Filing Your First CT600: New Company Guide

Congratulations on starting your limited company! One of your first responsibilities as a director is filing a CT600 Corporation Tax return with HMRC. Don't worry — while it might seem daunting, your first return is often simpler than you'd expect.

This guide walks you through everything you need to know about filing your first CT600, from registering with HMRC to submitting your return.

When Is Your First CT600 Due?

Your first CT600 deadline depends on when you incorporated and when your first accounting period ends.

Understanding Your First Accounting Period

When you incorporate, Companies House sets an accounting reference date (ARD) — the end date of your financial year. By default, this is the last day of the month in which you incorporated, one year later.

Example:

Incorporated: 15 March 2024
Default ARD: 31 March
First accounting period: 15 March 2024 to 31 March 2025

Your First Accounts Can Be Longer

Unlike subsequent years, your first accounting period can be up to 18 months long. This is common when directors want to align their year-end with a specific date (like 31 March for tax efficiency).

First Year Deadlines

For your first CT600:

Filing deadline: 12 months after the accounting period ends
Payment deadline: 9 months and 1 day after the accounting period ends

Example continued:

Period ends: 31 March 2025
Payment due: 1 January 2026
CT600 due: 31 March 2026\n

Step 1: Register for Corporation Tax

Before you can file, you must register your company for Corporation Tax with HMRC.

When to Register

You should register within 3 months of starting to trade. "Starting to trade" means:

Buying or selling goods/services
Employing staff
Advertising your business
Earning any business income

If your company is dormant (not trading), you still need to register if you expect to trade in the future.

How to Register

Go to HMRC's Corporation Tax registration
You'll need:

- Company registration number (from Companies House) - Date of incorporation - Date you started trading - Nature of business (SIC code) - Accounting period dates - Company address - Director details

Submit the registration online
HMRC will send your Unique Taxpayer Reference (UTR) by post to your registered office (usually within 14 days)

Keep your UTR safe — you'll need it for all HMRC correspondence and to file your CT600.

Step 2: Enrol for Online Services

To file your CT600 online, you need a Government Gateway account linked to your company.

Setting Up Access

Create a Government Gateway account (if you don't have one)
Enrol for Corporation Tax online services
HMRC posts an activation code to your registered office
Use the code to complete enrolment

Allow at least 2 weeks for postal delivery of activation codes. Don't leave this until the last minute — you can't file without completing enrolment.

Step 3: Gather Your Financial Information

Before preparing your CT600, you'll need:

Income and Expenses

All sales/income during the accounting period
All business expenses (rent, utilities, supplies, etc.)
Bank statements covering the full period
Invoices and receipts

For Your Balance Sheet

Bank balance at period end
Money owed to you (debtors)
Money you owe (creditors)
Value of any assets (equipment, vehicles, etc.)
Outstanding loans

Directors' Details

Any salary or dividends paid
Loans to/from directors
Benefits in kind\n\n

Step 4: Calculate Your Corporation Tax

Your CT600 reports your taxable profit and the tax due on it.

Taxable Profit ≠ Accounting Profit

Start with your accounting profit, then make adjustments:

Add back:

Depreciation (replaced by capital allowances)
Entertainment costs
Fines and penalties
Non-business expenses

Deduct:

Capital allowances on qualifying assets

Corporation Tax Rates

Profit Level	Rate
Up to £50,000	19%
£50,001 - £250,000	Marginal relief (effective 19-25%)
Over £250,000	25%

Most new companies fall into the small profits rate of 19%.

No Profit? You Still File

Even if your first year shows a loss, you must file a CT600. The return confirms your position and allows you to carry the loss forward to offset future profits.

Step 5: Prepare Your Accounts and Computations

Your CT600 must be accompanied by:

Statutory accounts — A balance sheet and profit & loss account
Tax computations — Showing how you calculated taxable profit

Both must be in iXBRL format (a machine-readable format HMRC's systems can process). You cannot submit PDF or Word documents.

Software like TinyTax generates these automatically from your financial figures.

Step 6: Submit Your CT600

With everything prepared, you can submit:

Review all figures for accuracy
Check accounts, computations, and CT600 are for the same period
Submit electronically via HMRC Gateway
Receive instant confirmation

What Happens After Submission?

HMRC processes your return and may:

Accept it without query
Open an enquiry (they have 12 months to do this)
Issue a refund if you've overpaid

Keep your submission confirmation safe — it proves you filed on time.

Common First-Year Mistakes to Avoid

1. Missing the Registration Deadline

2. Wrong Accounting Period

Ensure your CT600 covers exactly the same period as your statutory accounts. Mismatched dates will cause rejection.

3. Extended Period Not Split

If your first accounting period exceeds 12 months, you need two CT600 returns. The profits are apportioned between the periods.

4. Forgetting About Payment

The payment deadline (9 months + 1 day) comes before the filing deadline (12 months). Don't wait until you file to pay — you'll owe interest.

5. No Prior Year Data

Your first CT600 won't have prior year comparatives. Leave prior year boxes empty or enter zero where required.

Filing a Dormant Company's First CT600

If your company hasn't traded since incorporation, you'll file a dormant company CT600:

Confirm the company was dormant throughout the period
Report zero income and zero tax
No accounts required (just the CT600)

The process is simpler, but you still have deadlines to meet.

What If You Made a Loss?

Many new companies make a loss in year one — it's perfectly normal while you're getting established.

A loss doesn't mean you avoid filing. Your CT600:

Reports the loss
Shows no tax due
Carries the loss forward to offset future profits

Losses can be valuable — they reduce your tax bill in profitable future years.

Frequently Asked Questions

I incorporated months ago but haven't traded. Do I need to file?

It depends. If you're within your first accounting period, you won't need to file yet. But once the period ends, you'll need to file a CT600 — even if it shows zero activity. If you don't expect to trade, consider telling HMRC the company is dormant.

Can I do my first CT600 myself?

Yes. Many directors file their own CT600, especially for simple companies. Software like TinyTax handles calculations and generates the required iXBRL files automatically.

How long does it take to file?

With TinyTax, most straightforward returns take 15-30 minutes. More complex situations (multiple income streams, capital allowances calculations) take longer.

What if I make a mistake?

You can amend your CT600 within 12 months of the filing deadline. Log in to HMRC online services or file an amended return through software.

Do I need an accountant for my first return?

Not necessarily. For simple trading companies and dormant companies, filing software makes the process accessible to non-accountants. Consider professional help if you have complex transactions, significant capital allowances, or want peace of mind.

File Your First CT600 with TinyTax

Filing your first Corporation Tax return doesn't have to be stressful. TinyTax guides you through the process step by step:

Automatic tax calculations
iXBRL accounts and computations generated for you
Built-in validation to catch errors before submission
Direct submission to HMRC
Instant confirmation

Join thousands of new company directors who've filed their first CT600 with TinyTax.