This study aimed to determine the legal relationship of the parties in the Agreement on Sale and Purchase (PPJB) Flats and legal protection for buyers of Flat Housing Unit if the seller / developer defaults. This research is a normative juridical research, namely research on laws that are in the legislation in force in Indonesia. This research focused on document research or literature which essentially looks for theories, views that have correlation and are relevant to the problems to be studied. The results of the study indicated that the legal relationship between the parties in the PPJB starts from the signing of the PPJB, so that a legal relationship is established which creates the rights and obligations of each party. Legal protection for buyers of Flat Units if the seller / developer defaults is through arbitration at the cost of joint responsibility by the parties. It meant that if there is a dispute between the parties regarding the sale and purchase agreement unit apartment, the settlement is not through public court or litigation, but the solution is taken through arbitration (non litigation)