Question25: One of my client is having Residential Rent s. 16,20,000/= and Commercial Rent 12,00,000/= Total Rent Received 28,20,000/=. Whether GST registration is necessary or not'

Answer: Renting of residential dwelling for use as residence is exempt from GST vide S.no 12 of Notification No. 12/2017-Central tax and renting for commercial purpose is a taxable supply under GST.
According to section 23 (1)(a) if you are engaged exclusively in the business of supplying goods or services or both that are not liable to tax or wholly exempt from tax under this Act or under the Integrated Goods and Services Tax Act then not liable for registration. Otherwise as per section 22 you are required to registration if your " aggregate turnover" exceeds Rs. 20 lakh in a financial year.
Section 2(6) defines "aggregate turnover' which means the aggregate value of all taxable supplies (excluding the value of inward supplies on which tax is payable by a person on reverse charge basis), exempt supplies, exports of goods or services or both and inter-State supplies of persons having the same Permanent Account Number, to be computed on all India basis but excludes central tax, State tax, Union territory tax, integrated tax and cess.
Thus according to above provisions aggregate turnover in your case is Rs 2820000 (i.e taxable supply Rs. 1200000 + Exempt supply Rs 1620000) which exceeds Rs 20 lakh . so you are required to registered under GST. (Reply dt. 18/3/2018)