Hand with a pen marking off a checklist with the caption "Common ADU Laws and Ordinances"

Are you planning to build an accessory dwelling unit or ADU on your property?

ADUs, also known as granny flats or backyard cottages, are becoming more and more popular with every passing year. They are a great way to provide housing for an elderly relative or an adult child who’s still living at home.

Once the ADU is constructed, you can also rent it out to earn some extra income each month. Alternatively, the ADU can be used as a detached guest room or home office. (Click here to learn more about the ways in which an ADU can add value to your home.)

Various states and cities have been encouraging the construction of ADUs, as a potential solution to the urban housing crisis. But that doesn’t necessarily mean that building your own ADU is going to be a walk in the park.

If you build an ADU, you will need to follow all the local zoning laws, ordinances, and building codes. Otherwise, you will not be able to obtain the permits and licenses needed for building a legal ADU on your property.

The laws and ordinances governing ADU construction vary between different states, counties, and cities. Parking spot requirements, permissible height of the ADU, interior layout, and the quality of building materials are some of the things commonly regulated.

For instance, an attached ADU in California can only be up to 50 percent of the size of the primary residence. But a detached ADU can usually be up to 1,200 square feet, regardless of the size of the primary residence.

In the state of Vermont, an ADU can have a maximum habitable floor area of 900 square feet or 30 percent of the floor area of the primary residence, whichever is greater. For an ADU to be approved for construction in Orange County, Florida, at least one additional off-street parking space must be available. A driveway or carport of the primary residence can be used to meet this requirement.

Types Of ADU Laws & Regulations That Govern ADU Construction

Based on various ADU ordinances and regulations passed over the last few years, we have compiled a list of some of the common things that you should keep in mind when planning the construction of an ADU.

1. Density, Height, and Setback

The most common ADU laws and ordinances deal with factors such as the height of the ADU, the density constraints of the zoning district, and the required setback, which refers to the distance between the edge of the ADU and the adjacent property line.

Residential density is essentially the number of dwelling units that can be built on any given plot of land. Different zoning districts, even within the same county or city, may have different density constraints. So, you will need to do some research on the density restrictions in your area.

Depending on the state and county in which you’re constructing your ADU, there might be some restrictions on the maximum height of the structure. You will need to abide by these restrictions in order to receive a building permit.

The city of Los Angeles, for instance, mandates that an ADU must be less than 16 feet tall. In Atlanta, an ADU can be no more than 20 feet high. Other cities and counties throughout the country have their own regulations regarding the permissible height of any potential ADUs.

The only exception to this is if the ADU is attached to or inside of the primary residence. In that case, the ADU can usually be built to be as tall as the primary residence.

Alleyway in a residential neighborhood with backyard fences backing up to it.
It will depend on your neighborhood how close to the edge of your property you can build.

Finally, you should pay attention to the minimum setback requirements in your zoning district. Most zoning districts require that the distance between a detached ADU and the adjacent property lines be sufficient to ensure fire safety.

San Francisco, for instance, requires the distance between an ADU and the rear setback to be around 25 percent of the depth of the lot. Many zoning districts in Los Angeles, on the other hand, mandate that the rear and side setbacks be no less than five feet. The minimum rear and side setback allowed for ADUs in Atlanta is four feet.

In California, the existing garage conversion laws ensure that homeowners who convert their garage into an ADU do not need to worry about setbacks. (Learn more about garage ADUs here.) This is, however, an exception. You should check with your local Planning and Building Department to learn about the ADU setback regulations in your area. This will help you abide by all the relevant zoning laws and ordinances when designing your ADU.

2. ADU Size Limitations

Your state, city, or county can impose certain limits on the size of your new ADU. Most jurisdictions in California, for instance, will allow you to build a detached ADU of up to 1,200 square feet. There are more than 390 cities and counties that use the state code, ensuring that an ADU built there can be as big as 1,200 square feet.

However, some 150 cities and counties have more specific restrictions relating to ADU size. They might impose a limit of 850 square feet for one-bedroom ADUs and 1,000 square feet for 2+ bedrooms.

Still, all jurisdictions in California will allow a minimum of 800 square feet for a detached ADU, as long as it is less than 16 feet in height and has a minimum side and rear setback of 4 feet.

In California, an attached ADU can be 50 percent of the size of the primary residence, at most. This means that if you own a 2,000-square-foot home, then you cannot build an attached ADU that is bigger than 1,000 square feet.

In Seattle, Washington, an ADU built on the same plot of land as a single-family home can be no larger than 1,000 square feet. If the ADU is built alongside a townhouse, then it can be no larger than 650 square feet. However, this square footage does not include the garage area.

a small house showing just the framing and waterproof material on the outside of this ADU build
My friend Chad’s ADU going up here in Saint Petersburg, Florida!

In Saint Petersburg and Pinellas County here, as another example, ADU’s must be under 50% of the principal property’s (your main house) square footage. You also need to construct it the same, and have it look the same as well. So if your main house is a wood frame home with a shingle roof, the ADU needs to be also. We also have other rules here like:

  • a required concrete or cement walkway
  • a dedicated single parking
  • a 9ft wide driveway both for the ADU spot and the main house spots
  • ADU cannot exceed 50% of main house square footage as mentioned above
  • setbacks – 10ft from the back yard lot line, and 5ft from each side lot line

In Salt Lake City, Utah, an ADU cannot take up more than half of the home lot’s footprint, nor can it be larger than 650 square feet. The height limit for ADUs in this city is 17 feet (or as high as the primary residence, in case of attached ADUs).

Due to these size restrictions, space is a major concern for those constructing an ADU, as well as those planning to live in one. A living room, kitchen, bathroom, and at least one bedroom must all be squeezed inside the limited space allowed by zoning laws and ordinances.

Some homeowners choose to build studios, in order to avoid cramping and maximize the available space. This might be particularly desirable if the allowed size of an ADU on your lot is less than 1,000 square feet. Wall-mounted storage (and furniture) are another great way to save space.

Build plenty of shelves into the walls, have lots of upper cabinets (and hooks or pegs for utensils) in the kitchen, and fit the bathroom with wall-mounted racks that can serve as a linen closet. These wall-mounted storage options will ensure that the floor of the ADU remains uncluttered. Murphy beds and folding tables can also help make the ADU feel larger and more spacious.

3. Utility Connections

Utilities like electricity, water, and gas are some of the most important things you’ll need to consider when planning an ADU on your property. Depending on your own needs and preferences, you can connect the ADU to the electrical panel of the main house or have it metered separately.

If the ADU will be inhabited by your family members, then having a single fuse box might make more sense. Having the main dwelling and the ADU separately metered, on the other hand, will make it easier to divide the electric bill if you’re planning to rent the ADU out. The drawback of a separate electrical meter for the ADU is that you might have to spend a few thousand dollars to set it up.

Blue water meter with black pipe. ADU laws govern whether you need a separate water meter for your ADU.
You’ll have to decide whether it’s worth the cost for a separate ADU water meter.

Similarly, for the water supply, you can either have the ADU share the water connection with the primary residence, or get a separate meter. As with the electricity supply, setting up a new water connection for separate metering can be quite expensive. It is more cost-effective for the ADU to share most of its utility services with the primary residence.

However, the sewer line of the ADU cannot connect with that of the primary residence, under Los Angeles zoning laws. Instead, the ADU’s sewer line must connect to the one that is further downstream from the house.

A septic tank connection for your new ADU, if allowed in your zoning district, will first need to be approved by the Health Department. The ADU can only be connected to the primary residence if the existing septic tank is not too old, and is large enough to handle sewage from additional bathrooms. An older septic tank should be upgraded before being connected to the new ADU, to increase its capacity.

In general, the state of California does not require homeowners to install separate gas meters for the primary residence and the ADU. So, homeowners from this state can tap into the existing gas lines of their primary residence to power the new water heater, stove, or gas furnace that has been installed in the ADU.

4. Additional Parking Requirements

Earlier, California required homeowners to build new off-street parking for each ADU they constructed. However, this rule has now been modified, since it was difficult for the owners of smaller lots, who had little space to spare. Now, if you plan to build an ADU in California, you will not have to create any new parking space if­

  • Your property is located in a district that is considered to be historically or architecturally significant.
  • Your home is no more than half a mile from the nearest public transit system.
  • A car-sharing company (or vehicle) is located within a block of the ADU.

However, you will have to provide an extra parking spot for the ADU if none of these conditions are met. In San Francisco, you can add indoor bicycle spaces on your property to reduce some of the parking requirements for new ADUs. If you convert your garage into an ADU in Los Angeles – or demolish the garage to build an ADU in its place – you will not be required by law to replace the lost parking spaces.

Bicycles parked in front of a house.
You will probably have to provide parking for cars, not just bicycles, for your ADU tenants.

Before 2019, the city of Seattle required homeowners to provide at least one off-street parking space for their ADU. However, this regulation has now been removed, offering relief to homeowners who would previously have had to spend thousands of dollars even to build a simple gravel parking space.

While you do not need to build additional parking space to construct an ADU in Seattle, you are also not allowed to remove any required off-street parking space that already exists on the property. If you remove an existing required parking spot to build your ADU, you will have to replace it with another parking space on a different part of your property.

The city of Boise mandates that a two-bedroom ADU should come with at least one additional, onsite parking space for the use of its occupants. This is in addition to the two mandatory parking spots for the primary residence. Therefore, a property with a two-bedroom ADU in Boise will need a minimum of three parking spots.

Since the regulations vary so widely from one jurisdiction to another, you should check with your local Planning and Building Department to ensure that your property is in line with the parking requirements in your area.

In Conclusion

Building an ADU can be a great way to increase the value of your home and help alleviate the housing crisis at the same time. ADUs provide many of the benefits of traditional single-family homes, while being smaller and significantly cheaper to rent. And many prospective tenants prefer them over apartments or condos. (Click here to learn more about how much you can rent out your ADU for).

However, from applying for a building permit to receiving the occupancy certificate, the ADU construction process can be quite expensive and time-consuming. You’ll need some professional contractors to help you complete your project successfully.

Knowing about the ADU laws and ordinances that apply to your property will help you increase the efficiency of the building process, while avoiding any costly mistakes that might bring your ADU construction plans to a halt. So be sure to research them thoroughly before you spend any money on the new ADU.

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