Urban buildings are emerging as critical nodes in the circular water economy, particularly in the context of rising urbanisation, vertical housing, and water scarcity. With over 80% of consumed water converted into wastewater, the reuse of treated domestic sewage presents a significant opportunity for water savings and sustainability in high-rise and mixed-use developments. This chapter explores the classification, treatment, and reuse potential of building-generated wastewater, emphasising the roles of greywater and blackwater segregation, compact treatment systems such as membrane bioreactors (MBRs) and moving bed biofilm reactors (MBBRs), and integration of dual-plumbing architecture. Design considerations for new and retrofitted buildings are discussed, highlighting technical challenges, energy needs, and spatial planning. Various reuse applications are covered, including toilet flushing; heating, ventilation, and air conditioning (HVAC) supply; and irrigation, supported by successful case studies from India, Singapore, and Germany. A comparative analysis of treatment technologies, regulatory frameworks, and cost-energy implications provides a practical guide for stakeholders. The chapter also presents emerging trends such as IoT-based smart reuse systems, integration with rainwater harvesting, and life cycle-based assessment approaches. Research gaps related to antibiotic resistance, public perception, and decentralised governance are also identified. Together, these insights establish a foundation for implementing intra-building wastewater reuse as a mainstream solution to enhance water resilience, environmental compliance, and operational efficiency in urban infrastructure. The chapter contributes to the growing body of knowledge that bridges policy, design, and innovation for decentralised wastewater reuse at the building scale.