The objective of carrying out assessment under section 147 is to bring under the tax net any income which has escaped assessment in original assessment.

Original assessment here means an assessment under sections 143(1), 143(3), 144 and 147 (as the case may be).

In other words, if any income has escaped (*) from being taxed in the original assessment made under section 143(1) or section 143(3) or section 144 or section 147, then the same can be brought under tax net by resorting to assessment under section 147.

(*) In the following cases, it will be considered as income having escaped assessment: Where no return of income has been furnished by the taxpayer, although his total income or the total income of any other person in respect of which he is assessable during the previous year exceeded the maximum amount which is not chargeable to income-tax.

Where a return of income has been furnished by the taxpayer but no assessment has been made and it is noticed by the Assessing Officer that the taxpayer has understated the income or has claimed excessive loss, deduction, allowance or relief in the return.

Where the taxpayer has failed to furnish a report in respect of any international transaction which he was required to do under section 92E.

Where an assessment has been made, but:

i. income chargeable to tax has been under assessed; or
ii. income has been assessed at low rate; or
iii.  income has been made the subject of excessive relief; or
iv.excessive loss or depreciation allowance or any other allowance has been computed;

Where a person is found to have any asset (including financial interest in any entity) located outside India.

For making an assessment under section 147, the Assessing Officer has to issue notice under section 148 to the taxpayer and has to give him an opportunity of being heard. The time-limit for issuance of notice under section 148 is discussed in later part.

If the Assessing Officer has reason to believe that any income chargeable to tax has escaped assessment for any assessment year, then he may assess or reassess such income and also any other income chargeable to tax which has escaped assessment and which comes to his notice subsequently in the course of the proceedings under this section. He is also empowered to re-compute the loss or the depreciation allowance or any other allowance, as the case may be, for the assessment year concerned.

Items which are the subject matters of any appeal, reference or revision cannot be covered by the Assessing Officer under section 147.

Notice under section 148 can be issued within a period of 4 (*) years from the end of the relevant assessment year. If the escaped income is Rs. 1,00,000 or more and certain other conditions are satisfied, then notice can be issued upto 6 years from the end of the relevant assessment year.

In case the escaped income relates to any asset (including financial interest in any entity) located outside India, notice can be issued upto 16 years from the end of the relevant assessment year.
Notice under section 148 can be issued by AO only after getting prior approval from the prescribed authority.