Historically, the main focus of the study of housing in advanced economies has been on houses that meet the accommodation needs of households: houses as the main residence of families. In recent decades there has been the growth in the numbers of houses used for purposes other than as a main residence, for example in the forms of the recent global spread of Airbnb and of foreign engagement in housing as an investment tool; alongside a set of ‘for housing’ houses (FHH) another, overlapping, set of ‘not for housing’ houses (NFHH) is emerging. The present paper begins by identifying four types of NFHH, and considers the significance of their growth. It argues that while the NFHH sector is relatively small it has large impacts, and these are such that they challenge housing researchers and policy makers to develop additional ways of looking at housing systems.