The Japanese spatial concept of Ma is an experiential space-time interval mediating human consciousness and the environment that has historically structured the vernacular Kyomachiya (traditional townhouse) through the horizontal layering of intermediary thresholds. However, high-density urbanization and the loss of traditional exterior space have rendered the conventional Doma (earthen floor), especially in form of Toriniwa, a dark, linear, and passive earthen corridor, functionally and spatially obsolete in contemporary domestic architecture. Addressing a significant gap in the study of modern vernacular dwellings, this thesis investigates the morphological evolution of the Doma from a horizontal service threshold into an internalized, vertical volumetric void within small-scale Japanese residential architecture. Through a comparative architectural analysis, this study examines the historical baseline of the traditional Machiya, the transitional renovation methodologies of Shigenori Uoya and the complete internalization strategies in new constructions by Suppose Design Office and ALTS Design Office. Ultimately, this research concludes that while the geometric manifestation of the Doma has radically transformed from a two-dimensional plane into a three-dimensional vertical volume to accommodate the spatial constraints of the modern metropolis, its inherent phenomenological function which is to dynamically mediate the boundary between exterior and interior domains, has successfully remained intact.