Yes you can make modifications with full responsibility and permissions as prescribed by building department of MCGM. Moreover most of the societies have their own dos and don'ts ,duly passed by the general body, over above statutory approvals. Strictly modifications can not to be done to any load bearing structural member of the building including all RCC works. During internal modifications, removing and redoing of waterproofing work of toilets, Kitchens, dry balconies , etc. are most important . Also methodology of removing existing masonry and making new ones must be executed using all good construction practices. It is recommended to use light weight concrete blocks or use of dry walls and partitions.Notwithstanding , there is yet another concern - the occupants just below your apartment or office is likely to be most affected.... so have a good rapport with your neighbours!