Running a startup often feels like juggling flaming torches while riding a unicycle. You’re building your product, chasing customers, managing cash flow, and constantly solving new problems. In the middle of all that, corporation tax can feel like an afterthought – until a deadline is missed or a brown envelope from HMRC lands on your desk.
This guide walks you through how corporation tax works for UK startups, what deadlines you need to know, how to pay, and how to avoid common mistakes that cost money and peace of mind.
What Corporation Tax Actually Is
Corporation tax is the tax your company pays on its profits. In the UK, this is paid to HM Revenue and Customs (HMRC). If your business is making taxable profits, corporation tax is not optional – it is a legal requirement.
You may need to pay corporation tax if you are:
- A limited company
- A foreign company with a UK branch or office
- A club, co-operative, or other unincorporated association such as a community or sports organisation
Taxable profits usually include:
- Trading profits from your main business activities
- Profits from investments
- Chargeable gains when you sell assets for more than they cost
If your company is UK-resident for tax purposes, it pays corporation tax on worldwide profits. Non-UK resident companies with a UK branch or office generally only pay on profits attributed to UK activities.
Why Corporation Tax Matters So Much For Startups
For a young business, paying corporation tax on time is about more than just staying out of trouble. Consistent, accurate compliance:
- Helps you avoid penalties and interest
- Builds financial credibility with investors, lenders, and partners
- Forces you to keep proper records and understand your numbers
- Reduces stress when year end comes around
On the flip side, ignoring your responsibilities can lead to mounting fines, legal headaches, and a big distraction from growing your company.
Penalties For Late Corporation Tax Returns
HMRC can be strict when it comes to late filing. Here’s what happens if you miss your company tax return deadline:
- 1 day late: £100 penalty
- 3 months late: another £100 penalty
- 6 months late: HMRC makes its own estimate of your corporation tax bill and adds a 10% penalty on the unpaid tax
- 12 months late: a further 10% penalty on any remaining unpaid tax
If you file late three times in a row, the £100 penalties increase to £500 each time.
If your return is more than 6 months overdue, HMRC will issue what’s called a “tax determination” – an estimated bill which you cannot appeal. To fix things, you must submit your company tax return, pay the tax due, and settle all interest and penalties.
Key Dates: Your Tax Year, Filing Deadlines, And Payment Dates
Every startup has an accounting period for corporation tax, usually matching its financial year. Two main deadlines matter:
- Filing deadline: Your company tax return (CT600) must be filed within 12 months of the end of your accounting period.
- Payment deadline: Corporation tax must be paid 9 months and 1 day after the end of your accounting period.
For example, if your accounting period ends on 31 December 2024:
- Your corporation tax payment is due by 1 October 2025.
- Your company tax return must be filed by 31 December 2025.
The better you understand those dates, the easier it becomes to plan your cash flow and avoid last-minute panic.
When Exactly Do You Need To Pay Corporation Tax?
For most startups, the payment schedule is tied closely to the accounting period:
- New companies: Once you incorporate and start trading, you must tell HMRC that your business is active. Your first corporation tax payment will be due 9 months and 1 day after the end of your first accounting period.
- Existing companies: For ongoing businesses, the pattern repeats each year. Your tax payment is typically due 9 months and 1 day after your accounting period end. If you change your financial year, your accounting period and payment dates may also change, and HMRC must be informed.
While most accounting periods are 12 months long, your first year may be slightly different, especially if your incorporation date and accounting reference date don’t line up perfectly.
Why Organisation Is Your Best Defence
Corporation tax becomes far less intimidating when your dates and records are under control. To stay ahead:
- Add key filing and payment deadlines to your calendar as soon as you know them
- Keep track of your accounting period and any changes to your year end
- Work with an accountant or use reliable software if bookkeeping isn’t your strength
Good organisation turns tax from a fire drill into a routine task.
Your First Accounts And First Tax Return As A Limited Company
The first year of trading often brings a bit of extra complexity. Your first set of accounts and your first corporation tax return do not always cover exactly the same dates.
Annual accounts
When you form a limited company, Companies House sets an accounting reference date, which becomes your year end. Your first set of statutory accounts often covers slightly more than 12 months – from the date of incorporation up to that reference date.
For example: if your company is incorporated on 11 May, and Companies House sets 31 May the following year as your accounting reference date, your first accounts will cover around 12 months and 3 weeks.
After that, your accounts usually line up neatly with your financial year, such as 1 June to 31 May.
Company tax return
The period covered by a corporation tax return (your “accounting period for corporation tax”) cannot be longer than 12 months. That means if your first accounts run for more than 12 months, you will probably need to submit:
- One tax return for the first 12 months
- A second tax return for the remaining days covered by your first accounts
This also leads to two separate payment deadlines in your first year. In later years, things usually stabilise: you file one return and make one payment per year, aligned to your financial year.
The exact dates depend on whether your company started trading on the day it was incorporated or began trading later. Delayed trading can shift the start of your first corporation tax accounting period.
How Corporation Tax Affects Cash Flow And Budgeting
For a startup, cash is king. Corporation tax can feel like a sudden hit if you haven’t planned for it throughout the year. To avoid a nasty surprise:
- Estimate your profit for the year as early as you reasonably can
- Calculate a rough corporation tax figure based on those profits
- Put money aside regularly, rather than trying to find the full amount just before the deadline
Treating your tax provision like any other recurring cost (such as payroll or rent) helps smooth out your cash flow and keeps you from scrambling when the due date arrives.
Step-By-Step: How To Pay Corporation Tax
The mechanics of paying corporation tax are straightforward once you understand the sequence. Here is a simple roadmap for new businesses:
- Register with HMRC
After incorporation and once your company starts trading, let HMRC know your business is active for corporation tax purposes. - Keep proper records
Maintain up-to-date records of income, expenses, assets, and liabilities. Good bookkeeping is the foundation for an accurate tax calculation. - Calculate taxable profits
Using your accounting records, work out your profit after allowable expenses and deductions. This figure forms the basis of your corporation tax liability. - File your company tax return
Submit your CT600 to HMRC within 12 months of your accounting period end. The return includes your calculations, accounts, and any supporting information required. - Pay your corporation tax
Make your payment by the deadline (9 months and 1 day after the end of your accounting period), using the correct 17-character payment reference number provided for that period.
Reusing the earlier example: if your accounting period ends on 31 December 2024:
- Pay corporation tax by 1 October 2025.
- File your tax return by 31 December 2025.
Payment Options For Corporation Tax
You have several ways to settle your corporation tax bill:
- Online or telephone banking: Use Faster Payments, CHAPS, or Bacs.
- Direct debit: Set up a direct debit in your HMRC online account. Allow around 5 working days for the first payment and about 3 days for later payments.
- Debit or corporate credit card: You can pay online; note that corporate credit cards usually incur a fee.
- Bank or building society: Use a paying-in slip issued by HMRC.
- Group Payment Arrangement: If your startup is part of a group of companies, the group may use a group payment arrangement for corporation tax.
Two important points to remember:
- Your 17-character payment reference changes for each accounting period. Using the wrong reference can delay the allocation of your payment.
- If a deadline lands on a weekend or bank holiday, your payment needs to arrive with HMRC by the last working day before that date.
Paying For Multiple Accounting Periods
If your startup has two accounting periods in its first year (for example, because your first set of accounts spans more than 12 months), you will:
- File two separate company tax returns
- Make two separate corporation tax payments, each with its own deadline and reference number
If no Corporation Tax Is Due
Even if your company has no corporation tax to pay, you cannot simply ignore the process. You should let HMRC know there is “nil to pay”, usually by filing a return or submitting the relevant information through your online account.
You can also log into your HMRC online portal to check that payments have been received and allocated correctly.
Why Good Record-Keeping Makes Everything Easier
Accurate records are essential for calculating corporation tax properly and defending your position if HMRC ever asks questions. You should keep:
- Sales and revenue records
- Business expense records, including invoices and receipts
- Payroll and employee wage records
- Bank statements and loan documentation
Organised records help you:
- Stay compliant with HMRC requirements
- Understand the financial health of your business
- Identify legitimate deductions, such as office costs, equipment purchases, or research and development (R&D) expenditure that may qualify for relief
Common Corporation Tax Mistakes Startups Make
Startups are often learning as they go, which makes certain errors particularly common:
- Missing deadlines: Filing or paying late leads to penalties and interest that eat into your cash.
- Underestimating the tax bill: Failing to plan for tax can cause cash flow problems when a large bill appears.
- Overlooking allowances and reliefs: Many early-stage companies miss out on schemes such as the Annual Investment Allowance (AIA) or R&D tax relief, which can significantly reduce their tax burden.
Avoiding these mistakes starts with awareness, planning, and, where needed, getting professional support.
Bringing It All Together
Corporation tax can feel complex when you first launch a business, but it becomes far more manageable once you understand the rules and deadlines. By registering promptly, keeping solid records, planning for your tax bill, and making use of available allowances, you can stay on the right side of HMRC and focus on what really matters: building and growing your startup.
If you are unsure about anything, working with a qualified accountant or tax adviser can save time, reduce risk, and often lower your overall tax bill by ensuring you are claiming everything you are entitled to.
