After the fall of Phnom Penh to the Khmer Rouge on April 17, 1975, Chantaraingsey is thought to have retreated with his men to the area around the former hill station of Kirirom, where he had considerable support from the local peasantry.
He supervised the design and construction of new towns such as Tioulongville (Kirirom) and Sihanoukville (Kompong Som) and important town plans such as the Bassac development in Phnom Penh, where a mix of cultural facilities such as the National Theatre Preah Suramarit and the Exhibition Hall neighboured with large housing experiments.