Single vs 2 Story ADU in San Diego: Which One Makes More Sense?

“Should you build up or build out?” is one of the first big questions San Diego homeowners ask about ADUs. Accessory Dwelling Units are small, separate homes that exist on the same property as your primary residence. Homeowners increasingly build them to create additional living areas for their parents who need assistance and to generate rental income. San Diego residents must decide between constructing a single-story ADU or a two-story ADU as their primary building option. One design solution extends horizontally across the ground space, while the second design solution creates vertical space by constructing upward. Your selected option will depend on your lot dimensions, your financial resources, the local building regulations, and your intended usage of the area.

What Is a Single‑Story vs 2 Story ADU in San Diego?

A single‑story ADU sits on one level. It spreads out sideways, taking more of the lot footprint. It’s often simpler to build and easier to move around. This works well if you have room in the yard or care about easy access.

A 2 story ADU in San Diego is built up in two levels. It uses less ground area but gives you more total square footage. This is helpful when your lot is small, but you want meaningful space.

Are 2 Story ADUs Allowed in San Diego?

Most cities in the San Diego region do allow 2 story ADUs under local codes. California state rules, including updates like SB 897, support them in many cases.

Key points:

• Many areas permit ADUs to reach heights of approximately 18 feet and allow heights of 20 feet near transit when considering roof pitch.  
• Cities establish different height limits for attached ADUs that connect to houses and garages which enable buildings to reach heights of about 25 feet. 

What can change the rules:

• The local zoning laws define the regulations which apply to your specific city and neighborhood. 
• The distance people can walk to public transportation and high-population areas determines the accessibility of these locations. 
• Height assessment procedures rely on specific measurement points which include wall heights and roof ridge heights and parapet heights. 

You should perform a feasibility or zoning check at the beginning of your project because it will help you identify potential problems that may arise later.

Key Differences: Single vs 2 Story ADU in San Diego

1. Land Use & Footprint

• Single‑story ADU: Uses more lot space, fine if you have extra yard.
• Two‑story ADU: Needs less ground area, so it keeps more yard open and covers less of the lot.

2. Design Flexibility

• Single‑story: Layouts are usually simple and straightforward.
• Two‑story: Lets you separate living areas—common space downstairs, bedrooms, and private areas upstairs.

3. Accessibility

• Single‑story: Better for seniors, mobility issues, or aging‑in‑place. No stairs to worry about.
• Two‑story: Requires stairs and railings, which can be harder for some users over time.

4. Privacy & Rental Potential

• Single‑story: Often better for family use or long‑term living.
• Two‑story (especially over‑garage or stacked units): Can be more attractive for rental income or multi‑family use, since it feels like a full, separate home.

Cost Comparison: Is It Cheaper to Build Up or Out?

Construction Cost Overview

• A typical 2 story ADU in San Diego often runs around $300–$350 per square foot, depending on finishes, site, and design.
• Total cost changes with the layout and how much work is needed on the lot.

Where Two‑Story ADUs Save Money

• Smaller foundation and roof because the building is narrower.
• Less lot coverage, which can mean less grading, dirt moving, and site prep.

Where Two‑Story ADUs Cost More

• Staircases, railings, and extra structural support.
• More complex labor—lifting materials, scaffolding, and tighter spaces.

Bottom Line

The difference between single‑story and two‑story costs is often fairly small, roughly in the $10K–$15K range, depending on size and finish. For many owners, lot size and zoning matter more than the raw price.

Zoning, Height & Legal Considerations

Height limits and setbacks vary from city to city around San Diego. Many places require:

• Minimum 4‑foot setbacks from side and rear walls.
• Height limits that change if you’re near transit or in certain zones.

Some zoning quirks:

• Extra height allowances near transit or dense multifamily areas.
• More flexibility on multifamily lots or when using bonus programs.

An early feasibility or design review with a planner or architect can help you avoid issues later.

Types of 2 Story ADUs in San Diego

1. ADU Over Garage (Carriage House)

• Uses space above the garage that’s often underused.
• Keeps parking on the ground level.
• Frequently used for rental income, since it feels like a separate unit.

2. Traditional Two‑Story ADU

• Looks like a small home.
• Living and common areas downstairs, bedrooms and private rooms upstairs.
• Works well for family members or tenants who want a full‑home feel.

3. Stacked ADUs

• In some cases, two separate units can be stacked, especially on multifamily properties or under bonus rules.
• Offers maximum rental potential but only in certain situations.

When a Single‑Story ADU Makes More Sense

A single‑story ADU may be the better fit if:

• You have a larger lot and enough yard space.
• Accessibility is a priority,  aging in place or someone with mobility limits.
• You want simpler construction and easier permitting.
• You prefer a less complex layout and straightforward flow.

When a 2 Story ADU in San Diego Is the Better Choice

A 2 story ADU in San Diego often makes more sense when:

• Your lot is small or tight, and you want to keep the yard open.
• You want to maximize square footage without covering more ground.
• You care about rental income or multi‑family use.
• Local zoning allows vertical expansion, and you’re okay with stairs and extra structure.

Design Considerations for Both Options

Whether you choose single or two‑story, good design helps both work well:

• Efficient floor plans that use space smartly.
• Natural light and airflow through windows and doors.
• Privacy from the main house, think about sight lines and entry locations.
• Parking and access that don’t feel cramped.
• Fire safety and building codes, especially for two‑story setups.

Modular or prefab options can help simplify complex two‑story builds, making construction faster and more predictable.

Common Mistakes to Avoid

• Ignoring local zoning differences between cities in the region.
• The layout needs your proper evaluation of height and setback rules before you proceed to create a final design. 
• The project suffers from stair and circulation planning faults which create dangerous and awkward pathways. 
• The design process fails to incorporate future needs for tenants who will require accessibility solutions and family members who will need to use the space.

Choosing the Right ADU for Your Property

The right choice needs to consider budget needs and space availability and zoning requirements and your future objectives. Bathsystem USA provides a systems-driven solution which helps homeowners who want to build their homes more efficiently as they focus on quality through their two-story ADU construction projects which include complex bathroom and wet-area designs. The construction process should focus on building durable assets which provide permanent worth.

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Not Sure Whether to Build Up or Out? Let’s Help You Decide.
If you’re torn between a single‑story ADU and a 2 story ADU in San Diego, we can walk through your lot, zoning, and goals to see what fits best.

Get in touch today to talk about your ADU plans and next steps.

FAQs

Are 2 story ADUs allowed in San Diego?

Yes. Most cities in the region allow them under local codes, with some height and setback rules.

Is a 2 story ADU in San Diego more expensive than a single‑story?

Usually only a bit more expensive, often around $10K–$15K more, depending on size and finish. Site conditions often matter more than the price.

What is the height limit for a 2 story ADU in San Diego?

Many areas allow about 18 feet, and up to 20 feet in some transit‑zone cases, with around 25 feet for certain attached ADUs.

Is it better to build up or out for an ADU?

Build up if your lot is small, you want to keep the yard open, or you care about rental income. Build out if you have room, want easier access, or simpler construction.

Can I build an ADU over my garage?

Yes, many owners choose an ADU over the garage (carriage‑house style) to add usable space while keeping parking on the ground level.