ADU statute limits the size of an attached ADU to 50% of the size of the existing primary dwelling (Gov. Code Section 65852.2(a)(1)(D)(iv)). The local agency is generally required to allow an ADU of at least 850 sq. ft, or 1000 sq. ft. if it contains more than one bedroom. In a situation where the local agency’s development standards, as noted in an adopted ADU ordinance, constrain the area for an ADU, it could be limited to no less than 800 sq.ft. (Gov. Code Section 65852.2(c)(2)(B) & (C)).