Just Incorporated? Your First CT600 Timeline: Key Dates You Can't Miss

Just Incorporated? Your First CT600 Timeline

You've just set up a limited company. Congratulations. Now comes the bit Companies House doesn't spell out clearly: your corporation tax obligations start immediately, and the deadlines are unforgiving.

Here's your complete timeline from incorporation to first CT600 filing.

Day 1: You Incorporate

The moment your company is registered at Companies House, several clocks start ticking:

What happens automatically

• Companies House notifies HMRC of your new company
• HMRC will send you a CT41G form (notice to deliver a company tax return) — usually within a few weeks
• Your first accounting reference date (ARD) is set to the last day of the month, one year after incorporation

What YOU need to do

• Register for Corporation Tax with HMRC within 3 months of starting to trade (or receiving any income)
• You can register online at gov.uk
• You'll need: company name, registration number, date of incorporation, date trading started, accounting period end date

Don't wait for HMRC to contact you. Register proactively. Waiting too long is the #1 mistake new companies make.

Your First Accounting Period

The default

Companies House sets your first ARD as the last day of the month, one year after incorporation.

Example:

• Incorporated: 15 July 2025
• First ARD: 31 July 2026
• First accounting period: 15 July 2025 – 31 July 2026 (12.5 months)

The catch: More than 12 months

If your first period is longer than 12 months (which it almost always is for the first year), you'll need to file two CT600 returns:

• CT600 #1: 15 July 2025 – 14 July 2026 (12 months)
• CT600 #2: 15 July 2026 – 31 July 2026 (0.5 months)

Yes, a 16-day CT600. It's annoying, but HMRC's rule is absolute: no accounting period for corporation tax can exceed 12 months.

How to avoid the two-return problem

Shorten your first ARD at Companies House so your first period is exactly 12 months or less.

Example fix:

• Incorporated: 15 July 2025
• Change ARD to: 31 March 2026 (a common choice — aligns with the tax year)
• First period: 15 July 2025 – 31 March 2026 (8.5 months) — just ONE CT600

You can change your ARD by filing form AA01 at Companies House (free, takes 5 minutes online).

The Complete Timeline

Here's the full picture for a company incorporated on 1 April 2025 with a 31 March year-end:

The "first year extension"

For your first set of annual accounts, Companies House gives you 21 months from incorporation to file. After that, it's 9 months from the year-end.

Key Deadlines Explained

Corporation Tax Registration: 3 months

You must register within 3 months of the earliest of:

• The date you started trading
• The date you received any income
• The date you acquired any assets

Dormant companies don't need to register until they become active, but it's good practice to register anyway.

CT600 Filing: 12 months after period end

Your CT600 must be filed electronically with HMRC within 12 months of the accounting period end.

Corporation Tax Payment: 9 months + 1 day

Your tax payment is due before your filing deadline. Many new directors miss this — they think they have 12 months to sort everything out, but the payment is due at 9 months.

This catches a LOT of first-time directors.

Companies House Accounts: 9 months (or 21 months first year)

Your annual accounts must be filed at Companies House within 9 months of the year-end. For the first year, you get 21 months from incorporation.

What You Need for Your First CT600

Financial records

• Bank statements for the entire period
• Invoices for all income
• Receipts for all expenses
• Payroll records (if you paid yourself a salary)
• Asset purchases (equipment, computers, etc.)

Accounts

• A set of statutory accounts (can be prepared by you, your accountant, or software)
• These must be in iXBRL format for submission with the CT600
• Micro-entity accounts (FRS 105) are the simplest format for small companies

Company details

• UTR (Unique Taxpayer Reference) — HMRC sends this after registration
• CRN (Company Registration Number) — from Companies House
• Registered address and principal business activity (SIC code)

First-Year Tax Planning

Your first year offers some unique opportunities:

1. Choose your year-end wisely

• 31 March: Aligns with the tax year, simplifies everything
• 31 December: Calendar year, intuitive for international businesses
• Custom date: Can be strategic (e.g., after your busy season when cash is highest)

2. Claim all startup costs

Costs incurred before incorporation (up to 7 years before) can be claimed as pre-trading expenses if they would have been deductible during trading. Common examples:

• Market research
• Professional fees (legal, accounting)
• Training courses
• Travel for business planning

3. Capital allowances are time-apportioned

If your first period is shorter than 12 months, your AIA limit is reduced proportionally:

• 12-month period: £1,000,000 AIA
• 9-month period: £750,000 AIA
• 6-month period: £500,000 AIA

4. Employer pension contributions

You can make employer pension contributions from day one. These are:

• A deductible company expense (reduces corporation tax)
• Not subject to employer NI
• A tax-efficient way to build personal wealth

5. Register for VAT if appropriate

If you expect turnover to exceed £90,000 in 12 months, you must register for VAT. Voluntary registration can be beneficial if you sell to VAT-registered businesses (they reclaim the VAT, so it doesn't affect your pricing).

Common First-Year Mistakes

1. Not registering for Corporation Tax

HMRC will eventually catch up with you, but by then you'll have accumulated penalties. Register within 3 months of starting to trade.

2. Mixing personal and business money

Open a separate business bank account immediately. Don't run personal expenses through the company or vice versa. It makes your CT600 much harder and creates director's loan complications.

3. Thinking the company and you are the same

A limited company is a separate legal entity. Money in the company isn't your money until you formally take it out (as salary, dividends, or pension). This distinction matters for your CT600.

4. Missing the payment deadline

Corporation tax is due at 9 months + 1 day. The filing deadline is 12 months. Many first-timers focus on filing and forget they should have paid 3 months earlier.

5. Not keeping receipts

From day one, save every receipt for business expenses. Digital photos are fine. You'll need them for your CT600 and if HMRC ever enquires.

6. Forgetting you need TWO returns

If your first period exceeds 12 months, you need two CT600 returns. This is HMRC's #1 penalty trigger for new companies.

Filing Your First CT600

When you're ready to file:

1. Prepare your accounts (or have your accountant do it)
2. Calculate your tax (profit × applicable rate)
3. File electronically — HMRC no longer accepts paper CT600s
4. Pay your corporation tax via HMRC's online payment service

Taxpipe makes first-time filing simple. Our step-by-step wizard guides you through every box with plain-English explanations. No jargon, no confusion — just answer the questions and we file with HMRC. £59 per return.

Frequently Asked Questions

Do I need to file a CT600 if my company hasn't traded?

If your company is registered with HMRC for Corporation Tax, yes — you file a nil return showing zero profits. If you haven't registered because the company is dormant, you don't need to file until you start trading.

What if I can't afford an accountant?

You don't need one. Services like Taxpipe are designed for directors filing their own CT600. At £59 per filing, it's a fraction of accountancy fees.

Can I change my accounting year-end after incorporation?

Yes. File form AA01 with Companies House. You can shorten your period at any time, or extend it (subject to restrictions).

What if my company made a loss in its first year?

File the CT600 showing the loss. You can carry the loss forward against future profits. If you had any other income in the company, the loss can offset it.

When will I get my UTR?

HMRC typically sends your UTR within 2-3 weeks of registering for Corporation Tax. If you haven't received it after 4 weeks, call HMRC on 0300 200 3410.

Filing your first CT600? Don't stress. Start with Taxpipe — built for first-time filers. Guided entry, automatic calculations, HMRC submission. £59.

Related Articles

• First Year Corporation Tax: Complete Guide for New Companies

Related Articles

• Corporation Tax Deadlines 2026: When You Need to File and Pay
• HMRC Penalties for Late Corporation Tax Filing