Maintenance charges in apartment

I am living in a apartment in which 6 flats was constructed G+2 (Ground floor Full car parking, Each floor 3 flats,880,890,1040sq.ft) by the builder as per the plan. We have fixed the maintenance charges as Rs.500 per flat. One of the owner brought two flats 880+890 sq.ft and converted into a single flat with two car parking, one kitchen, one Main door, one EB connection and Four bathrooms. Now he is refusing to give the maintenance charges for the two flats and he is arguing, it a single flat. How to solve this dispute legally? With Regards, S.Narayanan, Chennai