The agreement is important because it relates to the interests of the parties. In making an agreement, it should be made in writing in order to obtain legal certainty, so that the objectives of the law can be realized. In article 1313 BW states that: "An agreement is an act by which one or more people bind themselves to one or more other people". The transfer of rights to land which is a joint property should be agreed upon by both parties, in this case husband and wife, because from the joint property both parties have partial rights. The Land/Building Sale and Purchase Agreement is proof of the existence of a legally valid sale and purchase transaction of land/building as long as there is an agreement by both parties and has fulfilled the legal requirements of the agreement as stipulated in the Civil Code. The author raises the issue of how the legality of the transfer of joint property which is carried out unilaterally and what are the legal consequences of the transfer of joint property which is carried out unilaterally. The type of research method used is normative law type, which is based on a study of theoretical secondary data to examine primary materials, secondary materials and primary materials, normative research can also be interpreted as research used to examine the rules, legal principles, doctrines and the existing legal system. An agreement is said to be valid if it meets the conditions of the agreement as regulated in the Civil Code. If any of the objective conditions are not met, then the agreement is invalid. Such is the case in the case of the High Court Decision Number: 577/PDT/2019/PT.BDG. As a result of an agreement that does not meet the objective requirements, the agreement is null and void. As a result of being null and void, the agreement is deemed to have never existed. As in the case of the High Court Decision Number: 577/PDT/2019/PT.BDG. There should be no transfer of land ownership rights to the buyer.