For decades, residential redevelopment in South Korea has been synonymous with high-rise apartment complexes, providing modern living and handsome profits for developers. So when South Korean architect Seung H-Sang recently proposed a revamp of a Seoul slum with simple low-rise homes, he faced strong opposition.

Mr. Seung, 52, is pushing to preserve a part of Baeksa Village, one of Seoul’s last remaining shantytowns. In a collaboration with 12 architects, he hopes to rebuild the makeshift houses with modern, low-rise structures,...