Accounting Periods Explained — When Does Your Corporation Tax Year Start and End?

Your accounting period determines when you file your CT600 and when you pay Corporation Tax. Get it wrong and you could miss deadlines or file for the wrong dates. Here's how it works.

What is an accounting period?

An accounting period is the time window your CT600 covers. It's the period for which you report your company's income, expenses, and Corporation Tax liability.

Most accounting periods are exactly 12 months long, but they can be shorter (never longer than 12 months for Corporation Tax purposes).

Accounting period vs financial year

These terms are often used interchangeably, but they're slightly different:

• Financial year (Companies House): The period your annual accounts cover. This can be up to 18 months for a new company.
• Accounting period (HMRC): The period each CT600 covers. This can never exceed 12 months.

If your Companies House financial year is longer than 12 months (common for new companies), HMRC will split it into two accounting periods, each requiring a separate CT600.

When does your accounting period start?

For a new company, the first accounting period starts on the date of incorporation. After that, each new period starts the day after the previous one ended.

For existing companies, accounting periods run continuously — one ends, the next starts immediately. There should be no gaps.

Common accounting period end dates

You can choose any year-end date, but the most popular are:

31 March is the most common for UK companies because it aligns with the Corporation Tax financial year, simplifying rate calculations when rates change between financial years.

How accounting periods affect your deadlines

Your accounting period end date determines everything:

Example

Accounting period: 1 April 2025 – 31 March 2026

• Corporation Tax payment due: 1 January 2027
• CT600 filing deadline: 31 March 2027
• Companies House accounts due: 31 December 2026

First year accounting periods

Your first accounting period is special:

1. It starts on the date of incorporation
2. It ends on the date you chose as your company's year end
3. It can be shorter or longer than 12 months (up to ~18 months)

If it's longer than 12 months

HMRC splits it into two periods. Each requires its own CT600.

Example: Company incorporated 1 October 2024, year end 31 March 2026 (18 months).

HMRC creates:

• Period 1: 1 October 2024 – 30 September 2025 (12 months) → CT600 #1
• Period 2: 1 October 2025 – 31 March 2026 (6 months) → CT600 #2

You'll need to apportion your income and expenses between the two periods. Taxpipe handles this split automatically.

If it's shorter than 12 months

No problem. File one CT600 for the shorter period. Your deadlines are still calculated from the period end date, so a shorter period means earlier deadlines.

Changing your accounting period

You can change your company's financial year end with Companies House (form AA01). Common reasons:

• Aligning with the tax year (31 March)
• Matching a parent company's year end
• Moving to a calendar year

When you change your year end:

• Your next accounting period will be longer or shorter than 12 months
• If longer, HMRC splits it into multiple CT600 periods
• You can only extend your year end once every 5 years (Companies House rule)

Straddling tax years

If your accounting period spans two Corporation Tax financial years (which run 1 April – 31 March), and the tax rate changed between them, your profits are apportioned by days.

Example: Accounting period 1 January 2023 – 31 December 2023.

• 1 Jan – 31 Mar 2023: 90 days at the old rate
• 1 Apr – 31 Dec 2023: 275 days at the new rate

Taxpipe's auto-computation handles this apportionment automatically.

What triggers a new accounting period?

A new accounting period starts when:

1. The previous one ends (normal succession)
2. The company starts trading (if previously dormant)
3. The company stops trading (this ends the current period)
4. The company enters or leaves a group
5. The company starts or ceases to be UK-resident

Dormant company periods

Dormant companies still have accounting periods. If your company is dormant, your accounting period is determined by:

• HMRC's records (they'll send you a notice to deliver a CT600)
• Or, if you've notified HMRC of dormancy, they may pause issuing notices

Even if HMRC pauses notices, you still have accounting periods for Companies House purposes.

Short periods

A short accounting period (less than 12 months) can occur when:

• You're in your first year and your year end comes quickly
• You've changed your year end
• The company started or stopped trading mid-year

Short periods have the same filing requirements as full-year periods — you just have fewer months of activity to report.

Multiple CT600s in one year

It's possible to file multiple CT600s in a calendar year if:

• Your first accounting period was extended beyond 12 months
• You changed your year end
• The company started and stopped trading in the same year

Each CT600 covers one accounting period. Each has its own filing deadline and payment deadline.

Getting your dates right

The most common mistake with accounting periods is using the wrong dates on your CT600. Here's how to check:

1. Check HMRC's letter — they send a "notice to deliver" that specifies the exact period they expect a CT600 for
2. Log in to HMRC — your online account shows the periods they have on record
3. Check Companies House — your confirmation statement shows your current accounting reference date

If you're unsure, HMRC's Corporation Tax helpline (0300 200 3410) can confirm your accounting periods.

Filing with Taxpipe

When you create a return with Taxpipe, you enter your accounting period start and end dates. We validate them for you:

• Checking the period doesn't exceed 12 months
• Warning if dates don't match standard patterns
• Calculating your filing and payment deadlines automatically

File your CT600 now →

Related articles:

• CT600 Deadlines: When Is Your Company Tax Return Due?
• Corporation Tax Rates 2025/26 and Marginal Relief Explained
• Your First Company Tax Return — A New Director's Guide

Related Articles

• Changing Your Accounting Period: How It Affects Your CT600
• Your First CT600 Timeline: Key Dates
• First Year Corporation Tax: Complete Guide for New Companies
• Accounting Period Longer Than 12 Months: What Happens