This document provides guidelines for the design and management of gray water reuse system in buildings. This document is applicable to newly built, expanded, or renovated residential buildings, hotels, apartments and similar structures, with the aim of achieving advanced technology, safety, health, and economic efficiency. This document covers gray water reuse including both standard and high grade gray water. It provides the recommendations on specific design methods, process, and parameters, and can be directly used to guide engineering design. This document includes: (1) Scope of the guidelines; (2) Normative references; (3) Terms, definitions, and abbreviations; (4) Raw-water, quantity and quality of gray water; (5) Collection system of gray water; (6) Treatment process of gray water; (7) Reuse system of gray water; (8) Safe guarding and monitoring control.