Sustainability in the conservation of architectural heritage is now more extensively considered than it was decades ago. The introduction of the concept of sustainability to the field marked hopes to overcome problems threatening heritage sites. There are general concepts guiding sustainable conservation. However, heritage specifics play important roles in achieving sustainability, and may direct the formulation of sustainable concepts to be applied. This paper is an attempt to add to the discourse of heritage sustainability by discussing buildings managed through a tradition of Islamic waqf in the World Heritage Stone Town of Zanzibar. It examines sustainability in terms of its financial, social, managerial, and environmental aspects. The relatively good survival rate of waqf buildings in the old town over several centuries suggests sustainable and transmissible ‘genes’ within the tradition. Waqf was found to elaborate ways to strike a balance between heritage consumption and use, avoiding gentrification, and enabling collective urban conservation. It further suggests that sustainable conservation cannot avoid monetary sacrifices, and if it is to be sustainable in the long term it should be inherent in the heritage itself.