What is the 60 day capital gains rule?

You have 60 days from completion to report a taxable gain on UK residential property to HMRC and pay the tax.

It is a standalone return, separate from Self Assessment, and it applies even if you file a return anyway. The gain then appears again on your annual return, with the payment already made credited against it.

If there is no tax to pay, for example because the gain is covered by your allowance, there is nothing to file.

Penalties start once the 60 days pass, so the time to work out whether there is a gain is before completion rather than after.