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
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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