Last Updated on August 13, 2026 by DYM Builders Group
If you’ve spent any time in the real estate or design world, you may have heard the term accessory dwelling unit, or its much easier-to-say abbreviation: ADU. But what exactly is an ADU? In short, they’re additional residential units on the same lot as a single-family home. They come in many forms, from in-law suites attached to houses to entire apartments in the backyard of a property. Sometimes homeowners have extended family members live in their ADU, or they rent it out to generate passive income.
However, if not permitted properly, ADUs can spell big trouble. Whether they’re rent-producing units or separate dwellings, ensuring you have the necessary permits and allowances to build such a unit is the difference between smooth sailing and lots of fines. Here are the questions to keep in mind before you make the leap to construct an ADU.
What Are the Types of ADUs?
ADUs come in several shapes and sizes depending on your lot layout and property goals:
- Detached ADUs: Standalone residential units built completely separate from the main home, such as backyard cottages or guest houses.
- Attached ADUs: Living units built directly onto the side or back of the main home, sharing at least one common wall.
- Interior Conversions: Units created inside the existing structure of the primary house, like a finished basement or converted attic space.
- Garage Conversions: Transforming an existing attached or detached garage into a livable apartment.
- Junior ADUs (JADUs): Smaller units created within the main house footprint that are capped at 500 square feet, often sharing a bathroom with the main home.
Why Construct an ADU?
ADUs are a great way to add more rental units to a neighborhood. Not only are these beneficial for the homeowner, who will receive rental income, they are also beneficial for the renter. There’s a shortage of rental properties in California, especially affordable ones.
ADUs can also help families stay close to their aging loved ones. Often called “in-law suites,” ADUs provide a separate yet conveniently nearby option for parents and grandparents.
In addition, adding a well-built ADU boosts your overall property value and home equity. The extra square footage also gives you flexible space that can double as a quiet home office, private guest house, or hobby studio when you do not have long-term tenants.
Where Are ADUs Legal?
The legality of ADUs varies state by state and even county by county. Some municipalities also have strict rules about ADUs. These laws also vary by the type of ADU you’re constructing: internal, attached, or detached.
- Internal ADUs: Units within the single-family home, such as in the basement or on the second floor.
- Attached ADUs: Units that are connected to the main home but not necessarily in the same structure.
- Detached ADUs: Completely separate units, separate from the structure of the single-family home.
In California, laws passed in recent years make it much easier to construct an ADU than it was in the past. For example, you no longer need to be the primary occupant of the home to build an ADU on the same property (convenient for landlords). You can now also build in communities with Homeowner Associations that ban ADUs. Additionally, municipalities can no longer prohibit ADUs based on lot size.
Of course, you’ll still need to permit your ADU, but that should be significantly easier than it was in 2019. Check your local laws to ensure building ADUs is possible in your state, county, and municipality.
How Much Does It Cost to Build an ADU?
The cost of ADUs varies greatly depending on where you live, how big the unit will be, and how well-equipped it is. For example, building a 5,000 square-foot detached ADU with in-unit laundry, two state-of-the-art bathrooms and several skylights will cost more than a 1,000 square-foot attached or internal ADU with modest features.
Calculating the cost of an ADU is also different from calculating the cost of new construction, because there are more factors to consider. For example, the price will differ depending on how far away the ADU is from the main house’s power, water, and sewage lines.
Additionally, the cost of materials and labor has varied widely due to supply chain issues and labor shortages. So calculating the cost of an ADU is difficult without getting an expert opinion from a contractor. However, you can estimate that an average-sized ADU will usually cost somewhere between $150,000 and $300,000 to build.
How Long Does It Take to Build an ADU?
Constructing an ADU can be a long or short process, depending on what kind of ADU you’re building. Working with a professional can also greatly reduce the time it takes to permit, plan, and build the ADU. Think of it like building a tiny house: if you don’t know what you’re doing and need to learn every step of the process, it’s going to take a long time. But if you work with a professional contractor who builds houses for a living, the structure can go up in a relatively quick amount of time.
DYM Builders specializes in accessory dwelling units, and can help make your dream a reality. With architects, engineers, and designers all under the same roof, DYM Builders is a one-stop shop for California construction and contracting.
Conclusion
Building an ADU is a smart way to add living space, keep family close, or earn steady rental income. Whether you want to convert an existing garage or build a detached cottage in your backyard, following local regulations and getting proper permits is key to a smooth project.
If you are planning to add an ADU to your property, DYM Builders is here to help. Our team handles every step from architectural design and city permitting to full construction. Contact DYM Builders today to schedule a consultation and bring your ADU project to life.
