If you’ve been hearing the term “ADU” more often lately, you’re not alone. Accessory Dwelling Units (or “ADUs” for short) are rapidly growing in popularity across California and beyond — and for good reason.
In simple terms, an ADU is a secondary housing unit built on the same lot as a primary residence. They’re sometimes called backyard cottages, in-law suites, or guest houses, but no matter the name, they all share the same idea: adding flexible living space without the need to buy a new property.
If you’re wondering about the ADU meaning, it’s simple: it’s a self-contained home that can offer independent living for family, guests, or renters. Another way to think about it: the ADU definition is a smaller, secondary home that shares a lot with a main house — but functions as its own complete living space.
A Short History of ADUs
Accessory Dwelling Units have been around much longer than most people realize. Early versions, often called “granny flats” or “carriage houses,” date back to the early 20th century when families built extra small homes for family members or hired help.
However, strict zoning laws after World War II made building secondary units difficult in many communities. As cities expanded outward and real estate prices soared, policymakers began rethinking the potential of ADUs.
In recent years, particularly in California, ADUs have come roaring back. New state laws have made them easier and faster to approve, helping homeowners add much-needed housing while preserving neighborhood character.
Today, ADUs are a smart solution to the challenges of affordability, family needs, and sustainable land use.
Different Types of ADUs
Not all ADUs look the same. Here are a few common types:
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Detached ADU: A completely separate structure, like a small house in the backyard. Detached ADUs offer the most privacy and flexibility for homeowners and their family members.
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Attached ADU: Built as an addition to the existing home, sharing at least one wall.
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Garage Conversion: Turning an existing garage into a full living unit.
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Basement or Interior Conversion: Finishing unused space inside the home to create an independent living area.
Each type of ADU can be customized depending on your property size, zoning rules, and personal needs.
Why Are ADUs So Popular?
ADUs are being built everywhere — and it’s not just a trend. Here’s why homeowners love them:
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Family Flexibility: Give aging parents, adult children, or extended family members a private place to live close by.
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Rental Income: Turn your ADU into a long-term rental or a short-term Airbnb to generate extra income.
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Increase Property Value: Homes with ADUs typically sell for more — often much more — than those without.
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Smarter Land Use: As housing shortages grow, ADUs offer a way to create more homes without expanding neighborhoods.
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Manageable Property Taxes: Because an ADU shares the lot with your primary residence, it often won’t trigger a full reassessment of property taxes — though some additional value may be added.
Plus, in places like California, new laws have made it easier than ever to get permits, finance construction, and even secure favorable loan terms.
Key Laws and Regulations in California
California has led the nation in promoting ADUs as part of its broader housing strategy. In 2016 and 2020, several bills were signed into law — including Senate Bills 9 and 10 — to remove many of the traditional barriers to building an accessory dwelling unit (ADU).
Some key highlights:
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Streamlined Permits: Cities and counties must approve ADU applications within 60 days if they meet state standards.
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Reduced Fees: Many impact and utility connection fees have been lowered or waived.
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Relaxed Parking Requirements: In many cases, no additional parking spaces are required.
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No Minimum Lot Sizes: Most single-family and multifamily lots are now eligible.
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More Units Allowed: On some properties, you can add both an ADU and a Junior ADU (JADU).
It’s important to check your local city or county rules, as some specific design standards may apply, but statewide, the door to building an ADU has never been more open.
Cost to Build an ADU
One of the first questions homeowners ask is: how much does it cost to build an ADU?
Here’s a general breakdown:
| Type of ADU | Estimated Cost Range |
|---|---|
| Garage Conversion | $80,000 – $150,000 |
| Attached ADU | $150,000 – $250,000 |
| Detached ADU | $180,000 – $300,000+ |
Costs can vary based on factors like:
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Site conditions (grading, utility hookups, access)
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Size and design choices
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Permit and impact fees in your city
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Materials and finishes
At SideHaus, we work closely with homeowners to deliver beautiful, move-in ready ADUs at competitive prices — and we make the entire process as simple and transparent as possible.
Financing Options for ADUs
Building an ADU is a major investment, but there are more financing options available than ever before:
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Home Equity Line of Credit (HELOC): Use the equity in your real estate to fund your project.
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Cash-Out Refinance: Refinance your mortgage to take cash out and fund the build.
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ADU-Specific Loans: Some lenders now offer special construction loans just for ADU projects.
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Government Grant and Subsidy Programs: In some counties, grants or deferred loans are available, especially for creating affordable housing units.
At SideHaus, we can connect you with trusted lending partners offering rates starting as low as 4.5%, making your accessory dwelling unit (ADU) project even more accessible.
Common Misconceptions About ADUs
There’s a lot of outdated information floating around. Let’s clear up a few common myths:
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“ADUs are too small.”
Many ADUs can be up to 1,200 square feet — larger than some standalone homes! -
“Getting a permit is impossible.”
Thanks to new laws, permits are streamlined, and approval is often granted in just a few months. -
“ADUs are only good for rentals.”
While renting your ADU can create passive income, many people build them for family members, guest houses, or lifestyle upgrades. -
“You need a huge lot.”
Minimum lot size requirements have been eliminated statewide, making it possible to add an ADU even on modest properties alongside your primary residence.
How Long Does it Take to Build an ADU?
Timelines can vary depending on your property and the permitting process, ranging between six months to two years.
Because we build the house in the factory while the site is getting prepared, we are able to drastically reduce timelines and complete a project within about six months.
Our streamlined process — from permitting to final construction — is one of the fastest ways to add a brand-new living space to your property.
Is Building an ADU Right for You?
Adding an ADU isn’t just about gaining extra space — it’s about creating more freedom, flexibility, and financial security for your future.
Whether you’re dreaming of a cozy backyard home for family members, an income-producing rental, or simply a smarter investment in your real estate, an ADU could be one of the best moves you make.