All residentially-zoned properties are eligible to construct one ADU and one Junior ADU (JADU), regardless of lot size. For studios and one-bedroom ADUs, 850 square feet is allowed and up to 1,000 square feet for 2 or more bedrooms. A JADU is created within the walls of a single-family residence and may share central systems and restrooms with the primary residence, and must contain a basic kitchen. JADUs are allowed up to 500 square feet. Refer to the Development Code Section 22.42.120 for additional ADU requirements - https://library.municode.com/ca/diamond_bar/codes/code_of_ordinances?nodeId=CICO_TIT22DECO_ARTIIISIPLGEDEST_CH22.42STSPLAUS_S22.42.110REACUSST.
Once you have plans ready, you may submit through the Planning inquiry form. No additional forms are required at this stage. Please allow a few days for the Planning and Building and Safety Divisions to review for completeness. Once authorization is received, a planner will instruct you on the next steps for formal building plan check submittal to the Building and Safety Division. There is a 15-working day turnaround once accepted by the Building and Safety Division. Every subsequent submittal is 10 working days. When all comments/corrections have been addressed, your permit will be issued for the ADU.