Yes you can do that but if you are doing the business as a company you will have make the changes in objects of business and then rent them out and then you will have to also register the same under GST as you will be doing it as business.
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you can convert it to business or can offer the same income under housing property. it depends on the ownership of flats its with individual or with company....regarding registration there is no restriction under GST Law before taking application kindly insure you enjoy the threshold exemption allowed under GST law....Happy2help email yogesh.kanojiya485@gmail.com

Yes, you can do it, but for that you have to create a corporate entity and one of the main Object clause in Momorandum of Association needs to be renting of immovable property.