Can My ADU Be Bigger Than My House?

Key Takeaways

• ADUs are usually required to be smaller than the main house, with attached ADUs often limited to 50% of the home’s size and detached ones capped around 800–1,200 sq ft.
• In rare cases, an ADU can be larger—such as when the main house is very small or when special permits or local zoning exceptions apply.
• Local laws set strict rules on ADU size, height, setbacks, appearance, and lot coverage to maintain neighborhood balance and property value.
• Always check your city’s ADU rules and consult professionals before designing or building to avoid costly mistakes or legal issues.

Table of Contents

Toggle

An ADU, or Accessory Dwelling Unit, is a small extra home built on the same property as your main house. People build ADUs to create extra living space for family members, such as aging parents or adult children. Others build ADUs to earn money by renting them out or simply to have more space for guests or hobbies. Because ADUs are smaller homes on the same lot, many people wonder if the ADU can be bigger than the main house itself. This is an important question because size rules ensure that the neighborhood stays balanced and that ADUs do not overpower the main home or the block.

What Are the Rules for ADU Size?

Most cities and towns have clear rules about how large an ADU can be. These size limits help protect your property, your neighbors’ homes, and the neighborhood’s overall look and feel. There are two main types of ADUs: attached and detached. An attached ADU is physically connected to your house, like a converted garage, basement apartment, or an extra room added on. A detached ADU is a separate building, such as a small guesthouse or granny flat built somewhere in your backyard. Many places have different size limits for attached versus detached ADUs because detached ones typically take up more land space.

The limits usually come in the form of either a percentage of your main house’s size or a fixed maximum size allowed by local law. This keeps ADUs small enough to feel like a “secondary” home rather than a full replacement or oversized competitor to your main house.

Is It Possible for My ADU to Be Bigger Than My House?

In most cases, your ADU cannot be larger than your main house. For attached ADUs, many cities limit the size to up to 50% of the main house’s square footage. For example, if your house is 2,000 square feet, your attached ADU might only be allowed to be up to 1,000 square feet. Detached ADUs often have a fixed maximum size, typically between 800 and 1,200 square feet.

It means that if your house is very small but the maximum allowed ADU size is large, your detached ADU might actually be bigger than your house in some rare cases. But for the majority of homeowners, ADUs are designed to be smaller and more modest.

Are There Any Exceptions or Special Cases?

There are a few exceptions or special cases where your ADU could be bigger than your house:

• Small Main House: If your main home is very small, such as a tiny house or a cottage less than 800 square feet, some cities allow a detached ADU that could be larger in size because of a fixed cap on ADU size.
• Special Permits or Waivers: Some cities or counties might let you apply for special permits or exceptions. These can happen if you want to use the ADU for special housing needs, like elder care or accessible housing.
• New Rules and Changes: Because ADU laws are changing fast in many places to solve housing shortages, some recent updates might permit larger ADUs in new developments or under certain zoning categories.

Despite these exceptions, it’s very important to check your local rules and speak with your city’s planning department or a professional before assuming your ADU can be bigger than your house.

Other ADU Size Rules to Know

ADU size is just one part of many important rules you need to follow:

• Height Limits: Many places limit ADUs to a certain height. This keeps the ADU smaller and less imposing than the main house.
• Setbacks: These rules say how close your ADU can be to property lines, fences, or your main house. Usually, there needs to be a few feet of space on all sides for privacy and safety.
• Appearance Rules: Some neighborhoods require ADUs to look like the main house. This helps the buildings blend together and makes sure the ADU fits with the style of the neighborhood.
• Lot Size and Coverage: ADUs cannot cover too much of your yard or lot. There are limits on how much of your property can be built on to protect outdoor space and neighborhood character.

Together, these rules make sure your ADU is a welcome addition to your property and community.

Why Are There Size Limits on ADUs?

Size limits exist because ADUs are meant to be “second homes” on the property, not the main home. They should be smaller to keep the balance between buildings and open space on the lot. Size limits help protect neighbors from large buildings blocking sunlight or causing privacy problems. They also protect your property’s value by keeping homes neat and well-sized.

Setting limits also helps local governments control growth and ensure neighborhoods remain comfortable and safe without becoming too crowded. ADUs are a flexible way to add housing, but they have to follow rules to make sure they fit in well.

How to Find Your Local ADU Rules

Local rules can be very different depending on where you live. To find your specific rules:

1. Visit Your City or County Website: Most places have planning or building department websites with information about ADUs.
2. Call the Building or Planning Department: Ask them about ADU size limits, height restrictions, setbacks, and other important rules.
3. Talk to Local Builders or Contractors: They know the rules and can help guide your plans.
4. Look at Your Zoning: Your property’s zoning type (residential, multifamily, etc.) often affects ADU rules.

Always gather this information before starting your ADU project to avoid surprises.

Quick Tips for Planning Your ADU

• Think carefully about how you will use your ADU: for family, rental, or hobbies. This helps decide the right size.
• Check local rules early on, even before designing, so your plans fit the law.
• Plan for future needs. Maybe the family grows, or you want to rent it in the future.
• Get advice from experts such as architects, builders, or planning officials.
• Remember that bigger isn’t always better—a right-sized ADU can be easier to build and maintain.

Summary and Final Advice

Usually, your ADU must be smaller than your main house. Attached ADUs often can’t be more than half the size of your home, and detached ADUs have set maximum sizes, commonly between 800 and 1,200 square feet. There are some rare exceptions for very small houses or special permits, but these do not apply everywhere. Knowing the rules early and following them is the best way to keep your project smooth and avoid costly fixes. If you’re thinking about building an ADU, always start by checking your local laws and getting expert advice.

FAQ

Can I build a two-story ADU?

Sometimes, yes. Many places allow two stories but usually limit height. Check your local rules.

Who decides ADU size rules?

Your city or county government makes these rules.

What happens if I build an ADU that’s too big?

You might have to change it or even remove part of it. It’s best to follow the rules from the start.

Can I rent out my ADU?

In most places, yes. But check local rules because some areas have special rental requirements.

Do ADUs require separate utilities?

Sometimes yes, depending on the size and local rules. Your builder or city can tell you.

Want to Learn More About Building an ADU?

You’ve just learned if your ADU can be bigger than your house—now take the next step in planning smart. Check out these helpful guides to avoid delays, stay on budget, and understand the latest laws:

How Long Does it Take to Build an Accessory Dwelling Unit (ADU)? – Discover realistic timelines so you can plan your project with confidence.

How Much Does it Cost to Build an ADU? – Get a clear idea of what to expect in terms of budget and hidden expenses.

What is the New ADU Law 2025? – Learn about the latest legal updates that could impact your ADU project.

How Much Is a Permit to Build an ADU in California? – Know the fees before you apply and avoid surprise costs.

How to Get an ADU Permit in California – Step-by-step guide to getting your permit approved faster.

What Is a State Exempt from ADU? – Find out if your property qualifies for any ADU exemptions that can save time and money.

👉 Start with the topic that matters most to you and keep building your ADU knowledge!