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Accessory Dwelling Units (ADUs) (also referred to as Granny Units, Companion Units and Second Dwelling Units) are generally defined as a dwelling unit on a residential lot in addition to a primary dwelling. The ADU can be attached to the primary dwelling or be detached. An ADU provides independent living facilities for one or more persons and includes permanent provisions for living, sleeping, cooking and sanitation. The State of California passed legislation requiring all jurisdictions to update their ADU ordinances by Jan. 2017, and then again by Jan. 2020, liberalizing requirements for ADUs. There is no longer any minimum lot size, and ADUs that comply with the ordinances must be approved ministerially. Mountain View passed a new ADU ordinance in March of 2017 and again in 2020 to comply with new state laws. Los Altos passed a new ADU ordinance in July of 2018 and by May of 2019 had seen 32 ADUs permitted, far more than Mountain View. Los Altos is still working on its 2020 revisions in order to be in compliance with state law. Its ordinance will likely be similar to Mountain View’s.

Generally, all single-family lots can have both a Junior Accessory Dwelling Unit (JADU) and an ADU, which can be attached or detached, assuming they can comply with rest of the ordinance. A JADU is created out of existing space in the main home, can be up to 500 sq. feet, plus an additional 150 sq. feet to accommodate ingress/egress, as a separate entrance is required. ADU’s up to 850 sq. feet for a 1-bedroom and up to 1,000 sq. feet for a 2-bedroom are allowed. No setbacks are required if the ADU is created from a legally established existing space. Otherwise, the rear and side setbacks must be at least 4 feet and ADUs up to 16 feet high are allowed. If a garage is converted to an ADU, the parking does not need to be replaced. Mountain View requires a 10-foot setback from the primary home, while Los Altos tentatively will require only 5 feet, hoping to keep the ADU from being too close to the property line.

Mountain View prohibits short-term rentals in all ADUs less than 800 sq. feet; Los Altos tentatively is prohibiting short-term rentals in all ADUs.

ADUs are also allowed in multifamily complexes, up to 1 for every 4 units.

Los Altos Hills uses ADUs to comply with its RHNA allocation, regardless of whether or not the units are occupied by low-income households. Los Altos is also hoping to count all new ADUs towards its lower-income RHNA allocation and probably will do a survey to attempt to see what the rent levels actually are, following the method of Los Altos Hills.

Granny Unit, Companion Unit, Second Dwelling Unit
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