GOV.UK
Roll-over relief: assessment and postponement
Except in cases within CG60660, any assessment on the gains accruing on the disposal of the old assets should be made at the normal time. YOU SHOULD NOT WAIT UNTIL THE THREE YEAR REPLACEMENT PERIOD HAS EXPIRED. You can amend the assessment if you subsequently receive a valid claim to relief.
You can accept an appropriate postponement application if the claimant can demonstrate an intention to acquire a qualifying asset shortly after the normal payable date for the tax and within the normal three year time limit. This is not to say that a postponement application should be accepted on the basis of such an intention during the normal three year time limit. If a qualifying asset is to be acquired some time after the normal payable date for the tax, no postponement application should be accepted. You should consider explaining that interest may be payable on any postponed tax that is not eventually rolled over.
The possibility that all or part of the asset disposed of may not have been a qualifying asset should be borne in mind when considering a postponement application.
If the claimant has entered into an unconditional contract for the acquisition of new assets provisional relief may be due, see CG60620. You can accept a postponement application until it is clear whether relief will be due.