An attached project may add new floor area to the primary building or convert qualifying existing space. A junior ADU is a narrower form contained within a qualifying single-family structure and is subject to its own size and occupancy rules.
Use current guidance from the authority reviewing the address. The California HCD handbook was updated in March 2026, and local rules can change as cities align with state law.
Separate the three concepts
| 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 | existing 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 provider use these labels interchangeably in a cost proposal.
Evaluate the main home as part of the project
Record rooms, circulation, windows, exterior access, shared walls, existing services and any area the household will lose. A plan that creates an ADU but leaves the primary home with poor privacy, storage or daylight may not be a good result.
For an attached addition, inspect the connection between old and new structure, roof geometry, drainage, foundation relationship and mechanical/electrical/plumbing routes.
Privacy and sound need a written assembly
Ask the drawings and scope to identify:
- 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 proposed assembly and penetrations.
Utilities may be shared or separated
The right metering and service strategy depends on the utility, jurisdiction, intended operation and existing capacity. Ask whether the proposal includes separate panels, meters, water heating, shutoffs and controls, and who confirms those arrangements.
See utilities and site work. A short physical route may still require substantial service or interior restoration work.
What should feasibility confirm?
The feasibility review should state the governing jurisdiction, existing legal space, intended unit type, plausible floor area, exterior access, structural implications, utility basis and any tenant or occupancy issues requiring qualified 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 detached-site work but may create difficult connections to the existing building. An interior conversion can preserve the envelope but require extensive systems, fire, sound and finish work.
Use the cost guide and ask for:
- 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
- restoration and closeout.
When is this path a poor fit?
Test a detached ADU when attached work damages the main home’s plan, creates extensive structural alteration or cannot provide the intended privacy. Test a garage conversion when a verified accessory structure 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 provider to price the same existing-condition information and proposed assemblies. Verify its CSLB licence, 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 construction beside living space. The proposal should explain temporary weather protection, dust separation, security, noise, working hours and how water, power, heating or cooling interruptions will be managed. If a wall or roof is opened, identify the sequence that restores protection and who responds outside normal site hours.
Map the routes workers and materials will use. A plan that depends on moving through occupied rooms needs a different protection and cleanup scope from one with direct exterior access. Record which rooms, yard areas, parking spaces and services will be unavailable during each stage.
Check long-term operation before finalising the layout
Decide how residents will reach entries, bins, meters, shutoffs, outdoor areas and parking without passing through private space. Locate equipment so maintenance does not require entry into the other dwelling. Consider mail, deliveries, lighting, address visibility and emergency access.
If systems are shared, document how costs and repairs will be handled. If they are separate, 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 specific 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 proposal 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.