Future cities will have to confront limited urban spaces and resources, undertake the preservation or conservation of sense of place, and continuously improve the existing urban environment. Accordingly, urban void spaces are likely to become key strategic places for ‘Green Urban Development’. Urban voids are spaces that are useless, underused, abandoned, or in-between spaces among public and private realms. This research looks into urban voids that can be found especially within the residential environment in Seoul, as a chance for sustainable urban design. Dispersed urban voids have been generated due to various reasons, such as intrinsic to policy and planning system, changing economic, social and functional aspect and further on. The study briefly evaluates the existing built environment especially the quality of urban spatial structure and public spaces in the residential area, which is made up of individual buildings. Existing urban voids are extracted, identified and then classified into three major categories- plot, block, and street condition. A crucial aspect would be showing how these urban voids could be used or reused in terms of ‘green urban development’, which should consider not only the importance of preservation, but also balancing with new developments. Conventional low-level technology which involves planting and greening and environmental high technology which includes fuel cell, electric car station, rainwater storage and so on can be potentially applied and integrated into these urban voids. Consequently this research paper will suggest that each improvement measure should be considered as a piecemeal ‘act’ of an integrated urban regeneration and transformation of a whole city with adequate development guidelines.