Full-scale commercial use of a residential flat is often opposed by co-operative housing societies. That said, utilizing 20% of the space in a residential flat is legally permissible as long as the business in question falls within certain categories.
These cases clearly indicate that there are some instances where professional activity in a residential flat would be considered legal. However, these should not be taken as blanket rulings. Apart from the individual bye-laws of housing societies, different state laws would need to be considered.
Whatever the other aspects of the case may be, it is certainly necessary to get the approval of the housing society in a general body before using a residential flat for commercial purposes. Depending on the nature of the business and also subject to the approval of the housing society, it may also be necessary to obtain permission from the local municipal authorities for conversion of the flat into a commercial establishment.
It may be possible to obtain permission from the housing society to conduct a business that does involve any movement of people or storage of goods in the residence. Nevertheless, it is not a good idea to start any kind of business in a residential flat without getting complete clarity on the legal aspects.
Even if a housing society permits business activity in a certain case, such activity can still be deemed illegal and be notified accordingly. It is not advisable to take anything for granted, and to have the legality of such an undertaking examined by a qualified lawyer.