Common ones include:

CT600A: Loans to participators (if your close company has lent money to directors or shareholders)
CT600C: Group relief (if part of a group)

iXBRL Accounts: The Technical Requirement Nobody Tells You About

Here's something that surprises almost every first-time filer: you can't just file a CT600 on its own. You must also submit your company accounts in a format called iXBRL (inline eXtensible Business Reporting Language).

This is a tagged version of your accounts that HMRC's computers can read. You can't create iXBRL files in Word or Excel — you need specialised software.

This is the main reason most directors either hire an accountant or use dedicated CT600 software. Creating iXBRL accounts manually is essentially impossible for a non-specialist.

For micro-entity (FRS 105) and dormant companies, Taxpipe generates your iXBRL accounts automatically as part of the £59 filing process — you enter your financial figures, and we produce the correctly tagged accounts and submit them alongside your CT600 in one go. If your company qualifies as a small company (FRS 102 Section 1A), you will need to produce your iXBRL accounts in third-party accountancy software and upload them to Taxpipe before submitting.

Read our full iXBRL explainer if you want to understand the technical side.

How to File Your CT600: Three Options

Option 1: Use Taxpipe (Best for First-Time Directors)

Cost: £59 | Time: 30-60 minutes

Taxpipe was built specifically for directors like you — people who've never filed a CT600 before and don't want to pay accountant prices.

Here's how it works:

Create your account — takes 2 minutes
Enter your company details — we auto-populate from Companies House
Enter your financial figures — guided step-by-step, with help text for every field
Review your CT600 — we generate it automatically from your entries
Review your iXBRL accounts — generated automatically for micro-entity (FRS 105) and dormant companies; small company (FRS 102 Section 1A) accounts must be produced in your accountancy software and uploaded
Submit to HMRC — one click, using your own Government Gateway credentials, directly through HMRC's secure API
Get your confirmation — official HMRC acknowledgement

No tax knowledge required. No accounting qualifications needed. If you can read a bank statement, you can use Taxpipe.

Start your first CT600 filing →

Option 2: Hire an Accountant

Cost: £200-£500+ | Time: 1-4 weeks

An accountant will handle everything for you. You send them your records, they prepare the accounts and CT600, and file it with HMRC.

Pros:

Completely hands-off
They may spot tax-saving opportunities
Useful for complex situations

Cons:

Expensive for a simple company
Communication back-and-forth takes time
You still need to provide all the information

For a straightforward small company, you're paying £200-500 for something you could do yourself in an hour. See our full cost comparison.

Option 3: Use Other Commercial Software

Cost: £100-300+ | Time: varies

There are other CT600 software options (EasyDigitalFiling, Andica, TaxCalc, etc.). Many are designed for accountants rather than directors, so they can be confusing for a first-time filer. We've written a comparison of CT600 software if you want to evaluate alternatives.

Step-by-Step: Filing Your First CT600 with Taxpipe

Let's walk through exactly what happens when you file with Taxpipe:

Before You Start

Make sure you have:

✅ Your company UTR (Unique Taxpayer Reference) — sent by HMRC after you registered for corporation tax
✅ Your HMRC Government Gateway ID and password (enrolled for Corporation Tax)
✅ Your profit and loss figures for the accounting period
✅ Your balance sheet figures as at the period end date
✅ Details of any dividends paid
✅ Details of any director's loan account balance

Don't Have a UTR?

Your UTR is a 10-digit number sent by HMRC, usually within a few weeks of incorporating your company. Check old HMRC letters, or call HMRC's corporation tax helpline: 0300 200 3410.

If you never registered for corporation tax, you must do so with HMRC before you can file.

The Filing Process

Step 1: Company Setup

Enter your company name and registration number. Taxpipe fetches your company details from Companies House automatically — registered address, incorporation date, SIC codes, and accounting reference date.

Step 2: Accounting Period

Confirm the start and end dates of the accounting period you're filing for. For most first-time filers, this is from your incorporation date to your first accounting reference date. If that span exceeds 12 months, you'll need to file two returns (see the earlier note on long periods).

Step 3: Financial Figures

Enter your:

Revenue/turnover
Cost of sales (if applicable)
Administrative expenses
Other income (bank interest, etc.)
Fixed assets and their values
Current assets (bank balance, debtors)
Current liabilities (creditors, tax owed)
Director's loan account balance

Each field has clear help text explaining what to enter and where to find the number.

Step 4: Capital Allowances

If you bought business equipment during the period, enter the details. Taxpipe calculates the available allowances — including the Annual Investment Allowance (AIA, up to £1,000,000 per year) and writing-down allowances on your capital allowance pools.

Step 5: Review

Taxpipe generates your complete CT600. For micro-entity (FRS 105) and dormant companies, your iXBRL accounts are generated automatically. For small companies (FRS 102 Section 1A), upload the iXBRL accounts produced in your accountancy software. Review everything. Our real-time validation catches common errors before submission.

Step 6: Submit

Click submit. Taxpipe sends your CT600 and accounts directly to HMRC via their secure API. You'll receive a confirmation with a unique HMRC transaction ID.

That's it. You've filed your first CT600. 🎉

Common Mistakes First-Time Directors Make

Learn from others' errors. Here are the most frequent mistakes we see:

1. Missing the Payment Deadline

Remember: you must pay corporation tax within 9 months and 1 day, but you have 12 months to file. Many directors focus on filing and forget to pay — resulting in interest charges.

2. Not Registering for Corporation Tax

Your company must be registered for corporation tax with HMRC before you can file. This should happen automatically after incorporation, but check. If you haven't received a UTR within 3 months of incorporating, contact HMRC.

3. Confusing Personal and Company Finances

Your limited company is a separate legal entity. Money in the company is not your money. If you take money out, it's either salary (subject to PAYE/NI), dividends (subject to dividend tax), or a director's loan (potentially subject to Section 455 tax at 33.75%).

4. Not Claiming All Allowable Expenses

Many first-time directors leave money on the table by not claiming legitimate business expenses. If you paid for something wholly and exclusively for business purposes, it's usually deductible.

5. Forgetting About Companies House

Filing a CT600 with HMRC is separate from filing annual accounts with Companies House. You must do both. Read our Companies House filing guide.

For a complete list of mistakes to avoid, see our guide to 12 common CT600 mistakes.

What Happens After You File?

Once your CT600 is submitted:

HMRC sends an acknowledgement — this confirms receipt (not acceptance)
HMRC processes your return — usually within a few weeks
Your tax account is updated — you can check it in your HMRC business account
If there's an issue, HMRC will write to you

For a detailed walkthrough of the post-filing process, read what happens after you file your CT600.

How to Pay Your Corporation Tax

After filing, you need to pay any tax owed. Payment methods include:

Bank transfer (fastest — same or next day)
Direct debit (set up through your HMRC account)
Corporate credit/debit card (fees apply)
CHAPS (same-day for large amounts)

Always use the payment reference shown in your HMRC online account or on the payslip HMRC sends you — it's a 17-character code based on your company's UTR. Full details in our payment guide.

HMRC's Free Filing Service Has Closed

HMRC's free CT600 filing service closed on 31 March 2026. All companies now need to use commercial software or an accountant — there is no longer a free government filing option.

At £59, Taxpipe is the most affordable HMRC-recognised option.

Read our full guide on the HMRC service closure →

Estimate Your Corporation Tax

Before you file, get an instant estimate of what you owe. Our free corporation tax calculator shows you:

Your estimated tax liability
Which tax rate applies to your company
How marginal relief affects companies earning between £50,000 and £250,000

Use the free calculator →

FAQ

Do I need an accountant to file a CT600?

No. There's no legal requirement to use an accountant. Many directors file their own CT600 using software like Taxpipe. An accountant is optional — useful for complex situations but unnecessary for straightforward small companies.

What is a UTR and how do I get one?

A UTR (Unique Taxpayer Reference) is a 10-digit number assigned by HMRC when your company registers for corporation tax. It's usually sent by post within a few weeks of incorporation. If you haven't received one, call HMRC on 0300 200 3410.

How much does it cost to file a CT600?

Options range from £59 with Taxpipe to £200-500+ with an accountant. HMRC's free service closed on 31 March 2026. See our complete cost comparison.

What if my company made no profit?

You must still file a CT600, even if your company made a loss or broke even. Your tax liability will be zero (or you may be able to carry the loss forward), but the filing obligation remains.

Can I file a CT600 for a dormant company?

If HMRC has issued your dormant company a notice to deliver a return (form CT603), you must file — Taxpipe handles dormant nil returns, which are simpler since there are few figures to enter. If you have formally notified HMRC of dormancy and they've acknowledged it, HMRC will generally not issue notices and no return is required unless asked.

What happens if I file my CT600 late?

The penalties depend on your filing deadline:

Filing deadline before 1 April 2026: £100 immediately, another £100 at 3 months late.

Filing deadline on or after 1 April 2026: The fixed penalties doubled — £200 immediately, another £200 at 3 months late (and £1,000 each for a third consecutive late return).

In both cases, if the return is still outstanding at 6 months, HMRC estimates your tax and adds 10% of unpaid tax; then another 10% at 12 months. Interest also accrues on unpaid tax from the payment deadline.

What's the difference between filing at Companies House and filing a CT600?

Companies House and HMRC are separate bodies. You must file annual accounts with Companies House and a CT600 with HMRC. They have different deadlines and different requirements. Most companies need to do both every year.

How long does it take to file a CT600?

Using Taxpipe, most first-time directors complete the process in 30-60 minutes. With an accountant, the process typically takes 1-4 weeks due to back-and-forth communication.

Related: HMRC penalties for late filing

Related: how to pay Corporation Tax

Ready to File Your First CT600?

It sounds more complicated than it is. Thousands of first-time directors file their own CT600 every year using Taxpipe. The guided process walks you through everything, generates your iXBRL accounts automatically (micro-entity and dormant) or accepts your upload (small company FRS 102 Section 1A), and submits directly to HMRC.

£59. No subscription. No hidden fees. No accountant needed.

File your first CT600 now →