ADU Contractor Answers What’s The Difference Between A Room Addition and an ADU?

What are some of the key differences between an ADU and a room addition?

ADUs Are Usually Separate But Not Always Completely Separate

An ADU is a detached unit separate from the primary dwelling unit on the same property, or in some cases, especially in California, inside or attached to an existing home. In other words, an ADU is not always attached to your house like a bonus room or converted garage might be. Here’s the primary difference, an ADU must have its own kitchen and usually its own bathroom facilities although in California in-law purposed ADUs may share the home’s existing bathrooms. If you build inside or expand your existing house, it is the conversion of the space to full living quarters complete with bath and kitchen facilities that qualifies it as an ADU. Another person or family can live in an ADU independent of the family that is already in the home. This will of course include separate entrances as well. These factors mean the cost or investment for an ADU is more substantial.

An ADU can be a room addition but a room addition alone isn’t an ADU. In California, you can build an ADU without expanding your current home structure, especially if you convert all or part of your garage into an accessory dwelling unit.

2 BR 2 Bath addition ADU

Not Enough Intermediate Housing? Add It With An ADU

First and foremost, an ADU is a completely separate living space from your main house. This may sound tricky because some ADUs are built within converted or expanded space in an existing home. An ADU has its own entrance, bathroom, kitchen, etc whereas room additions are typically attached or integrated into the existing house. You might be able build an ADU where zoning allows detached structures as big as 1,200 square feet. A so-called tiny house is often a qualifying structure with its own full-time living facilities.

A room addition with its own bathroom does not alone qualify as an ADU. Adding a wet bar and a microwave oven still doesn’t get you there. You need a fully furnished kitchen and bathroom and separate entrance to meet the requirement. Guidelines for ADUs in California can be found at

In addition, an ADU is often a depreciable asset and can be rented out. In contrast, a room addition doesn’t provide you with rental income except as perhaps just a room-to-rent. Plus, if zoning allows it, you could potentially build an ADU on your property even if your city or town doesn’t allow additions to existing homes. This might be especially appealing if you’re looking to upgrade your home, add value or build an income-producing asset.

ADUs are also a great way to meet the growing housing demand in many communities. But they must be constructed the right way – and that’s where hiring a professional ADU builder comes into play. To learn more about ADU construction and design, contact us or schedule a consultation.

Permission Granted

In California, there are a number of special permissions that make it easier for property owners to build an ADU. The state recognizes the need to provide more housing options, and has created regulations designed to encourage the construction of these new living spaces.

The California Department of Housing and Community Development (HCD) provides guidance on the construction and use of ADUs. According to their website, “Local governments can allow both attached and detached accessory dwelling units in all zoning districts.” This means that property owners have the ability to construct an ADU whether or not they are allowed to expand their existing home.


Junior ADUs, living spaces attached to an existing dwelling. A JADU is typically a sleeping and cooking space in an existing dwelling, where the resident has both access to a bathroom facility in the main structure, and a separate entrance to their apartment from outside.

In The Zone and Permits

There are still zoning issues that impact weather and where you may build an ADU. Of course you must comply with all city and Local standards and obtain the proper permits and plans for constructing an ADU. HCD also has some helpful guidelines for building regulations regarding safety, parking, and other important considerations. For example, they note that “Accessory dwelling units must be built to comply with all applicable state building, mechanical, plumbing, and electrical codes.”

In addition, HCD suggests that local governments consider incentives for property owners who build ADUs on their properties. These can include reduction of fees and easing permit requirements, as well as preferential placements on the waiting list for affordable housing.

If you’re interested in building an ADU of your own, it’s important to consult with local authorities and HCD for guidance. With the right permissions and knowledge of regulations, you can create a new living space that is both functional and affordable.