ADU BASICS
What Is an ADU in Real Estate? Understanding Accessory Dwelling Units & ADU Homes
Just starting to think about building an accessory dwelling unit (ADU) on your property? Wondering the ins and outs of what is an ADU in housing? What does ADU stand for in real estate? You’re not alone. ADUs are popping up everywhere as homeowners look for flexible, high-value ways to add space, generate rental income, or support multigenerational living.
What is the accessory dwelling unit meaning, and is an ADU the same thing as a companion unit, granny flat, casita, guest house or in-law suite? What kinds of ADU units can you build? And how do you get started?
In this guide, we’ll walk you through everything you need to know about ADU housing: what an ADU is, the different types of ADU houses available, and what it takes to build one on your property.
What is an ADU in Real Estate? And what does ADU stand for?
What are accessory dwelling units? In real estate, ADU stands for “accessory dwelling unit.” An ADU is a secondary housing unit located on the same lot as a primary residence.
ADUs can be detached from the main house, attached to the main house, or part of the main house. ADUs include a full kitchen, a bathroom, and a separate entrance.
ADUs are also commonly referred to as granny flats, in-law units, casitas, backyard cottages, secondary units, or guest houses. These compact, efficient homes provide a versatile option for property owners, enabling them to make the most of their existing space in several ways, including:
- Rental housing
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Multigenerational living (e.g. aging parents or housing younger family members)
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Work-from-home space or home office
Understanding what is an ADU in real estate helps explain why so many homeowners are choosing this as a viable option for building.
ADU terminology FAQs
What is the difference between an ADU and a guest house or in-law suite?
These terms are often used interchangeably. However, an ADU usually includes a kitchen and is legally approved for independent living or rental. Guest houses might not include a kitchen and may not be eligible for long-term rental. Read more about guest houses.
What does an ADU house include?
A typical ADU home includes:
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Living/sleeping space
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Full kitchen
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Bathroom
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Private entrance
Local regulations will dictate what an ADU must include, as well as how large certain amenities – like the kitchen – must be.
Are tiny homes considered ADUs?
In some areas, yes. When placed on a foundation and connected to utilities, tiny homes may qualify as ADUs. Read more about tiny homes vs ADUs.
What is a Junior Accessory Dwelling Unit (JADU)?
Junior ADUs (JADUs) offer an option to add another unit (in addition to an ADU) within your existing home.
JADUs must be contained within an existing or proposed single family home (think garage or spare bedroom), under 500sqft (you can add on up to 150 sqft to get to 500), and have a full bathroom and efficiency kitchen (appliances + cabinets).
Types of ADUs
There are several types of ADU housing in terms of where on your property they are constructed. The most common ADU types include detached ADUs, attached ADUs, and conversion ADUs.
Understanding each type can help you select the best ADU for your needs, your property layout, and local regulations. Note that each state and city will have different rules for what you can build.
Detached ADU
ADUs can be built as stand-alone structures on your property – known as a “detached” ADU – to add more space that is separate from the existing home. Each state and city will have different rules for the ADU setbacks, which determine how far your ADU must sit from property lines and your main house.
Attached ADU
You can choose to add on to your main home to create an “attached” ADU. Expansions to the existing structure might be a bump out of the house, or an addition above a garage. This ADU home addition option involves some additional uncertainty since the construction involves the existing building.
Conversion
You can convert existing space like a garage, basement or other building on your property, into an ADU. Conversion ADUs can be attached or detached from your main home, and offer a practical way to repurpose underutilized areas without changing your home’s existing footprint.
Considering Attached vs. Detached ADU
As you begin your journey to add an accessory dwelling unit to your property, consider how the various ADU house styles can impact your living space and lifestyle.
Each type offers unique benefits, whether for guest accommodations, rental income, or multi-generational living. Read our guide for key considerations of attached vs. detached ADUs.
Why build an ADU?
ADU homes offer homeowners tremendous flexibility. Some of the most common reasons people build accessory dwelling units include:
- Rental income – Offset your mortgage or generate passive income
- Multigenerational housing – Keep family close, but with privacy
- Guest accommodations – Offer out-of-town guests a private space
- Home office or studio – Create separation between work and life
- Long-term investment – Increase your property’s value and versatility
- Aging in place – Downsize without leaving your property
Want more inspiration? Check out our deep dives below.
Rental Income
Rent out your ADU or downsize and move into the ADU so you can rent out your main house!
Family Housing
Ready to move your parents onto your property? College students need a place to live? ADUs have you covered for family housing needs.
Guest House
Live-in nanny or au pair? Frequent guests visiting from out of town? An ADU can increase flexibility & add to your property value.
How Many ADUs Can I Build?
The answer depends first on what kind of property you have.
Single Family Properties
On a single-family property in California, homeowners may now be able to add:
- one standard ADU
- one detached ADU up to 800 square feet
- one conversion ADU created from existing space, such as part of the home or an accessory structure
- one JADU up to 500 square feet inside the main home
Keep in mind not every lot will physically or financially support that full combination, and this combination was recently adopted by the state; local governments are still adapting to this change in interpretation to allow more ADUs.
Read more: How many ADUs you can build on a single-family property
Multifamily Properties
On a multifamily property, the rules are different. California law allows:
- up to eight detached ADUs, not to exceed the number of existing units. So if you have a triplex, you could build three detached ADUs.
- ADUs may be created from existing non-habitable space within the existing structures.
Remember that local jurisdictions can still apply certain objective standards depending on the unit type. Site layout, utility capacity, and development limits still matter… so not every property can support every theoretically allowed maximum.
Read more: How many ADUs you can build on a multifamily property
ADU Housing Laws and Regulations
Regulations play a big role in defining what is an ADU in real estate, based on your region. Each state and city has its own rules for ADU development.
It is important to work with an ADU provider that is local or very familiar with your area’s regulations as they can vary greatly depending on where you live.
In California, for example, the state has passed laws making it easier to build ADUs. The state rules set the “floor” for the local jurisdictions, meaning some cities will just allow that minimum, while others (like the City of San Diego), have created even more ADU-friendly rules. Below are some of the most important California state-wide ADU regulations… and be sure to check your local ADU ordinance for specifics rules that apply to your property.
Size & Reduced Setbacks
No minimum lot size requirement. Local municipalities must allow a minimum of a 850 sqft 1BR or 1000 sqft 2BR+ with only 4′ side and rear setbacks, and many cities allow up to 1200 sqft. Also, at least an 800 sqft ADU must be allowed “by right,” even if it must encroach on the front setbacks or exceed FAR restrictions.
Lower Permitting Fees
Many impact and development fees for ADUs have been waived, and some remaining fees can be minimized by being aware of the important thresholds for square footage that trigger higher fees (e.g. 500 sqft, 750 sqft, 1000 sqft). Some cities also offer additional waivers.
No Owner Occupancy
Owner occupancy requirements for ADUs are no longer required, meaning you can add ADU housing on rental properties. State regulations recently evolved to ban owner occupancy requirements indefinitely.
HOAs
HOAs (Home Owners Associations) and CC&Rs (Covenants, Conditions & Restrictions) cannot reasonably prohibit development of ADUs. We design exteriors of ADUs to blend with the main home to conform to standards for exterior finishes.
Parking
Most parking requirements have been waived (none required if within 1/2 mile from public transit or car shares), or are readily addressed (parking spaces may be designated on the driveway).
Application Review
Building departments are required to act on any ADU application within 60 days (and we see most acting much sooner). Note though this doesn’t mean you’ll be approved that soon – most permitting processes still take 3-8 months.
How much does an ADU cost?
We’re so glad you asked! For many people, learning what is an ADU in real estate, cost is obviously a factor. We are big on transparency and like to get into the numbers with you right away.
Check out our ADU Costs page with the link below for everything you need to know about the full cost to design, permit & build an ADU.
How long does it take to build an ADU?
The entire process of designing, permitting, and building a new construction accessory dwelling unit (ADU) will take 10-18 months depending on complexity of your design, the city in which you are located, and your site conditions.
This additional dwelling unit design is roughly 3-4 months, permitting 3-8 months, and build out is 6-9 months (highly variable based on jurisdiction and also unit size). Read more about the three phases and how long each takes.
How do I start to build an ADU?
If you’re still asking what is an ADU in real estate, start with reviewing a project outline. Getting started on your ADU project can feel overwhelming as there are many moving parts to consider. Should you talk to an architect first? A contractor? A lender? Or begin at your city’s planning department?
The truth is, building an ADU home addition is a multi-step process that requires coordination across design, permitting, and construction. That’s why many homeowners choose to work with a specialized ADU builder or design-build firm who can manage the entire project from start to finish.
At Snap ADU, we act as the “quarterback” of your ADU project, guiding you through every step from feasibility and design to permitting and construction. We also coordinate with your lender (if you’re financing the project) to ensure your plans meet all requirements and timelines.
Whether you’re working with custom plans or selecting from our library of pre-designed ADU floor plans, we’re involved from day one to make the process as smooth, efficient, and successful as possible.
Common ADU Challenges and How to Tackle Them
Building an accessory dwelling unit is an exciting way to add space and value to your property, but it’s not without a few hurdles. Here are some of the most common issues homeowners face when building an ADU home and how to get ahead of them.
Permitting & Approval Delays
ADU permitting can feel like a black box, especially when every city has different rules. Between zoning setbacks, utility requirements, and shifting local interpretations, the process can take longer than expected.
✔ Tip: Start early and work with a team that understands your city’s requirements (look up your local ADU regulations). We’ve seen just how different ADU rules by city can be, and we tailor every project to fit the local process. You can also speed things up by using a pre-approved plan where available.
Design Constraints
Not every property can support every kind of ADU. Lot size, setbacks, and slope can all limit what’s possible, and some cities restrict features like roof height or outdoor space. A difference of a few feet can also mean thousands of dollars of costs. Elements like slope & fire rated construction can be affected by ADU location.
✔ Tip: Choose an ADU designer who doesn’t just draw plans, but understands what will actually get approved and what will drive costs. At SnapADU, we run a feasibility study up front so you’re only designing what’s realistic for your site and goals.
Budget & Financing
Sticker shock is real, and many people underestimate what it takes to build a full legal unit. Financing can be another sticking point, especially for homeowners without equity.
✔ Tip: Get transparent pricing early on. Our ADU Cost Guide breaks down what to expect and where the money goes. We also share financing options that have worked for other clients, from HELOCs to renovation loans. Also check out our Guide to Comparing Bids.
Construction & Contractor Issues
A smooth design means nothing if the build goes sideways. Delays, miscommunications, poor workmanship, or lack of warranty support can derail even the best ADU plan.
✔ Tip: Work with an experienced ADU builder who handles the full process — not just a GC you found online. We’re a design-build firm, which means our team stays with you from kickoff through final inspection. No handoffs, no finger-pointing. Read more about How To Compare ADU Companies.
Deciding To Move Forward
Permitting, design, budget, construction—all of it can be figured out with the right team and process. But the hardest part for many homeowners is simply moving from idea to action.
✔ Tip: You don’t need every detail figured out. But you do need to take the first real step. Read more in ADU Planning: What Actually Moves Homeowners from Idea to Action.
Drawbacks of ADUs
There are pros and cons of building an ADU home addition, and it’s important to assess what those tradeoffs are for you. Opponents of ADUs argue you won’t get the value out of the ADU, or that you’ll ruin the neighborhood. Which are legitimate concerns and which are just noise? Read more about potential drawbacks to ADUs.
Watch our ADU 101 Webinar
Thinking about building an ADU and want to learn more about the whole process? We’d love to help make sure you know exactly how it all works in our on-demand webinar. Learn from a top ADU contractor.
ADU FAQs & Resources
How do you build an ADU?
Hear about the whole process in about 25 minutes from the SnapADU team and six homeowners, plus local ADU experts in this special San Diego edition of Builder of Excellence that aired on CBS8.
What can you build on your property?
Residentially zoned properties – both single and multifamily – may add ADUs. Single family may add one ADU and JADU, while multifamily may add 2-8 ADUs (up to existing number of units) plus convert existing space. In San Diego, you can add even more ADUs if they are deemed affordable.
How much does an ADU cost?
It varies drastically based on size of the unit and how much sitework you need. In general, all-in project costs will be around $300K for a 1BR/1BA, $350K for a 2BR/1BA 750sqft, $400K for a 2BR/2BA 1000 sqft, and $425K for 3-4BR/2BA 1200sqft.
What makes a good ADU property?
Several factors make a property more ADU-friendly, namely things that keep sitework costs down. Read on to know what to look for.
How can I finance an ADU?
There are several options for financing an ADU, including some that don’t require a refinance of your current low mortgage rate.
Where can I see ADU Tours?
We have dozens of ADU tours and homeowner interviews for you to view. Pop over to our YouTube channel to see our latest videos, updated regularly with new content.
What is Design Build?
Read about why a design-build approach to an ADU project leads to increased predictability, cost efficiency, and timeliness. Decide if this approach makes sense for your project.
Can I see ADU plans?
We have dozens of plans that you can build as-is or customize to suit your tastes. Each plan includes pricing information on the build cost.
Is an ADU a good investment?
We break down all of the different costs and considerations so you can analyze how attractive it is to add an ADU to your property.