Accessory Dwelling Units: What Exactly Are They?

Accessory Dwelling Units: What Exactly Are They?

Mother-in-law cabins, secondary suites, cottages, and self-contained apartments are synonyms for an accessory dwelling unit (ADU). It's an old and simple idea to have a small, second home attached to or on the same property as your primary residence such as:

  • a small house on a foundation in the back or side yard
  • an apartment over or attached to the garage
  • a basement apartment

The demand for ADUs is steadily increasing and many are wanting to understand these units and their benefits further. In this article, we discuss:

  • Types of ADUs
  • Public and city interest in more ADUs
  • Common traits shared by all ADUs
  • Types of Accessory Dwelling Units

    1. An ADU above a workshop, above a garage, or attached to a garage—known as carriage houses and garage apartments.
    2. A detached ADU new construction—known as granny flats, mother-in-law cabins, backyard cottages, or laneway houses. It depends on the jurisdiction.
    3. Garage conversation ADUs
    4. Bump-out ADUs also known as Addition ADUs
    5. Basement conversion ADUs
    6. Internal ADUs - a part of the home besides the basement is converted.

    Related: How to Build the Perfect In-Law Unit 


    detached house

    Commonalities of Accessory Dwelling Units

    There are various structural forms of accessory dwelling units. However, there are a handful of traits, design aspects, and development obstacles that ADUs share. Their first commonality is that they're in a unique housing category. Take a look at a few other commonalities of ADUs:

  • They are considerably smaller than the average home or unit
  • They are an accessory to or adjacent to the central household unit
  • They are usually developed and built at a different time than the primary home
  • ADUs are one out of two units that are owned by the same owner on a single-family lot (residential)
  • ADUs are differentiated from other types of properties because of zoning regulation and the range of municipal land use
  • More informal ADUs exist than permitted and documented ADUs

  • It is all of these distinctive characteristics and more that make ADUs a unique type of housing. With a more cohesive language and understanding of accessory dwelling units, there's now a deeper understanding of the best practices in ADU development than ever before.

    Are you looking for residential and commercial site plans for permits? Contact My Site Plan today!

    Increased Demand for ADUs

    In the recent past, cities have been more eager than ever to improve the ADU codes in their area. It begs the question: why all this sudden demand?

    The average size of a household in America used to be 4 or 5 people. These days, the average is 1-2. Despite this considerable difference, most of our past and present housing is designed for families of 4-5. That may have made sense 80 years ago, but not today because households with 3-4 bedrooms are a poor match for the American demographic for two reasons:

    1. Our average household size is no longer that large.
    2. The average household income is nowhere near enough to support such a large home, as it's not just houses that are more expensive but also the upkeep for it.

    The statistics are quite staggering. Only 38% have 3+ people living in them. The other 62% are 1-2 person households. That means that 1-2 person households make up 2/3rds of the US population!

    Those who live in smaller households have had trouble securing housing that's an appropriate size. Often these folks have to rent or buy a larger home than they need because it's simply all that's available on the housing market.

    For these reasons, the demand for accessory dwelling units is only expected to rise.

    Related: 10 Best Backyard Workspaces 


    green house with car parked

    City Interest in ADUs

    Cities have a variety of reasons why it is in their best interest to encourage ADU development.

    Environmental - The environmental footprint of ADUs is barely noticeable compared to larger family homes. A new ADU that's detached is about 44% smaller than a standard home. Overall, new ADUs are about 33% lower than the standard. Smaller homes, apartments, and living spaces use far less energy regarding habitation, construction, and deconstruction through the building lifecycle.

    Economic - Accessory dwelling units can offer more affordable and flexible real estate options in metropolitan cities. Usually, the dwelling units utilize governmental infrastructure that already exists like schools, sewers, and roads. For this reason, accessory dwelling units reduce the need for expansion in already overcrowded central cities.

    Social - ADUs benefit the neighborhoods they're in because they can offer growth without much physical expansion or change in the neighborhood character.

    ADU Tips for Looking Good

    ADUs are incredibly beneficial to those who they're appropriate for. These units can be great fun to aesthetically design and decorate--and, it's much more economical. Because a renter or an owner has less to buy, they can usually spend nicely on what they do choose to have in the home. This extra bit of economic and decorative flexibility leaves some ADUs looking far nicer than regular homes!

    Here are a few tricks to make the most out of your accessory dwelling unit:

    • Coordinate the outside of the ADU with the main home's exterior.
    • Place your windows as high as possible to let the max amount of light enter.
    • Contemplate vaulted ceilings. They create more room.
    • Cultivate open spaces--don't waste any square feet!

    Frequently Asked Questions

  • Is a permit needed to build an ADU?

  • Yes. A permit is required from your local jurisdiction to permit you to build. In some areas, the exception to the rule is if the space is under 200 sq ft and uninhabitable, but in all other cases, a building permit is required.

    As always, check with your specific jurisdiction to get informed of the regulations in your area.

  • Does an ADU have a kitchen?

  • Yes, the kitchen is what makes it an accessory dwelling unit. The kitchen is what sets it apart from other living spaces. They are a requirement in ADUs.

  • Can I build it in my backyard?

  • That depends entirely on your local laws and regulations.

    small blue house

  • Who designs ADUs?

  • There are countless drafting companies to choose from, but My Site Plan has the experience and manpower to help you best.

    My Site Plan is a veteran-owned team of experienced CAD designers who specialize in site plans. We’re able to provide drafting services for anyone who may need non-certified site plans.

    Though site plans are our specialty, we can provide 3D renderings, floor plans, conceptual layouts, and nearly any other CAD-related projects that come across your desk.

    Learn more about how we operate here.

    Related: How to Get a Permit for an In-Law Unit 

    Final Thoughts

    This form of housing has limitless potential, and we will see more of it in the future. They can provide rental income for many, offer affordable accommodations for others, and offer families a way to stay together while maintaining a comfortable distance.

    Are you looking for residential and commercial site plans for permits? Contact My Site Plan today!



    Question Answer
    What are the environmental benefits of building an ADU? ADUs have a smaller environmental footprint compared to larger homes, as they use less energy during habitation, construction, and deconstruction phases.
    How do ADUs contribute to economic benefits in cities? ADUs utilize existing governmental infrastructure, reducing the need for expansion in crowded cities, and provide more affordable real estate options.
    What are the social advantages of ADUs in a neighborhood? ADUs can increase neighborhood density without significant changes in character, offering growth opportunities without extensive physical expansion.
    What are some design tips for optimizing the space in an ADU? Consider maximizing natural light with high windows, utilizing vaulted ceilings to create more space, and designing open floor plans to use every square foot.
    How do ADUs affect the real estate market in urban areas? By offering smaller, more affordable housing options, ADUs can help alleviate the mismatch between available housing and the needs of smaller households.

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    • Annie Rosellini