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What is an ADU?

You might have heard of a granny flat, an in-law enclave, or a backyard home. If yes, then you know what an ADU is. In a nutshell, in your area, an Accessory Dwelling Unit, or ADU, is a secondary, independent living unit built on the same premises as the primary residence. Generally, ADUs are constructed in one of the following four configurations:

  • Detached: An ADU built in the yard or somewhere else on your lot, separate from your home.
  • Attached: Also, a separate structure, but attached to the primary dwellings.
  • Existing Space Conversions: These ADUs entail repurposing a space, usually fully contained within the primary home, e.g., the main bedroom, into fully independent living units. 
  • Junior Accessory Dwelling Units (JADUs): Similar to conversion, the JADU designation was invented to allow less strict restrictions in certain conditions, usually to help reduce costs.
Full kitchen with marble countertop and hidden refrigerator/freezer in the ADU

Am I allowed to build an ADU?

Before planning an ADU, it’s probably a good idea to check with the relevant local authorities whether you’re permitted and eligible to build a secondary unit. Municipalities look at factors such as lot size and zoning laws to establish whether a homeowner can build an ADU.

As a first step, we recommend using our free Property Check to see your options.

* This is not meant, in any way, to replace speaking with your local planning authorities.

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What will it cost me to build an ADU?

Ranging from detached ADUs that are half the size of the primary dwelling to a single bedroom, interior conversion ADUs, ADU construction costs naturally depend on the type and scope of the ADU you intend to build. That said, per-square-foot construction costs are usually the same for ADUs and new build residential structures. As such, your contractor or architect can probably give you a fair estimate once you’ve decided on your basic ADU plans.

Where do I get the money to fund my ADU project?

Many, if not most, homeowners opt to fund their ADUs by borrowing against the equity of their homes. While home equity loans or lines of credit are the most common and straightforward form of financing, sometimes they aren’t practical or feasible. Such cases warrant more creative solutions, as in the following examples:

  • Refinance your mortgage – Often, refinancing a mortgage can decrease your debt burden and free up some space to take a larger loan than you might have without refinancing. You can then apply the new loan proceeds toward your ADU.
  • Private construction loan – If neither home equity nor refinancing is feasible, another option is to apply for a personal construction loan (PCL). These are bridge loans, usually short-term (up to 1 year), that bear higher interest rates and require that ADU construction is completed within the loan period. You can pay off the PCL upon completion by obtaining a new mortgage with the ADU as collateral.

Whether any of the above or some other financing alternative, you should begin the process by speaking with the financial institution where you took out your home mortgage.

How can I find out about nearby public transportation options?

Public transportation is vital to many ADU tenants, especially if the ADU doesn’t come with permanent parking. Consequently, it’s probably a good idea to familiarize yourself with the nearby transit options to share this information with potential lessees. Most urban areas publish transit maps that include bus, subway, train, ride-sharing, and other information, making it easy to obtain the information you need.

In addition, check with your local authorities responsible for ADUs to see if there are any regulations regarding minimal distances to public transportation and to find out if you may be required to provide a parking space to your ADU tenant.

Detached new construction ADU A construction site photo showing the framing process of a newly built Accessory Dwelling Unit (ADU). The photo shows a wooden structure being built on a concrete foundation, with workers using tools and machinery to construct the base.

I’d like to sell my ADU. Can I do so separately from my home?

The short answer is no; you can’t sell your ADU independently of your home.
The ADU concept came about mainly to provide lower rent options in highly populated areas with insufficient housing and to allow homeowners to generate additional income from their existing properties. The very reason a municipality will give you a license to build an ADU in the first place is so that you can rent it out or use it for your own family. Therefore, the authorities only allow ADUs to sell with the primary residence.

As the homeowner and owner of the ADU, am I required to live on the property?

You don’t have to live on your property and may rent out your primary home and your ADU.

I’m considering converting the garage at the front of my lot into an ADU. Is that allowed?

Save, perhaps, for particular circumstances, most municipalities won’t allow you to convert a garage located in front of the primary residence. However, you should check with your city’s planning department to see if you might obtain a permit to construct some other form of ADU elsewhere on your property.

A beautiful backyard view of an Accessory Dwelling Unit (ADU). The ADU is a small one-story building located in the back corner, with a flat roof and large windows. The exterior is painted a light color with a contrasting dark trim.

I have an “unofficial” unit on my property. Can I get a license for it?

Please note that The ADU Guide provides information on new construction ADU units and does express any opinions on the current legal status of existing ADUs or propose to offer legal advice of any kind. With that said, some municipal authorities may allow you to legalize an unlicensed unit and reach out to your local planning authority to find out more.

Are there any limitations on the size to which I can build my ADU?

As you might expect, the sizes of the various types of new-construction ADUs are regulated by local authorities and will likely differ from city to city. Here are a few general guidelines, but you SHOULD NOT rely on them and should check with your municipal planning department before commencing any construction:

  • ADUs can usually be at most 50% of the primary home.
  • The total area (square footage) of all structures on any given lot can generally be at most 45% of the entire room.
  • ADUs will also have absolute size limitations, e.g., “your ADU cannot exceed 1,200 sq. ft.”.

Does having a small or irregular property affect whether I can obtain an ADU building permit?

In many areas, each lot is different from its neighbors; even where not, other homeowners have different visions for their properties and ADUs. For instance, you may be thinking of an above garage ADU, but building one would block the view of your home, such that a side attachment might be your better bet, or perhaps demolishing your garage and rebuilding it elsewhere on your lot with the ADU on top.

To help you decide, try to put your ideas down on paper and then discuss them with your planning authority.