Changing Your Company Year End: CT600 & Tax Implications
Changing your accounting reference date (year end) creates a short or long accounting period — with implications for your CT600.
How to Change Your Year End
File form AA01 with Companies House. You can:
- Shorten your accounting period (to a minimum of 1 day)
- Extend your accounting period (to a maximum of 18 months, once every 5 years)
HMRC is automatically notified by Companies House.
CT600 Implications
Short Period
If you shorten your year end, you'll have a period shorter than 12 months.
Example: Year end was 31 December, changing to 31 March. Your next period is 1 January 2025 to 31 March 2025 (3 months).
Effects:
- Corporation Tax thresholds are prorated (e.g., small profits rate: £50,000 × 3/12 = £12,500)
- Capital allowances are prorated (AIA: £1M × 3/12 = £250,000)
- Filing deadline: 12 months from the short period end
- Payment deadline: 9 months + 1 day from the short period end
Long Period (Over 12 Months)
HMRC doesn't allow accounting periods longer than 12 months. If you extend your year end, you'll need to file two CT600s:
Example: Year end extended from 31 December 2024 to 31 March 2026.
- CT600 #1: 1 January 2024 to 31 December 2024 (12 months)
- CT600 #2: 1 January 2025 to 31 March 2026 (15 months → split into 12 + 3)
Actually, HMRC splits it as: first 12 months + remainder.
Common Reasons for Changing
- Aligning with the tax year — 31 March year end simplifies tax planning
- Seasonal businesses — align year end with quiet period
- Group alignment — match parent company's year end
- First year adjustment — new companies often change from the default
Tax Planning Opportunity
Changing your year end can be a powerful tax planning tool:
- Short period = lower thresholds — but also lower tax if profits are falling
- Timing capital allowances — bring AIA claims into a specific period
- Loss utilisation — create a short period to isolate losses for carry-back
Filing with Taxpipe
Taxpipe handles short accounting periods correctly — we prorate the Corporation Tax thresholds, marginal relief bands, and capital allowances automatically.
Changing your year end? File your CT600 with Taxpipe — we handle short periods, prorating, and marginal relief automatically. £59.