Can I enter a note in comments box of ct600

There isn't a general free-text comments box on the CT600 return itself — the return is a structured form of numbered boxes, and neither HMRC's own CT600 schema nor our filing form has an "any other information" field for arbitrary notes to HMRC.

A couple of things worth knowing:

- If a note relates to a balance sheet figure (debtors, creditors, fixed assets etc.), the relevant statutory disclosure note in your accounts is generated automatically from the figures you enter — you don't need to add anything manually there either.
- If you need to explain something to HMRC that isn't covered by the standard boxes or accounts notes, the usual route is to contact HMRC directly after filing (by phone or via your Corporation Tax online account), quoting your UTR and accounting period.

If none of your entries need further explanation via the standard boxes, you don't need to add anything extra — HMRC processes the structured figures as submitted.