This project builds on and leverages NREL's Solicitation #1 project titled "BEopt-CA (EX): A Tool for Optimal Integration of Energy Efficiency, Demand Response, Energy Storage and PV for California Homes." The BEopt software tool finds the least cost combination of energy efficiency and solar PV, providing support to integrated demand side management (iDSM) and Zero Net Energy (ZNE) for residential buildings in California. For this project, the NREL team will extend the tool and develop multifamily (including duplexes, triplexes, townhouses, flats and low-ride apartment buildings) residential modeling capabilities in BEopt for use in California. Specific multifamily modeling additions include developing multifamily-specific operating conditions, adiabatic surfaces, enhancements to input screens (geometry, option and site screens), enhancements to output screens (aggregate results), utilize multithreading (parallel simulations and use of multiprocessors for faster simulations) and multifamily HVAC and DHW (including solar) systems. Throughout the project, team members will test the software framework to ensure that the BEopt multifamily enhancements are accurate and reliable.