Corporation Tax

Your first CT600, and the long-first-year trap

The single most confusing thing about a company's first year: one long accounting period becomes two tax returns.

7 min readUpdated 13 July 2026

The short version

The rule
A Corporation Tax accounting period can't exceed 12 months (CTA 2009 s.10). Your first accounts usually run longer.
What you file
One CT600 for the first 12 months, one for the remaining days. Both to HMRC; one set of accounts covers both.
Two of everything
Two payment deadlines (9 months + 1 day after each period ends) and two 17-character payment references.
How it splits
Turnover and profit apportioned by days across the periods, unless you know the real attribution.

Two calendars that don't line up

Your company keeps two clocks. Companies House measures your accounting reference date — by default the last day of the month you incorporated in, a year later. HMRC measures your Corporation Tax accounting period, which the law caps at 12 months and can never stretch beyond it.

Because your first Companies House period runs to a month-end, it's almost always a little longer than 12 months — 12 months and a handful of days, sometimes a full 13. That few days is the whole problem.

Why one long year becomes two returns

A Corporation Tax accounting period can't exceed 12 months (CTA 2009 s.10). So when your accounts cover more than that, HMRC splits the time into two accounting periods:

  • Period 1 — the first 12 months. Its own CT600.
  • Period 2 — the leftover "stub", from day 366 to the end of your accounts. Its own CT600 too.

One set of accounts covers both, and you attach those accounts to the first return. But the tax computation happens twice, once per period.

The stub period is where the tax gets fiddly

Corporation Tax rates and the marginal-relief thresholds are scaled to the length of the period. A 30-day stub gets 30/365ths of the profit limits — so a modest profit in a short stub can fall into the marginal band and be taxed at a higher effective rate than you'd expect. The split isn't just administrative; it changes the number.

How the money is divided

By default, turnover and profit are apportioned across the two periods by the number of days in each — HMRC's time basis (CTM01405). If you genuinely know the real attribution — the trade only started in month eight, say — you can allocate on that basis instead, and you should, because it's usually more favourable and it's correct.

Two deadlines, two payment references

Two periods means two of the things people assume they have one of:

  • Two payment deadlines — Corporation Tax is due 9 months and 1 day after each period ends, so the two dates are about a month apart.
  • Two 17-character payment references — each period has its own reference (your 10-digit UTR plus a period suffix). Paying both with the same reference, or with the bare UTR, is a common way to have a payment land against the wrong period.

How WrenTax does it

WrenTax refuses to compute a single return over more than 12 months — because that quietly produces the wrong tax — and instead plans the two returns for you: day-apportioned by default, oldest first, each with its own deadline and payment reference laid out plainly. You can see both CT600s in full before anything is filed. Our corporation tax calculator runs the very same engine, so you can check the split yourself. General information, not tax advice.

Common questions

Why do I have to file two tax returns in my first year?

Because a Corporation Tax accounting period can never be longer than 12 months, but your first Companies House accounting period usually is — a company incorporated on the 12th has a first period running to the end of that month a year later, so 12 months and a bit. HMRC splits that into a 12-month period and a short 'stub' period, and each needs its own CT600.

How is profit split between the two periods?

By time, unless you know better. Turnover and profit are apportioned across the two periods by the number of days in each, following HMRC's default basis (CTM01405). If you actually know when the money was earned — the trade started late, say — you can allocate on that real basis instead.

Do I pay Corporation Tax twice?

Yes — two. Corporation Tax is due 9 months and 1 day after the end of each accounting period, so the two periods have two payment deadlines. Each payment also has its own 17-character reference (your 10-digit UTR plus a suffix for the period) — paying both with the same reference is a common and costly mistake.

Keep reading

Corporation Tax deadlinesReadHMRC error 1046, fixedReadFiling dormant accountsRead

This guide is general information about UK company filing, not tax or legal advice. Figures and deadlines are current for 2026; always check your own dates against Companies House and HMRC. Register data © Companies House.