ॐ असतो मा सद्गमय
The proper officer, in relation to assessment of taxes on the unregistered taxable person, shall issue the assessment order in FORM GST ASMT-15 within 5 years from the date specified under Section 44 for furnishing of the annual return for the financial year to which the tax not paid relates.
(ICAI FAQ PUBLICATIONS 06-09-2017 ASSESSMENT OF UNREGISTERED PERSONS: FAQ NO. 21)
If a taxable person fails to obtain registration even though liable to do so or whose registration has been cancelled under section 29(2) but who was liable to pay tax, the proper officer may assess the tax liability to the best of his judgement after providing opportunity of being heard to such person.
The proper officer shall issue a notice to a taxable person in FORM GST ASMT-14 containing the grounds on which the assessment is proposed to be made on best judgment basis. The registered person will be allowed a time of fifteen days to furnish his reply.
(ICAI FAQ PUBLICATIONS 06-09-2017 ASSESSMENT OF UNREGISTERED PERSONS : FAQ NO. 20)