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. New Company's First Period
When you incorporate a new company, your first accounting period often isn't a full year. For example:
- Company incorporated: 15 July 2025
- First year-end: 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. 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.
4. 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, and then prorated for the short period.
Common HMRC Rejection Errors
Error 9101: Period Too Long
HMRC counts dates inclusively. A period from 1 April 2025 to 31 March 2026 is actually 366 days (including both dates), which exceeds 12 months. The correct end date would be 30 March 2026 for exactly 365 days.
This catches many people out — especially when aligning year-ends to 31 March.
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
Make sure the total is not more than 12 months (counting inclusively).
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 length calculation (inclusive dates)
- Threshold prorating
- Financial year apportionment
First CT600 for a New Company
If you've just incorporated, your first CT600 will almost certainly be for a short period. Here's what to expect:
- HMRC will send a CT600 notice — usually within a few months of incorporation
- Your accounting period runs from incorporation to your first year-end
- All thresholds are prorated — a 6-month period means £25,000 lower limit (not £50,000)
- File within 12 months of the end of the period
Choosing Your Year-End
Popular year-end dates:
- 31 March — aligns with the tax year (avoids financial year splits)
- 31 December — calendar year
- Your incorporation month — simplest, exactly 12 months
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 tax rate.
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
- ✅ Handles the inclusive date counting correctly
You just enter your period dates and figures — Taxpipe does all the complex calculations.
Filing a CT600 for a short period? Taxpipe handles the complexity for £59 — automatic threshold prorating, financial year splits, and HMRC validation.