Filing a Limited Company Tax Return (CT600) is one of the most important responsibilities for any UK company director. Whether you’re running a new startup, a small consultancy, or an established business, understanding how Company Tax Returns work will help you avoid penalties, claim the right deductions, and legally reduce your tax bill.
This guide covers everything a UK director needs to know from deadlines and expenses to penalties, loss relief and best practices.
What Is a Limited Company Tax Return?
A Limited Company Tax Return officially called the CT600 is the form your company files with HMRC to declare:
- Your company’s income
- Allowable business expenses
- Taxable profits
- Corporation Tax due
- Any tax reliefs or losses being claimed
It is completely separate from your Self Assessment as a director.
Who Must File a Company Tax Return?
You must file a CT600 if your company is:
- Active and trading
- Receiving income, even if not trading (e.g., interest, rental income)
- In its first year, regardless of profit
- Previously dormant but has now resumed activity
Dormant companies do not file a full tax return unless HMRC specifically requests one, but they must still file simplified accounts to Companies House.
What Needs to Be Submitted?
A complete Company Tax Return includes:
1. CT600 Form
Summary of:
- Turnover
- Profit
- Allowances or reliefs
- Corporation Tax calculation
2. Statutory Accounts (iXBRL format)
Includes:
- Profit & Loss account
- Balance Sheet
- Notes to the accounts
- Director’s approvals
3. Tax Computations
A detailed breakdown showing how taxable profits were calculated, including:
- Add-backs
- Capital allowances
- Adjustments
- Use of losses
Corporation Tax Rates (2024/25)
Corporation Tax is not one flat rate. It depends on profit levels:
| Taxable Profit | CT Rate |
| Up to £50,000 | 19% (Small Profits Rate) |
| £50,001–£250,000 | Marginal Relief applies |
| Above £250,000 | 25% (Main Rate) |
If your company has associated companies, these thresholds are divided, so the 19% and 25% bands reduce.
Important Deadlines for Limited Companies
1. Corporation Tax Payment
Tax must be paid 9 months + 1 day after the end of your accounting period.
Example: Year-end 31 March – Tax due 1 January.
Large companies with taxable profits over £1.5m (adjusted for associated companies) must pay in quarterly instalments, not at the 9-month deadline.
2. Company Tax Return Filing (CT600)
Must be filed within 12 months of the end of the accounting period.
3. Companies House Annual Accounts
Due 9 months after the year-end.
(First-year companies have slightly different deadlines.)
What Expenses Can a Limited Company Claim?
Claiming the right expenses reduces your taxable profit. Allowable costs include:
- Staff and director salaries
- Employer NI & pension contributions
- Office rent & utilities
- Business insurance
- Advertising & marketing
- Professional fees (legal, accounting)
- Travel & accommodation (business-related)
- Software subscriptions
- Training related to your trade
- IT equipment
- Bad debts
- Business banking charges
- Use of home as office (apportioned)
Personal or mixed-use costs cannot be claimed unless apportionment is reasonable and documented.
Capital Allowances Your Company Can Claim
Capital allowances reduce taxable profits on assets like equipment, tech, and machinery.
Main Types
- Annual Investment Allowance (AIA) – 100% deduction on qualifying assets
- Full Expensing – 100% deduction on new plant & machinery
- Writing Down Allowances (WDA) – reduces profit over several years
Example: If you buy equipment worth £10,000 under AIA:
You deduct £10,000 instantly – reduces your taxable profit by £10,000.
Using Loss Relief
If your company makes a loss, you can:
- Carry the loss forward to offset future profits
- Carry it back to reclaim tax paid in the previous year (subject to limits)
- Use group relief (if you are part of a group)
Loss relief is a powerful tax planning tool for early-stage businesses.
Common Mistakes Directors Make
Avoid these frequent errors:
- Confusing personal tax with company tax
- Missing tax-deductible expenses
- Not using digital bookkeeping tools
- Filing late due to poor record keeping
- Forgetting about marginal relief calculations
- Not budgeting for Corporation Tax
- Incorrectly using the director’s loan account
- Assuming “no profit = no filing required”
Good bookkeeping prevents 90% of these issues.
Best Practices for Smooth Filing
- Use cloud software (Xero, QuickBooks, FreeAgent)
- Reconcile bank accounts monthly
- Keep digital receipts
- Track directors’ loan transactions
- Schedule quarterly profit reviews
- Put aside money for tax every month
- Work with a qualified accountant
- Plan year-end spending strategically to reduce taxable profits
Frequently Asked Questions
1. What is a CT600?
A CT600 is the official Company Tax Return form submitted to HMRC. It shows your company’s income, expenses, taxable profits, allowances, and the Corporation Tax owed.
2. Do all limited companies need to file a tax return?
Yes. Every active limited company must file a CT600, even if it makes no profit or has not traded much. Dormant companies file simplified accounts, but HMRC may still request a return.
3. What is the deadline for paying Corporation Tax?
Corporation Tax must be paid 9 months and 1 day after your company’s accounting year ends. Large companies may need to pay quarterly.
4. When is the CT600 filing deadline?
You must file your Company Tax Return within 12 months of your accounting period end.
5. Do I need to file a tax return if my company made no profit?
Yes. A zero-profit or low-activity company must still file a CT600 unless HMRC confirms the company is dormant.
Final Thought
A Limited Company Tax Return isn’t just a form it’s an opportunity to optimise your tax position, strengthen your financial systems, and stay fully compliant with HMRC. With accurate bookkeeping and the right support, you can legally reduce your tax bill and avoid costly penalties. If you need help preparing or filing your Company Tax Return, get in touch we can handle everything from bookkeeping to HMRC submissions so you stay stress-free and compliant.