Accessory dwelling units remain one of the most popular ways California homeowners add flexible living space to an existing property. An ADU can serve as a guest house, rental unit, home for family members, office, or long-term property improvement. The best results start with a careful feasibility review before design work begins.
What Counts as an ADU?
California recognizes several common ADU configurations, including detached units, attached units, and conversions of existing space such as a garage. The California Department of Housing and Community Development maintains the state’s current ADU guidance and updated its ADU Handbook in March 2026.
State law creates broad ADU rights, but the exact design and permitting path still depends on the property and local jurisdiction.
Step 1: Property Feasibility
Before spending heavily on plans, verify the basic site conditions. Important items can include lot configuration, existing structures, setbacks, access, utility locations, electrical capacity, sewer or septic conditions, parking conditions, fire access, grading, and whether the property is in a special planning or coastal area.
A good feasibility review helps identify expensive complications early rather than after drawings are complete.
Step 2: Choose the ADU Type
The right approach depends on your property and goals. A detached ADU offers separation and privacy. An attached ADU may work well when the home has a suitable side or rear area. A garage conversion can make use of existing square footage, although the existing structure may still require substantial upgrades.
Think about long-term use. A layout intended for rental income may differ from one designed for aging parents, adult children, or a private home office.
Step 3: Design and Plans
Once feasibility is established, the design can move into floor plans, elevations, structural details, energy requirements, and the other documents required for permit review. Design decisions should consider privacy, natural light, storage, mechanical systems, utility routing, exterior materials, and how the new unit relates to the main house.
Step 4: Permits and Jurisdiction Review
Los Angeles County includes many cities, and each project must follow the permitting process of the authority that has jurisdiction over the property. State ADU law has streamlined many approval rules, but homeowners should still expect plan review, code compliance, and inspections.
For current statewide rules, use the California HCD resources as the starting point. Local planning and building departments should then be checked for property-specific requirements.
Step 5: Utilities and Site Work
Utility planning is often one of the most important parts of an ADU project. Electrical service, plumbing, sewer connections, water supply, gas or electric appliances, HVAC, drainage, and trenching can materially affect the construction scope. Older properties may need panel upgrades or other infrastructure improvements.
Step 6: Construction
After permits are issued, construction generally moves through site preparation, foundation work, framing, rough plumbing and electrical, roofing and exterior work, insulation, drywall, cabinets and finishes, fixtures, final inspections, and punch-list completion. The sequence varies depending on whether the project is new construction, an addition, or a conversion.
Budgeting for an ADU
ADU costs vary widely because no two sites are identical. Size is only one factor. Site access, utility distance, foundation type, structural work, finish selections, plans, engineering, permit fees, electrical upgrades, drainage, and landscaping restoration can all affect the final investment.
A realistic proposal should explain assumptions and exclusions instead of relying on a single square-foot number.
Choose a Contractor With Relevant Experience
Verify the contractor’s California license through the CSLB license lookup. Ask who will coordinate plans, permitting, inspections, subcontractors, and project communication.
Ocean Home Builders provides ADU, addition, remodeling, and design-build construction services throughout Los Angeles and Orange County. Learn more about our ADU services.
Start With a Site Review
If you are considering an ADU, the most useful first step is to review the property and define the intended use of the space. Contact Ocean Home Builders to discuss your property, project goals, and next steps.