An attached project may add floor area to the main home or convert an existing space. A junior ADU is built within some single-family homes. It has its own size and occupancy rules.
Use current rules from the permit office for the address. California updated its HCD handbook in March 2026. Local rules can change as cities follow new state law.
Distinguish the three options
| Path | Basic concept | Main design question |
|---|---|---|
| Attached new ADU | new dwelling area shares a wall with the main home | can the addition fit without damaging the home’s layout and site use? |
| Interior/conversion ADU | current permitted space becomes an independent dwelling | what structural, fire, sound, egress and utility work is required? |
| JADU | compact unit within a qualifying single-family structure | do the location, facilities and owner-occupancy rules fit the household plan? |
Do not let a company use these labels interchangeably in a cost quote.
Evaluate the main home as part of the project
Record the rooms, paths through the home, windows and outside access. Add shared walls, current utilities and any space the household will lose. A new ADU is not a good result if the main home loses too much privacy, storage or daylight.
For an attached addition, check where the old and new parts join. Review the roof shape, drainage and foundation. Trace the heating, power and plumbing routes.
Privacy and sound need a written assembly
Use the drawings and written scope to answer the questions below.
- independent exterior access;
- doors or connections between units;
- wall and floor/ceiling assemblies;
- plumbing and equipment noise;
- window placement and private outdoor areas;
- refuse, mail and address handling; and
- how construction affects occupancy of the main home.
“Soundproof” is not a specification. Request the planned assembly and penetrations.
Utilities may be shared or separated
The right meter and service plan depends on the utility, permit office, planned use and current capacity. Ask if the quote includes separate panels, meters, water heaters, shutoffs and controls. Name who will confirm the plan.
See utilities and site work. A short physical route may still require substantial service or interior repair work.
What should property check confirm?
The early site review should name the permit office, legal space and planned unit type. It should give a likely floor area and show outside access. Add structural and utility issues. Flag any tenant or occupancy question that needs expert advice.
Do not remove bedrooms or protected tenant space based only on a conceptual drawing.
How should costs be compared?
An attached addition avoids some yard work but may be hard to join to the main home. An interior conversion keeps the outer shell. It may still need major work on utilities, fire safety, sound and finishes.
Use the cost guide and ask for the items below.
- design and engineering;
- demolition and temporary protection;
- structural alteration;
- envelope and waterproofing;
- fire/sound assemblies;
- electrical, plumbing and HVAC;
- kitchen, bath and finishes;
- work inside the main home;
- permits and inspections; and
- repairs after the work and final approval.
When is this path a poor fit?
Test a detached ADU if attached work would harm the main home’s layout. Do the same if it needs major structural work or lacks privacy. Test a garage conversion if a sound existing garage offers a better layout.
Do not proceed with a JADU solely because it appears cheaper. Confirm that its legal and operational limits match the long-term use.
What belongs in the builder comparison?
Ask each company to price the same existing-condition details and planned assemblies. Verify its CSLB license, relevant attached/conversion experience, who performs design and who manages occupied-home protection.
The builder comparison guide covers payment schedules, change orders, proof and warranty terms.
Plan work around an occupied home
Attached and interior projects put building work next to living space. The quote should cover weather protection, dust, security, noise and work hours. It should explain any loss of water, power, heat or cooling. If a wall or roof is opened, state how and when it will be made safe. Name who responds after normal work hours.
Map the routes for workers and materials. A path through occupied rooms needs more protection and cleanup than direct outside access. Record which rooms, yards, parking spaces and services cannot be used at each stage.
Check long-term operation before finalising the layout
Decide how residents will reach doors, bins, meters, shutoffs, yards and parking. They should not have to cross private space. Place equipment so repairs do not require entry into the other home. Plan for mail, deliveries, lighting, visible addresses and emergency access.
If systems are shared, document how costs and repairs will be handled. If they are different, price the extra equipment and utility work. Add these choices to the Los Angeles California ADU cost guide before comparing construction totals.
Frequently asked questions
Is a JADU simply a small ADU?
No. It is a distinct statutory form with exact location, size, facility and occupancy conditions. Use current state and local guidance.
Can an attached ADU share systems with the main home?
Some systems may be shared depending on design, code, utility and operational choices. The quote should state the actual arrangement and controls.
Is an interior conversion always the least disruptive path?
No. Work may occur directly within an occupied home and can affect structure, services, finishes and circulation.