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File CT600 split over two periods, capital allowance

Posted by Archie Hammett 16 Jun 2026 at 12:36
Tiny Tax have worked out my CT600 for just over a 12 month period, hence it is split into two. When I have tried to submit I get the following message from HMRC:

Submission blocked:

Please review the per-period capital allowance split - see the banner at the top of the capital allowance section.

To be honest, I can't find the section it means, also I can't seem to be able to go in and manually split the capital allowance over the two periods, 1 year and 26 days.

Any help and advice greatly appreciated.

Many thanks.

1 Comment

T
Tiny (AI Assistant) Staff 16 Jun 2026 at 13:32
The "Submission blocked" message is coming from TinyTax itself, not HMRC — it's a one-off check to make sure you've confirmed how your capital allowances are allocated across the two CT600 periods before anything is sent.

Here's how to find and resolve it:

1. Go back to the CT600 form (click "Edit Form" or the back button from the Review & Submit page). You're looking for the Tax Computation section within the form.

2. Scroll down to the Capital Allowances area. Because your accounting period runs from 6 Aug 2024 to 31 Aug 2025 (12 months and 26 days), TinyTax files two CT600 returns — one for the first 12 months and one for the remaining 26 days. The AIA / capital allowances row will have a split sub-row beneath it showing how the total claim is divided between Period 1 and Period 2.

3. By default the split is done by day-ratio — so Period 1 (365 days) gets roughly 93% of the allowance and Period 2 (26 days) gets around 7%. If your capital expenditure actually all happened in one period — say, all in the first 12 months — you can click either of the two £ amounts to edit them directly and put the correct figures in. The only rule is that the two amounts must sum to your total claim within £1.

4. Once you've reviewed the split (even if you keep the default day-ratio figures), the submission block should clear and you can continue to submit.

A couple of practical notes:
— The AIA annual limit is £1,000,000 per year, pro-rated for short periods. For the 26-day stub period the limit is around £71,000. For most small companies this is well above what's claimed, so it's unlikely to be an issue.
— If you're not sure which period the expenditure fell in, the day-ratio default is the safe option and is accepted by HMRC.

Let us know if the section doesn't appear as expected or the block doesn't clear after you've reviewed the split.