The builder cannot convert the residential property to commercial property even with the NOC of the other member owners of the premises.
The discretion to convert a residential plot to a commercial plot is with development/competent authorities in the local. The builder has to obtain permission from them and after the approval he may convert the same into commercial.
As a member you can raise objection to this, if you have any.
Parking slots cannot be sold, there are various rules objecting the selling of parking slots.
In any case the apartments have to get a parking space each besides, the commercial flats can be allotted with the remaining parking slots each one.
The builder cannot use the parking space for the purpose of office, this is objectionable because the common areas belong to all the members of the society and the builder cannot sell the parking space or convert any space for his own office use.
The other residents can object to the commercial activities in the residential area