The legal issues regarding the transactions of unlawful buildings have persisted for decades. The Supreme Court of Taiwan invented the idea of "right of de facto disposal" and provided flesh and blood to this idea over a series of cases, precedents, and resolutions. Nevertheless, these judicial constructs still fail to solve the problem of unlawful buildings, as evidenced by the large number of lawsuits nowadays. Using economic analysis of law, I propose a new theory to construct the nature and contents of right of de facto disposal. Specifically, I argue that in addition to strictly enforcing the building code and tearing down all the unlawful buildings, the best solution is to include unlawful buildings in the real estate registry and impose civil fines to owners of unlawful buildings. The second best solution is to construe right of de facto disposal as a type of property rights created by custom, and to provide notice through the building tax registry. Tax registration would serve as a necessary condition for acquiring a full-fledge, in-rem right of de facto disposal, which resembles a permanent usufruct. Holders of rights of de facto disposal are also subject to civil fines.