Filing a CT600 for a Short Accounting Period
Not every CT600 covers a full 12 months. If your company has a short accounting period (less than 12 months), the tax calculation works differently and there are specific rules you need to follow.
This guide covers when short periods happen, how they affect your tax, and how to file correctly.
When Do Short Accounting Periods Happen?
1. Shortening a New Company's First Period
By default, a new company's first accounting period runs to the last day of the month of its first anniversary — which is usually slightly longer than 12 months, not shorter (more on that below). But many founders shorten their accounting reference date to align with the tax year, and that creates a short first period:
- Company incorporated: 15 July 2025
- Accounting reference date shortened to: 31 March 2026
- Period length: 8 months 17 days
2. Changing Your Year-End Date
If you change your accounting reference date, you might have a short transitional period:
- Old year-end: 30 June
- New year-end: 31 March
- Transitional period: 1 July 2025 to 31 March 2026 = 9 months
3. The "Stub" Period After a Long First Year
If a company keeps its default accounting reference date, its first set of accounts covers more than 12 months — but a Corporation Tax accounting period can never exceed 12 months. HMRC therefore splits it into a 12-month CT600 plus a second, short CT600 covering the remaining days. That short stub return is one of the most common short periods in practice.
4. Company Cessation
When a company ceases trading or is wound down, the final period runs from the last year-end to the cessation date — often less than 12 months.
5. Joining a Group
Sometimes companies align their year-ends with a parent company, creating a short transitional period.
How Short Periods Affect Tax Calculation
Threshold Prorating
Corporation tax thresholds are prorated for short periods. This is crucial because it affects which rate you pay.
Example: 9-month period
| Threshold | Full Year | Prorated (9/12) |
|---|---|---|
| Lower limit | £50,000 | £37,500 |
| Upper limit | £250,000 | £187,500 |
This means if your company earns £45,000 profit in a 9-month period, you're above the prorated lower limit (£37,500) and will pay the main rate with marginal relief — even though £45,000 would be below the small profits threshold for a full year.
Financial Year Splits
If your short period crosses a financial year boundary (1 April), profits must be split between financial years:
Example: Period 1 January 2025 to 30 September 2025
- Financial Year 2024 (to 31 March 2025): 90 days
- Financial Year 2025 (from 1 April 2025): 183 days
- Total: 273 days
Profits are apportioned by the number of days in each financial year, and each portion is taxed at that year's rate.
Associated Companies Prorating
If you have associated companies, the thresholds are first divided by the number of associated companies plus one (i.e. including your own company), and then prorated for the short period. So a company with one associated company filing a 6-month return has a lower limit of £50,000 ÷ 2 × 6/12 = £12,500.
Common HMRC Rejection Errors
Error 9101: Period Too Long
A CT600 can cover at most 12 calendar months. A period from 1 April 2025 to 31 March 2026 is exactly 12 months — perfectly valid, and in fact the most common CT600 period there is. What gets rejected is a period that runs beyond 12 months, for example 1 January 2025 to 31 January 2026 (13 months).
If your accounts cover more than 12 months, you can't squeeze them into one return — you need two CT600s: one for the first 12 months and a second for the remainder. Also make sure your return dates match the period HMRC expects (the notice to deliver, form CT603) — a mismatch is another common cause of rejection.
Error 9106: Future Dates
You cannot submit a CT600 for a period that hasn't ended yet. The end date must be in the past.
Error 9043: Threshold Calculation Mismatch
If your tax computation doesn't correctly prorate the thresholds for a short period, HMRC's validation will reject it. This is one of the most common errors for short-period returns.
Capital Allowances in Short Periods
The Annual Investment Allowance (AIA) is also prorated for short periods:
| Period Length | AIA Available |
|---|---|
| 12 months | £1,000,000 |
| 9 months | £750,000 |
| 6 months | £500,000 |
| 3 months | £250,000 |
Writing Down Allowances (WDA) are similarly prorated — an 18% WDA for a 6-month period becomes 9%.
Step-by-Step: Filing a Short Period CT600
1. Determine Your Period Dates
- Start date: The day after your previous period ended (or incorporation date for first period)
- End date: Your chosen accounting reference date (or cessation date)
Make sure the total is not more than 12 months.
2. Prepare Your Figures
All income and expense figures should be for the actual short period — don't annualise them.
3. Check Threshold Prorating
Verify that your tax computation correctly prorates:
- The £50,000 lower limit
- The £250,000 upper limit
- The AIA limit
- Any associated company divisions
4. Verify Financial Year Split
If your period crosses 1 April, ensure profits are correctly split between financial years.
5. Submit and Verify
File your CT600 and check for HMRC validation errors. The most common issues are:
- Period longer than 12 months
- Threshold prorating
- Financial year apportionment
- Return dates not matching the period on HMRC's notice to deliver
Your First CT600 as a New Company
A common surprise: if you keep the default year-end, your first CT600 is usually not short — your second one is. Here's why.
When you incorporate, Companies House sets your accounting reference date to the last day of the month of your first anniversary. Incorporate on 15 July 2025 and your first accounts run to 31 July 2026 — that's 12 months and 17 days, slightly longer than a year.
Since a Corporation Tax accounting period can't exceed 12 months, that first long period needs two CT600s:
- Return 1: 15 July 2025 to 14 July 2026 (a full 12 months)
- Return 2: 15 July 2026 to 31 July 2026 (a short 17-day stub period)
What to expect in practice:
- HMRC will send a notice to deliver a return (CT603) — usually after you register for Corporation Tax or start trading
- Your accounting period starts when you start trading (often the incorporation date)
- All thresholds are prorated for the short return — the 17-day stub above gets roughly 17/365 of each threshold
- Each return must be filed within 12 months of the end of its period
Your first CT600 is only a short one if you shorten your accounting reference date — for example, to 31 March.
Choosing Your Year-End
Popular year-end dates:
- 31 March — aligns with the Corporation Tax financial year (avoids financial year splits); shortening to it gives you one short first period, then clean 12-month years
- 31 December — calendar year
- The default (last day of your incorporation month) — no Companies House paperwork, but your first accounts cover slightly more than 12 months, so you'll file two CT600s the first time around
Pro tip: 31 March is ideal if you want to avoid financial year split calculations. Your first period will be short, but every subsequent one will be a clean 12 months with a single set of rates.
How Taxpipe Handles Short Periods
Taxpipe automatically:
- ✅ Prorates all thresholds based on your period length
- ✅ Splits profits across financial years if needed
- ✅ Applies the correct tax rates per financial year
- ✅ Prorates capital allowances (AIA, WDA)
- ✅ Validates period dates before HMRC submission
You just enter your period dates and figures — Taxpipe does the calculations.
Filing a CT600 for a short period? Taxpipe handles the complexity for £59 — automatic threshold prorating, financial year splits, and HMRC validation.