Areas that are covered with natural vegetation have been converted into asphalt,concrete,or roofed structures and have increased surface impermeability and decreased natural drainage capability.Conventional drainage systems were built to mimic natural drainage pattems to prevent the occurrence of waterlogging in developed sites.These drainage systems consist of two major components: 1) a stormwater conduit system,and 2) a runoff storage system.Runoff storage systems contain retention basins and drywells that are used to store and percolate runoff,whereas conduit systems are combination of catch basins and conduit pipes used to collect and transport runoff.The construction of these drainage systems is costly and may cause significant environmental disturbance.In this study,low impact development (LID)methods that consist of extensive green roofs (GRs) and permeable interlocking concrete pavements (PICPs) are applied in real-world construction projects.Construction project documents were reviewed,and related cost information was gathered through the accepted bidding proposals and interviews of specialty contractors in the metropolitan area of Phoenix,Arizona.Results indicate that the application of both LID methods to existing projects can save an average of 27.2％ in life cycle costs (LCC) for a 50-year service life and 18.7％ in LCC for a 25-year service life on the proposed drainage system,respectively.