Last updated: 26 October 2026 · Written by Axel
Your first company accounts and corporation tax return almost never cover the same dates. Companies House counts from the day the company was incorporated to its accounting reference date, which is usually a little over 12 months later. HMRC counts from the day the company started trading (or first had income), and a tax period can never run longer than 12 months. So a new company files one set of accounts but can end up with one or two CT600s, each with its own payment date. Once you know which dates drive which filing, the first year is not hard. It just isn't the same as every year after it.
The Companies House side: your first accounts
When a company is incorporated, Companies House sets its accounting reference date (ARD) for you. Companies House guidance puts the first ARD at "the last day of the month in which the anniversary of their incorporation falls" (Life of a company: accounts). Your first financial year starts on the day of incorporation and ends on that date.
Incorporate on 10 March 2026 and your first ARD is 31 March 2027. The first accounts therefore cover a little over 12 months, and nobody chose that. It is simply the days left in the month you set up in.
The deadline is where most new directors get caught out. After the first year, private company accounts are due 9 months after the year end. The first accounts are different: GOV.UK gives the deadline as 21 months after the date you registered with Companies House. The fuller rule, for first accounts covering more than 12 months, is 21 months from incorporation or 3 months from the ARD, whichever is later. In practice that is almost always the 21-month date, and it usually lands a few days or weeks before the "9 months after year end" date people assume.
- Covers: incorporation day to the first ARD, including any weeks before you traded.
- Due: 21 months from incorporation (for a first period over 12 months).
- One set of accounts, however the tax side splits.
If the company did nothing at all before trading, that quiet stretch still sits inside the first accounts. If it stays dormant the whole period, see dormant company accounts: what to file instead.
The HMRC side: your first corporation tax period
HMRC does not care when you incorporated. Under the Corporation Tax Act 2009, an accounting period begins "when the company comes within the charge to corporation tax", which in plain terms means when it starts to do business. GOV.UK says that includes buying, selling, advertising, renting a property and employing someone, and that your first accounting period starts from the date you started to do business. Bank interest counts as income too, so a company with money sitting in an interest-paying account may not be as dormant as you think.
The second rule is the one that splits returns. GOV.UK is blunt: a corporation tax accounting period "cannot be longer than 12 months". When your accounts cover more than 12 months of trading, "you must file 2 returns to cover the period of your accounts".
So which is it, one CT600 or two?
- Started trading a while after incorporating: the tax period runs from the trading date to the ARD. If that is 12 months or less, it is one return.
- Traded from day one (or within the first few weeks): the trading stretch is over 12 months, so HMRC splits it. The first return covers the first 12 months, the second covers the few days or weeks left.
Telling HMRC you have started
Companies must notify HMRC of the beginning of their first accounting period "not later than three months after the beginning of the accounting period" (Finance Act 2004, section 55). When you register the company with Companies House, GOV.UK offers to set it up for Corporation Tax at the same time. If you skipped that, you do it by adding Corporation Tax services to your business tax account, where you give the date you started to do business and the date your first accounts are made up to. HMRC then sends an activation code and the Corporation Tax payment deadline. Check that deadline against your own dates, because your trading start date fixes the tax period.
Three months is the latest you can leave it, not the target. You can tell HMRC the day you start trading, and it is worth doing early, because you have to wait for the activation code before you can finish.
Worked example: incorporated in March, trading in June
A plasterer incorporates on 10 March 2026, spends the spring finishing jobs as a sole trader, and starts trading through the company on 1 June 2026. Nothing goes through the company account before then. Deadlines use the GOV.UK rules: tax paid 9 months and 1 day after the tax period ends, CT600 filed 12 months after it ends.
| Filing | Period covered | Deadline |
|---|---|---|
| Tell HMRC trading has started | Starts 1 June 2026 | By the end of August 2026 |
| First accounts to Companies House | 10 March 2026 to 31 March 2027 | 10 December 2027 |
| Corporation tax payment | 1 June 2026 to 31 March 2027 | 1 January 2028 |
| CT600 to HMRC | 1 June 2026 to 31 March 2027 | 31 March 2028 |
One set of accounts, one CT600. The quiet weeks in spring are inside the accounts but outside any tax period, so the tax period is 10 months long and needs only one return.
Now change one fact: the company starts trading on 10 March 2026, the day it was incorporated. The accounts do not change. The tax side does.
| Filing | Period covered | Deadline |
|---|---|---|
| First accounts to Companies House | 10 March 2026 to 31 March 2027 | 10 December 2027 |
| CT600 number 1, tax payment | 10 March 2026 to 9 March 2027 | 10 December 2027 |
| CT600 number 2, tax payment | 10 March 2027 to 31 March 2027 | 1 January 2028 |
| Both CT600s filed | Both periods | 31 March 2028 |
Two things to notice. First, the tax for the first 12 months is due on the same day as the accounts, weeks before most people expect a tax bill. Second, both returns are filed by the same date. The profit from the one set of accounts is shared between the two returns, normally by the number of days in each.
The same split happens if trading started on 1 June but HMRC was never told, and it works from the incorporation date instead. The second diagram at the top of this example shows the two side by side.
Mistakes that cost new companies money
- Working to "9 months after year end" for the first accounts. The 21-month date usually comes first.
- Letting the incorporation date stand as the tax start date when you started trading later. HMRC will usually go with it, but it can mean two returns instead of one and an earlier tax bill.
- Filing one CT600 for more than 12 months. A long first period needs two.
- Paying the tax when you file. The payment date comes months before the filing date.
Late filing costs more than you would think. Companies House charges £150 for accounts up to a month late, rising to £1,500 at over 6 months, and doubles it if your accounts are late two years running. HMRC adds £200 for a CT600 a day late and another £200 at 3 months (Company Tax Returns: penalties). The full picture is in late filing penalties at Companies House and HMRC, and there is a deadline lookup at ct600.axel.trade/filing-deadlines.
When the first period needs an accountant
The simple cases above are very doable yourself. Some first periods are not, and it is worth paying for advice:
- You extended your first year end towards the 18-month limit and the business changed a lot along the way.
- There were real costs or income in the "quiet" months before trading, and you are not sure how pre-trading spending should be treated.
- The company is part of a group, owns property, or took over an existing sole trader business with stock, vans and equipment.
- You do not know when trading started, and the honest answer could move the tax period by months.
Trading start is the one date only you know. Don't pick a convenient one. It fixes a legal filing period.
Getting your first filings sorted
Most first years come down to three dates: incorporation, ARD and the day you started trading. Get those right and the rest follows. A.X.E.L Filing reads your incorporation date and year end from the public register, asks when you started trading, and shows you whether your first year is one return or two. If it is two, it splits the profit between them by the day and files both with the one set of accounts, still for the one £49 fee. You file micro-entity accounts to Companies House and the CT600 to HMRC with your own Government Gateway login (how it works). There is a longer explainer of the split at ct600.axel.trade/first-accounting-period, and a walk-through in how to file a CT600 yourself.
Quick answers
Frequently Asked Questions
01When is my first corporation tax return due?
Your CT600 is due 12 months after the end of the tax period it covers. If your first accounts are split into two tax periods, both returns are due 12 months after the end of the accounts. The tax itself is due earlier, 9 months and 1 day after each tax period ends.
02Why do my first accounts cover more than 12 months?
Companies House sets your first year end to the last day of the month of your first anniversary. Incorporate on the 10th and your first accounts run 12 months plus the rest of that month. That is normal and needs no action.
03Does my first accounting period start when I incorporate?
At Companies House, yes. For corporation tax, no: it starts when the company starts to do business or first has income. Weeks before that are in your accounts but not in any tax period.
04Do I need two CT600s in my first year?
Only if the company traded for more than 12 months inside the first accounts, which usually means it started trading straight after incorporating. If you started trading later, one return often covers it.
05What if my company did nothing in its first year?
It is probably dormant for corporation tax, but you still file accounts with Companies House. Our guide to dormant company accounts covers what to file.
The first year is the odd one out. After it, accounts and tax periods normally line up and every year looks the same. When your dates are clear, start your first return at file.axel.trade: £49 covers the micro-entity accounts and CT600 together, filed to Companies House and HMRC under your own login.
A.X.E.L Filing
Accounts and CT600, filed in one sitting.
Prepare your accounts and Corporation Tax return with the tax worked out as you type, next to your own Self Assessment, and let your AI run the what-ifs. Then file to Companies House and HMRC yourself. No accountant fees, no subscription.
£20 for a dormant company
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