Yes, this is a current limitation — TinyTax does not support CT600 Box 45.
It's worth clarifying what Box 45 is for: it is specifically for terminal loss relief, which applies when a company has permanently ceased trading and wants to carry back losses from its final 12 months against profits from the preceding 3 years. If your company is still trading, Box 45 would not be the right box regardless — a standard carry-back of a trading loss uses Box 860 instead.
Neither Box 45 (terminal loss relief) nor Box 860 (standard carry-back) are currently available in TinyTax.
What TinyTax does handle:
- Filing your CT600 for the current loss-making year showing £0 tax due
- Carrying the loss forward automatically to offset future profits (Box 780 / Box 250 in the next period)
To make a carry-back claim, you have two options:
Option 1 — HMRC's Corporation Tax Online Service: Log in to your HMRC business tax account and file an amended CT600 for the prior profitable year, including the carry-back amount in Box 860.
Option 2 — Use an accountant: An accountant can submit the carry-back claim on your behalf, which is worth considering if the amounts are significant.
We have a full guide here with more detail on both options: https://tinytax.co.uk/support/loss-carry-back-claims
Update Post Status
Where did you first hear about TinyTax?
Helps us focus our time and budget. Thanks for sharing!
Your Free Access is Ending
Thanks for being an early TinyTax user!
We introduced paid subscriptions in November 2025, and you've enjoyed free access since then. From now on, you'll need a subscription to file new returns.
Your past submissions remain accessible.
Solo: £40/year (1 company) Portfolio: from £99/year (up to 100 companies)
Previous Period Required
Period ending is overdue and must be filed first.
Already filed elsewhere? Mark it as filed to proceed.
1 Comment
Log in to comment
It's worth clarifying what Box 45 is for: it is specifically for terminal loss relief, which applies when a company has permanently ceased trading and wants to carry back losses from its final 12 months against profits from the preceding 3 years. If your company is still trading, Box 45 would not be the right box regardless — a standard carry-back of a trading loss uses Box 860 instead.
Neither Box 45 (terminal loss relief) nor Box 860 (standard carry-back) are currently available in TinyTax.
What TinyTax does handle:
- Filing your CT600 for the current loss-making year showing £0 tax due
- Carrying the loss forward automatically to offset future profits (Box 780 / Box 250 in the next period)
To make a carry-back claim, you have two options:
Option 1 — HMRC's Corporation Tax Online Service: Log in to your HMRC business tax account and file an amended CT600 for the prior profitable year, including the carry-back amount in Box 860.
Option 2 — Use an accountant: An accountant can submit the carry-back claim on your behalf, which is worth considering if the amounts are significant.
We have a full guide here with more detail on both options: https://tinytax.co.uk/support/loss-carry-back-claims