In your first year you will probably file two corporation tax returns, not one — and the first thing you owe HMRC is not money, it is a notification within three months.
New directors tend to assume the company year works like a personal tax year: one period, one return, one deadline. It does not. Companies House sets your accounting reference date automatically, which usually makes your first set of accounts longer than twelve months. A corporation tax accounting period cannot exceed twelve months. So the first period of account splits into two, and each split gets its own return and its own payment date.
Here is the whole sequence, with dates.
Step 1: tell HMRC the company is active — within three months
When you register a company at Companies House you are usually offered the option to register for corporation tax at the same time. Take it. If you did not, you must add Corporation Tax services to your business tax account.
The three-month deadline comes from section 55 of the Finance Act 2004, which requires notice in writing to HMRC “not later than three months after the beginning of the accounting period” — that is, within three months of your company becoming active. GOV.UK’s guidance no longer states the deadline; it now says only that you should add Corporation Tax services to your account when you start to do business. The statutory obligation is unchanged. “Active” means carrying on a business activity: trading, providing services, buying and selling with a view to profit, earning interest, managing investments or receiving any other income.
Preliminary activity does not count. Writing a business plan, negotiating contracts, or incurring costs while deciding whether to start are pre-trading activities, not the start of trading.
You register online with the company’s Government Gateway credentials. Have four things to hand: the company’s registration number, the date you started to do business, the date your first accounts are made up to, and your 10-digit Unique Taxpayer Reference.
Step 2: work out your two accounting periods
Companies House sets your first accounting reference date (ARD) as the last day of the month in which the first anniversary of incorporation falls. Incorporate on 14 September 2026 and your ARD is 30 September 2027.
That means your first period of account runs from incorporation to the ARD — twelve months and a bit. Because a corporation tax accounting period cannot be longer than twelve months, that period splits:
| Period | |
|---|---|
| Period of account (the accounts) | 14 September 2026 to 30 September 2027 |
| CT accounting period 1 | 14 September 2026 to 13 September 2027 |
| CT accounting period 2 | 14 September 2027 to 30 September 2027 |
One set of accounts. Two CT600s. Two computations. Two payment deadlines.
The stub period is short — seventeen days here — but it is a real accounting period with a real return, and the £50,000 and £250,000 corporation tax limits are pro-rated for it.
Step 3: the deadline map
Using the same company, incorporated 14 September 2026:
| What | When | Rule |
|---|---|---|
| Tell HMRC the company is active | Within 3 months of the accounting period starting | HMRC notification |
| Annual accounts to Companies House | 14 June 2028 | 21 months from incorporation, or 3 months from the ARD, whichever is longer |
| Corporation tax payment — period 1 | 14 June 2028 | 9 months and 1 day after 13 September 2027 |
| Corporation tax payment — period 2 | 1 July 2028 | 9 months and 1 day after 30 September 2027 |
| Both CT600s to HMRC | 30 September 2028 | 12 months after the end of the period of account |
Three points that catch people out.
The Companies House deadline is not 9 months in your first year. For a first period of account longer than twelve months it is 21 months from incorporation, or 3 months from the ARD, whichever gives you longer. Here that is 14 June 2028 — not 30 December 2027.
Both returns are due on the same day. Where the period of account is eighteen months or less, both CT600s are due twelve months after the end of the period of account — not twelve months after each individual accounting period ends.
You pay before you file. Both payment dates fall months before the filing deadline. That is how corporation tax always works, but it lands harder in year one when nobody has told you.
Step 4: what actually goes to HMRC
A Company Tax Return is three documents, not one:
- The CT600 form itself
- Your statutory accounts, tagged in iXBRL
- A corporation tax computation, also tagged, showing how you got from accounting profit to taxable profit
HMRC’s free filing service closed on 31 March 2026. From 1 April 2026 you must use commercial software. Paper filing is only available with a reasonable excuse or if you are filing in Welsh. There is no longer a free DIY route for a company tax return — a change many first-time directors have not registered. Companies House accounts filing remains free and its web filing service still works.
If your company was dormant before it started trading
Very common: incorporate in one month, start trading a few months later. GOV.UK splits this on one question — did the company start trading on the day it was set up, or after?
If you started trading on the day the company was set up, your corporation tax accounting periods run from incorporation and the standard first-year split applies: a twelve-month period and a stub, each with its own return.
If you started trading after the company was set up, your corporation tax dates will not line up with your Companies House dates. The dormant stretch before trading began is treated separately from the trading period, so you may still end up filing more than one return.
Either way, the document that settles it is the notice to deliver a Company Tax Return that HMRC sends. It states the exact periods HMRC expects returns for — if it covers both the dormant and the trading period, you must file for both. And either way you still prepare one set of accounts covering the whole period from incorporation to the ARD.
What goes wrong in year one
Missing the notification. Three months passes quickly when you are launching a business, and HMRC can charge a failure to notify penalty.
Filing one return instead of two. The second return is not optional, and the £200 late filing penalty applies to each return that is late. That penalty doubled from £100 for returns with a filing deadline on or after 1 April 2026 — it turns on the deadline, not the date you actually file.
Assuming no profit means no return. If HMRC has issued a notice to deliver a Company Tax Return, you must file it. A loss-making or dormant company that ignores the notice still gets the penalty.
Confusing the Companies House and HMRC deadlines. Different filings, different bodies, different penalty regimes. Companies House late filing penalties start at £150 for a private company up to a month late and rise to £1,500.
Frequently asked questions
When do I need to tell HMRC my new company has started trading?
Section 55 of the Finance Act 2004 requires notice in writing to HMRC not later than three months after the beginning of your corporation tax accounting period — in practice, within three months of your company becoming active. GOV.UK’s guidance no longer states the deadline, but the statutory obligation stands. You do this online through your business tax account.
Why do I have to file two corporation tax returns in my first year?
Because a corporation tax accounting period cannot be longer than twelve months, and your first period of account usually is. Companies House sets your accounting reference date as the last day of the month containing the first anniversary of incorporation, so the first period splits into a twelve-month accounting period and a short stub period, each needing its own CT600.
When are my first company accounts due at Companies House?
Where your first period of account is longer than twelve months, the deadline is 21 months from the date of incorporation, or 3 months from the accounting reference date, whichever is longer. This is different from the 9-month deadline that applies in later years.
Do I still file a corporation tax return if my company made no profit?
Yes, if HMRC has issued a notice to deliver a Company Tax Return. The return is required whether or not there is tax to pay, and the £200 late filing penalty applies even to a company with no corporation tax liability.
Can I file my first Company Tax Return for free?
No. HMRC’s free filing service closed on 31 March 2026, so a Company Tax Return must now be filed using commercial software, with the accounts and computation tagged in iXBRL. Companies House accounts filing is still free and its web filing service still works.
Getting year one right
The first year is the one where the deadlines are least intuitive and the penalties are most avoidable. Two returns, two payment dates, one filing date, and a Companies House deadline that follows a different rule entirely.
SmartFiling prepares and files CT600 returns on a fixed fee, including first-year returns where two periods are involved, the corporation tax computation, capital allowances and HMRC submission, with a three-week turnaround from receiving your records. Every return is reviewed and signed off by an ICAEW Chartered Accountant.
