Detached ADUs
Standalone backyard units for family, guests or long-term rental use.
ADUs & Home Additions
Detached ADUs, garage conversions, room additions and second stories that add living space to your property.
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Request a Free EstimateTwo ways to add space
There are two fundamentally different ways to add space: expand the home you live in, or create a separate, independent dwelling on your lot. The right choice depends on who will use the space, your lot, your budget and local rules.
Converting an existing structure through a garage conversion often reuses the foundation and walls you already have. Expanding the main house can mean a ground-floor room addition or extension, or building up with a second-story addition when yard space is limited.
Before choosing a concept, use our ADU budgeting guide to identify design, utility and site-work allowances that should be compared in writing.
Ways to add living space
Garage conversions
Converting an existing garage into a comfortable, permitted living space.
Room additions
Extending the home's footprint for bedrooms, offices or larger living areas.
Second-story additions
Adding living space upward when lot size is limited.
Expanding your home vs. creating a separate dwelling
The first decision is whether the new space becomes part of the main house or a separate, independent living unit.
- Expanding the primary residence
- Room additions and second stories add bedrooms, offices or larger living areas that share the home's kitchen, entry and utilities. They follow the zoning rules for the main house.
- Creating an accessory dwelling unit (ADU)
- An ADU is a complete, independent home on the same lot, with its own kitchen, bathroom and entrance. ADUs follow their own set of state and local rules.
- Junior ADUs (JADUs) vs. other ADU types
- A Junior ADU (JADU) is a specific unit type created within the walls of an existing single-family home, and state law caps JADUs at 500 square feet. That cap applies to JADUs only. Other ADUs — detached, attached, garage or other conversions, including some created inside existing space — follow different size standards. Allowable size depends on the unit type, your property, state law and local regulations, and is confirmed for each project.
- Choosing between them
- Consider who will use the space, whether independence and privacy matter, and how the space might be used in the future.
Detached vs. attached ADUs
- Detached ADU
- A standalone backyard home offering the most privacy. Requires its own foundation and utility runs from the main house or street.
- Attached ADU
- Shares a wall with the main house and may simplify utility connections, but offers less separation.
- Garage conversion ADU
- Converts an existing garage into a dwelling, reusing the foundation and structure. See garage conversions for how that process differs.
- Above-garage ADU
- Builds a unit over a new or existing garage, which involves structural evaluation of the garage below.
Room additions and second stories
When the goal is more space in the main home, the lot usually decides whether to build out or up.
- Ground-floor additions
- A room addition or extension expands the footprint for a bedroom suite, family room or larger kitchen. Setbacks and lot coverage limit how far you can build.
- Second-story additions
- A second-story addition keeps the yard intact but requires structural evaluation of the existing foundation and walls.
- Connecting old and new
- Rooflines, floor heights and exterior finishes are planned so the addition looks original to the home.
Utilities, permits and structural considerations
Most of the early work on an ADU or addition happens before construction begins.
- Zoning and local standards
- California state law has expanded where ADUs are allowed, and each city or county applies its own standards for size, height and setbacks. Rules are confirmed for your specific property.
- Utilities
- Water, sewer, gas and electrical capacity are evaluated. A new unit may need a sewer connection route, a panel upgrade or a separate meter depending on the jurisdiction and design.
- Engineering
- Structural engineering is typically required for foundations, new framing and any changes to the existing structure. Soils reports may be required on some lots.
- Plan review and inspections
- Plans are submitted for review, and work is inspected at stages from foundation through final.
How the project typically unfolds
AAA All America Affordable Construction and Remodeling Inc. manages the agreed construction process, including applicable permit coordination, inspections, project management, subcontractor coordination, quality oversight, warranties and project completion. BH Signature personally reviews homeowner inquiries and introduces relevant project details to the identified licensed contracting partner, which handles construction planning and delivery.
- Step 1
Feasibility review
Review the lot, zoning, setbacks, utilities and goals to identify which options may work.
- Step 2
Design and engineering
Develop architectural plans, structural engineering and any required energy or soils documentation.
- Step 3
Plan review and permits
Submit plans to your city or county and respond to plan-check comments until permits are issued.
- Step 4
Foundation and framing
Build the foundation and framing, with inspections at required stages.
- Step 5
Systems, finishes and final inspection
Install plumbing, electrical, mechanical and finishes, then complete final inspection.
Responsibilities depend on the agreed project scope and contract. Permit approval rests with the relevant authorities; warranty coverage and terms are confirmed for your project. Completion timing depends on the work, site conditions and required approvals, not a promised date.
ADU and addition rules vary by city and county. Plans typically require design, engineering and permit approval before construction.
Comparing ways to add space
| Option | Independent living unit? | Best when | Key considerations |
|---|---|---|---|
| Detached ADU | Yes | You want privacy and have backyard space | New foundation and utility runs |
| Garage conversion | Can be | An existing garage is underused | Parking, insulation, ceiling height |
| Room addition | No | You need more space in the main home | Setbacks and lot coverage |
| Second-story addition | No | The lot has no room to expand outward | Structural capacity and longer construction |
Specific size, height and setback limits depend on your city and property.
Frequently asked questions
How do I know whether my property can have an ADU?
Start by checking the property address, lot conditions, access, utilities and applicable ADU rules. Easements, hillside conditions and other restrictions can affect the design. Feasibility and approval require a property-specific review.
How much does an ADU cost in Southern California?
Costs vary with site access, unit size, foundation, utilities, materials, design and permitting. Garage conversions and detached ADUs can have substantially different costs. The contractor can prepare an estimate after reviewing the proposed project.
What's the difference between an ADU and a room addition?
An ADU is a separate, independent dwelling with its own kitchen and entrance. A room addition expands the main house and shares its kitchen and entry.
Is a garage conversion cheaper than building a new ADU?
It often costs less because the foundation and structure already exist, but upgrades for insulation, windows, utilities and code compliance can be significant. Costs depend on the garage's condition.
How large can an ADU be?
Allowed size depends on the ADU type, the property and current state and local requirements. A property-specific feasibility review is needed to confirm limits and the approval process.
Do I need an architect or engineer?
Most ADUs and additions require drawn plans and structural engineering for permit submittal. The contractor can explain how design and engineering are coordinated.
How long does the process take?
Timelines vary widely by city, design complexity and plan-check rounds. The design and permit phase is often a substantial part of the overall schedule.
Start Your Project With a Personal Connection.
Considering this home improvement? Tell BH Signature what you have in mind. The licensed contractor can then discuss your property, project scope, consultation, and estimate directly with you.
Request Your Free EstimateFree estimates are provided by the licensed contractor, AAA All America Affordable Construction and Remodeling Inc..