If its an association of Apartments, then very limited rights can be exercised on the use of the 10 covered parking areas used by builder. Since the builder owns a flat in the building, he is also part of the association and am sure would have the documents to prove that he continues to be the rightful owner of the parking lots.
My suggestion: Try negotiation and mediation. Use the the open space available in the premises FOC.
Get the land conveyed as quickly as possible to the Association...Maharashtra govt has recently cleared some legislation on the same...
If negotiation and mediation fails, then approach a good lawyer and get ready for a legal battle to be fought on many fronts....the building sanction plans, urban development permissions obtained and rule applicable, recent HC rulings on the onus of builder to provide parking space and sale of such spaces, the corporation authorities, consumer forum etc.
all d best in resolving the problem (incidently its a problem faced by many flat owners....and egos cause obstacles)...