Notification No. 17/2017-Central Tax (Rate) dated 28/06/2017 made it clear that in case a person is not registered under GST then E-commerce portal have to pay GST.
Here is a cut from notification:
In exercise of the powers conferred by sub-section (5) of section 9 of the Central Goods and Services Tax Act, 2017 (12 of 2017), the Central Government, on the recommendations of the Council, hereby notifies that in case of the following categories of services, the tax on intra-State supplies shall be paid by the electronic commerce operator –
(ii) services by way of providing accommodation in hotels, inns, guest houses, clubs, campsites or other commercial places meant for residential or lodging purposes, except where the person supplying such service through electronic commerce operator is liable for registration under sub-section (1) of section 22 of the said Central Goods and Services Tax Act.
So, you are not required to take registration if your receipts do not cross 20 lakhs in a year. If you are running a business of renting your house as inn or lodge then you will need registration on crossing the limit of 20 lakhs.
If you are renting to individuals not as a business but on regular basis, then there be no GST as renting of house for residing purpose is exempted from GST.