Transaction cost theory is largely used to investigate the formal domain of land and housing. In the informal domain, however, this perspective has been employed largely as a supplement in addressing the other fundamental notion in new institutional economics—property rights—despite the possibility that informality in land development can emerge regardless of the informality or formality of such rights. To cover this gap, this study developed a theoretical framework based on transaction cost theory to explain the formal–informal dichotomy in land development. The proposed framework maintains that land development depends on engaging in transactions that involve total or partial ownership of a combination of capital and land through lease and/or sale contracts, which enable landowners to earn from the new rental prices produced by the increase in land prices. Landowners are afforded two avenues from which to reduce transaction costs, namely, formal and informal institutional frameworks, each defining and enforcing restrictive rules on agents’ actions. These avenues, however, are simultaneously a source of new transaction costs that can affect the expected financial return of land development. Landowners therefore tend to choose the institutional framework that entails lower transaction costs but enables higher gains. Thus, the higher transaction costs associated with a formal institutional framework are the primary deterrents to the selection of this structure. In turn, informal land development continues to expand, regardless of the existence of formal prohibitive measures. We investigated the formal–informal dichotomy in the rural land development process in Tehran Province, Iran. The results indicated that transaction costs cause inefficiency in formal institutions, thereby driving the perpetuation of informal development.