The Stacked ADU Guide

Imagine you’re running out of space at home. Maybe your family is growing, or you need a separate place for parents, kids, or rental income. Buying a new house feels too expensive, and your yard isn’t big enough to build out. What can you do?

Stacked ADUs could be your answer! These multi-story ADUs use vertical space to give you more room without eating up your yard. They’re affordable, smart, and fit perfectly into California’s housing rules. 

Here’s a detailed and updated Stacked ADU Guide based on California so that you can get a clear picture of all the rules and regulations in one setting.

What is a Stacked ADU?

A Stacked ADU is a small home built on top of another structure, like a garage, or as a two-story unit. Instead of spreading out on your property, you build upwards.

You can add a stacked ADU:

• Above a garage
• On top of an existing ADU
• As a two-story new structure

They are sometimes called “duplex ADUs” or “double-decker ADUs.” Each unit has its own entrance, kitchen, bathroom, and living area, so the people living there can be completely independent.

Why are Stacked ADUs Popular in California?

California has a big housing problem. There aren’t enough homes for everyone. Stacked ADUs are becoming popular because they help solve this problem. They let people build more homes on less land.

Benefits of Stacked ADUs

✅More Space: Perfect for small lots. You don’t lose garden or driveway space.

✅More Money: You can rent out the top unit for extra income.

✅Eco-Friendly: You can add solar panels, recycled materials, and energy-saving designs.

✅Flexible Living: Ideal for elderly parents, grown kids, or guests.

Legal Considerations for Stacked ADUs in California

California has made ADU rules easier, but some rules may be different depending on your city or county. 

Here are some basic guidelines:

• Local Zoning Laws: Check with your local office to learn about rules like setbacks, height limits, and more.

• Building Codes: Make sure your Stacked ADU follows all building codes, like safety and plumbing rules.

• Permitting Process: Get the right permits from your local building office. This includes submitting plans, paying fees, and having inspections.

• HOA Rules: If you belong to an HOA, check the rules to see if they limit ADU buildings.

If you want to know, you can visit this informative blog to learn the differences between attached ADU & detached ADU.

California’s 2024 Stacked ADU Guide/ Rules

California has made ADUs easier to build. Here are the main rules:

Detailed Explanation of the Key Laws for Stacked ADUs in California

1. Height Limits

• You can build taller stacked ADUs than before:

• Maximum height:

• Up to 25 feet if attached to your main house or in certain zones.

• ADUs built within half a mile of public transit, such as bus stops or train stations, can now be up to 18 feet tall. 

• Also, two-story multi-family homes can be 18 feet tall, offering more space and flexibility for people living there.

• Up to 16 feet for detached ADUs.

2. Front Setback Rules

Before 2024, California had rules that made it hard to build an 800-square-foot ADU. The setback rules required a lot of space between the ADU and the property edge.

Now, the setback rule is only four feet for the sides and back. There is also an exception for building an 800-square-foot ADU even if it doesn’t meet the front setback rule.

• You can build an 800 sq. ft. ADU, even if it sticks out a little in the front yard area.

• Cities can’t reject your project just because of this.

3. Permitting Process

• Cities must decide if they approve or deny your ADU within 60 days.

• If they deny it, they must explain why and what needs to be fixed.

4. Unpermitted Work

Before, homeowners couldn’t build ADUs if they had unpermitted work on their property. This included things like illegal structures or zoning problems.

Fixing these issues was expensive and slow. Now, the law has changed. Homeowners can build an ADU as long as the unpermitted work doesn’t create safety or health risks.

5. Fire Sprinklers

• No need to add fire sprinklers to your main house when building a stacked ADU.

6. Rental and Owner Occupancy Rules

• You don’t need to live on the property to build or rent out a stacked ADU.

• You can rent it out or sell it (as a condo) if local rules allow.

7. Size Limits

• 800 sq. ft. minimum must be allowed by all cities.

Maximum sizes:

• 850 sq. ft. for a 1-bedroom ADU.

• 1,000 sq. ft. for 2+ bedroom ADUs.

• For attached ADUs, they can’t be more than 50% of your main house size.

8. Parking Requirements

Before 2020, California had strict parking rules for new ADUs. It was hard for homeowners to meet these requirements.

Now, the rules are easier. Only one parking space is needed per bedroom or unit.

In some cases, parking rules are not needed at all. This includes when:

• The ADU is close to public transit (half a mile away)
• The ADU is in a historic area
• A car-sharing service is nearby
• The ADU is built in a garage or parking area

Additional Rules to Follow

• JADU Bathrooms: Junior ADUs (JADUs) no longer need a bathroom. As long as the main home has one bathroom, it’s enough for the JADU.

• Demolition Laws: Homeowners no longer face delays in getting demolition permits. If an ADU permit is approved, the demolition permit must be given, too.

• CalHFA Grants: The California Housing Finance Agency offers grants to help low-to-moderate-income homeowners build ADUs. This includes financial help with planning and development.

Besides, get more detailed info by reading what to know about ADU setbacks & fire ratings in California.

How Much Does It Cost to Stack Two ADUs?

Stacking ADUs is not cheap. The second unit costs about 85% of what the first one costs.

Here’s an example of costs in San Diego, California:

Shared elements, like one foundation or utilities, save money. But you’ll still need:

• Separate kitchens and bathrooms.
• Dedicated HVAC and electrical systems.

Challenges of Stacked ADUs

Building a stacked ADU has many benefits, but it also comes with some challenges. 

Here’s a detailed explanation of what you might face:

1. Noise Between Floors

When you have two levels, the sound from upstairs (like footsteps or moving furniture) can travel downstairs. This can be uncomfortable for the people living below.

To solve this, you’ll need soundproof materials, like special flooring or insulation. These materials add to your costs, typically around $12 per square foot.

2. Higher Building Costs

Stacked ADUs are more complex to build compared to single-story ADUs. Fireproofing is a must to meet safety rules, which increases expenses. 

For example, you’ll need fire-rated materials between the floors. Each unit requires separate plumbing, heating, and electricity systems. These extra steps mean the second unit will cost about 85% of the first unit’s cost.

3. Zoning and Compliance Rules

Every city has specific zoning laws and building codes. Some places have strict rules about:

• Height limits: You might not be allowed to build as tall as you want.

• Setbacks: This is the space required between your ADU and the property lines.

• Staircases: Outdoor stairs often have stricter setback rules than the ADU itself.

Always check your local regulations to avoid surprises.

4. Foundation and Structural Strength

Stacked ADUs are heavier than single-story units. The foundation needs to be strong enough to hold the weight of two levels. For that, you’ll need a structural engineer to inspect and reinforce the foundation, which can be costly.

5. Unexpected Costs

Construction projects often have hidden costs. With stacked ADUs, these might include:

• Extra materials for fireproofing and soundproofing.

• Adjustments to meet zoning or safety codes.

To avoid stress, plan an additional 10-15% of your budget for surprises.

6. Privacy

Living in a two-story ADU means sharing space. Without proper design, lack of privacy can become an issue. Adding features like enclosed staircases or strategic landscaping for privacy can help, but they add to the expense.

How to Tackle These Challenges

✅Do Your Research: Learn about your city’s ADU rules before starting.

✅Work with Professionals: Hire experienced architects, engineers, and builders to avoid mistakes.

✅Plan Your Budget Carefully: Include extra costs for soundproofing, fireproofing, and structural work.

✅Focus on Design: Use materials and layouts that maximize comfort and privacy.

Design Ideas for Stacked ADUs

1. Space-Saving Tips

• Use foldable furniture.
• Add built-in shelves for storage.
• Install large windows for natural light.

2. Sustainable Materials

• Bamboo flooring.
• Recycled wood.
• Solar panels.

3. Accessibility Features

• Add elevators or ramps for elderly family members.
• Use wider doors and hallways.

Besides, you can visit this informative blog to learn the basics of White Box DIY ADU Construction.

Wrapping Up

So, that’s actually an illustrative stacked ADU guide in California. Stacked ADUs are a great way to help with California’s housing needs. They save space, make money, and are good for the environment. If you want to use your property wisely, start planning your Stacked ADU now!

If you’re ready to build a Stacked ADU but feel overwhelmed by the rules and process, talk to an expert like ContractorDave. We’ll guide you through zoning, planning, and construction to ensure your project runs smoothly. 

What we do: We manage every step, from planning to execution, to create a space that fits your needs. With years of experience, we handle everything so you can enjoy your new space with peace of mind.

Plus, we offer long-term guarantees on our construction projects, which ensures peace of mind for our clients.

FAQs
Can You Stack Two ADUs in California?Yes, you can stack two ADUs in some areas of California, like San Diego and Encinitas. Stacking means building one ADU on top of another, using vertical space instead of spreading out on the ground.
Does an attached ADU count as square footage?Yes, an attached Accessory Dwelling Unit (ADU) adds to the total size of your property. However, it cannot be bigger than 50% of the main home’s living space.
What is the largest that a detached ADU could be?The maximum size for a detached ADU in California is 1,200 square feet. However, some cities and counties may have stricter regulations.
Can I build a two-story ADU in California?Yes, you can build a two-story ADU in California under certain conditions. However, you must adhere to local zoning regulations and building codes, which may limit height and size restrictions.
Can I build 2 ADUs on my property in San Diego?Yes, you can build two ADUs in San Diego if your property meets the rules. On single-family lots, you can add one ADU and one Junior ADU. 
On multifamily lots, you can build two detached ADUs and convert existing spaces into more units if allowed.