What is an Accessory Dwelling Unit (ADU)?

An Accessory Dwelling Unit (ADU) is a secondary residential unit located on the same lot as a primary single-family or multifamily home. These units are designed to provide flexible, independent living spaces that can serve a variety of purposes—such as housing for extended family, rental income, or downsizing while staying on your property.

ADUs are often referred to as granny flats, in-law units, backyard cottages, or casitas, and they come in a range of forms—from converted garages and basement apartments to newly constructed detached units. What they all share is the ability to function as a self-contained home, complete with a kitchen, bathroom, sleeping area, and private entrance.

ADUs are a key part of California’s strategy to address the housing shortage by increasing the supply of affordable and diverse housing options within existing neighborhoods—without changing the character of those communities.

Key Features

• Independent Living: Complete facilities for living, sleeping, cooking, and sanitation.
• Secondary to Main Home: Smaller in size and designed to complement the primary residence.
• Not for Short-Term Rental: Minimum rental period is 30 days.
• Owner-Occupancy: Required for single-family zones starting January 1, 2025.

Hermosa Beach regulations related to ADU’s can be found in the Municipal Code in Chapter 17.21. A graphic summary of the City’s regulations can be found here. This comprehensive ADU Handbook has been published by the California Department of Housing and Community Development (HCD).