CT600 Guide: UK Company Tax Return

What this article is about:
This guide explains the CT600 company tax return, which every UK limited company within the charge to Corporation Tax must file. It covers what the CT600 is, who needs to complete it, deadlines, penalties, allowances and reliefs, and how to stay compliant with HMRC.

 

Section A: Understanding the CT600 Form

 

Introduction
The CT600 form is the foundation of Corporation Tax compliance in the UK. Every limited company is required to complete one, and it plays a central role in HMRC’s assessment of tax liabilities. Before looking at how to file, it is important to understand what the CT600 is, who it applies to, and the type of information it requires.

 

1. What is the CT600?

 

Limited companies in the UK are required to file Form CT600, also known as the company tax return, to make their annual Corporation Tax submission to HMRC.

The CT600 is divided into various sections, requiring information such as:

• Company and return data: business name and type, the ten-digit company Unique Tax Reference (UTR), and accounting period.
• Tax calculation: turnover, profit-and-loss report, income and expenses.
• Declaration: completed business accounts and the name of the individual making the return.

Even if HMRC does not issue a CT603 notice, the company must still submit a CT600 if it is within the charge to Corporation Tax.

 

2. Who must file a CT600?

 

All UK limited companies must file a CT600 if they are liable for Corporation Tax. Other organisations such as associations, societies, clubs and charities may also need to submit the form.

Your company’s profits are calculated as turnover minus allowable expenses and capital allowances. Corporation Tax is then payable on the remaining profit.

Section Summary
The CT600 form is a statutory requirement for companies and certain other organisations. It provides HMRC with essential information about profits, income, and allowances. Every company within the charge to Corporation Tax must file a CT600, regardless of whether they receive a notice.

 

 

Section B: Filing the CT600

 

Introduction
Submitting the CT600 correctly and on time is one of the most important duties of company directors. Filing late can lead to penalties, and errors can result in HMRC enquiries. This section explains the key filing deadlines, how Corporation Tax Accounting Periods work, and how to submit the CT600 online.

 

1. Filing deadlines

 

The legal deadline is the later of:

• 12 months after the end of the accounting period, or
• 3 months after the company receives a notice to deliver a Company Tax Return (CT603).

 

 

2. Corporation Tax Accounting Period (CTAP)

 

The accounting period is the period covered by the tax return. It normally matches the financial accounts, but can differ in certain situations such as the first year of trading.

Key rules include:

• A CTAP cannot exceed 12 months.
• Companies can change their accounting reference date at Companies House using Form AA01.
• You cannot extend an accounting period beyond 18 months or extend more than once in five years, except in limited cases.

 

 

3. How to file the CT600

 

All company tax returns must be filed digitally through HMRC’s online gateway. CT600 (version 3) must be used for accounting periods beginning on or after 1 April 2015.

HMRC will acknowledge receipt but may amend the return or raise enquiries. If you later find an error, the return can be amended within statutory time limits.

 

4. Penalties for late filing

 

• 1 day late: £100
• 3 months late: £200
• 6 months late: HMRC estimates liability and adds a 10% penalty
• 12 months late: an additional 10% penalty

Inaccurate returns can also trigger penalties.

 

Section Summary
The CT600 must be filed within strict deadlines, with penalties escalating the longer it is delayed. Companies should plan ahead to meet filing dates, understand their Corporation Tax Accounting Period, and use HMRC’s digital filing system to remain compliant.

 

 

Section C: Corporation Tax Rates and Reliefs

 

Introduction
The CT600 does more than report profits. It also provides the mechanism to apply allowances and reliefs that reduce Corporation Tax liability. This section explains the current rates of Corporation Tax, as well as the most common allowances and reliefs available to businesses.

 

1. Corporation Tax rates

 

• From 1 April 2023, the main rate is 25% on profits above £250,000.
• Small profits rate of 19% applies up to £50,000.
• Marginal relief applies between £50,000 and £250,000.

The Patent Box regime may allow certain profits from qualifying patents to be taxed at 10%, but companies must elect into the scheme and meet strict conditions.

 

 

2. Capital allowances and reliefs

 

Capital allowances allow businesses to deduct the cost of capital assets from taxable profits.

• Annual Investment Allowance (AIA): £1 million.
• Full expensing (introduced April 2023, permanent): 100% first-year relief on qualifying plant and machinery.
• 50% first-year allowance for special rate assets.
• Writing down allowances: 18% main pool, 6% special rate pool.
• Structures and Buildings Allowance (SBA): 3% per year.

 

 

3. Other reliefs

 

• Creative industry tax reliefs – theatre, film, TV, video games.
• Goodwill and intangibles – relief restricted since 2015; some amortisation deductions allowed from 2019.
• Research and development (R&D) relief – SMEs and large companies can claim relief on qualifying R&D costs. Loss-making SMEs may claim payable credits.

Note: Disincorporation relief ended on 31 March 2018 and is no longer available.

 

Section Summary
Corporation Tax is charged at different rates depending on profit levels, but allowances and reliefs can significantly reduce the liability. From AIA and full expensing to R&D credits and the Patent Box, businesses should use the CT600 to claim all reliefs they are entitled to.

 

 

Section D: Expenses, Loss Relief & Compliance

 

Introduction
Corporation Tax compliance is about more than filing forms. HMRC expects companies to make accurate declarations and apply the rules on expenses and loss relief correctly. This section explains what expenses can be claimed, how loss relief works, and the compliance obligations around declarations.

 

1. Allowable expenses

 

Expenses can only be deducted if incurred wholly and exclusively for business purposes.

Examples of allowable expenses:

• Employer pension contributions to registered schemes
• Staff uniforms
• Sponsorship and charitable donations

Non-allowable expenses include:

• Business entertainment
• Ordinary clothing such as business suits

 

 

2. Loss relief

 

Companies making a loss can:

• Offset against other income in the same period
• Carry back against previous profits
• Carry forward against future profits
• Group relief within a group structure

 

 

3. Return declaration and compliance

 

The CT600 must include a declaration that the return is complete and correct to the best of the individual’s knowledge. Knowingly submitting false information can lead to prosecution.

 

Section Summary
Businesses must apply the “wholly and exclusively” rule for expenses and make full use of Corporation Tax loss reliefs. Compliance also depends on truthful declarations, as HMRC has strong enforcement powers in cases of inaccuracy or fraud.

 

 

FAQs

 

What is a CT600?
It is the company tax return form all UK companies within the charge to Corporation Tax must submit to HMRC.

When is the CT600 deadline?
The later of 12 months after the end of the accounting period or 3 months after HMRC issues a CT603 notice.

Can I file a CT600 without a CT603?
Yes. Filing is required even without a notice if the company is within the charge to Corporation Tax.

What are the penalties for late filing?
Fixed fines of £100 and £200, with additional percentage penalties if over 6 or 12 months late.

What allowances can reduce Corporation Tax?
AIA, full expensing, writing down allowances, SBA, R&D relief, Patent Box, and creative industry reliefs.

 

Conclusion

 

The CT600 is the cornerstone of Corporation Tax compliance for UK companies. Filing on time and declaring profits accurately are statutory duties, but companies can also use the return to access valuable reliefs and allowances. By understanding the filing process, deadlines, and available deductions, businesses can reduce their tax burden while avoiding penalties.

 

 

Glossary

 

Term	Definition
CT600	HMRC company tax return form used to report profits, claim reliefs, and calculate Corporation Tax due.
CTAP	Corporation Tax Accounting Period; the period covered by a company’s Corporation Tax return.
AIA	Annual Investment Allowance allowing 100% relief on qualifying plant and machinery up to the annual limit.
SBA	Structures and Buildings Allowance; gives a flat-rate annual deduction for qualifying new non-residential construction costs.
HMRC	HM Revenue & Customs, the UK’s tax authority.

 

Useful Links

 

Resource	Link
HMRC: File your Company Tax Return	gov.uk/file-your-company-tax-return
HMRC: Corporation Tax rates	gov.uk/corporation-tax-rates
Companies House: Change accounting reference date	gov.uk/change-your-companys-year-end
Taxoo: CT600 guide	taxoo.co.uk/ct600/
Taxoo: Corporation Tax payment deadlines	taxoo.co.uk/corporation-tax-payment-deadline/
Taxoo: Allowable expenses	taxoo.co.uk/allowable-expenses/
Taxoo: Capital allowances	taxoo.co.uk/capital-allowances/