FAQs

Accessory Dwelling Units (ADUs) come in many shapes and sizes but are always a self-contained home that is smaller than the main house and legally part of the same property. They must have a kitchen, bathroom, and place to sleep, and typically range from 150-square-foot studios to 1,200-square-foot homes with multiple bedrooms. They can be larger if you go through the process to get a Special Permit.

Junior Accessory Dwelling Units (JADUs) are smaller units of up to 500 square feet of converted space in a single-family home and must include an efficiency kitchen (sink, stove, fridge, and counter). Some JADUs have their own bathroom, while others share one with the main house. State law now allows homeowners to have both a JADU and a regular ADU on their property. Construction costs for JADUs are typically much lower.

An attached ADU can be new or converted from an interior space as long as it shares at least one wall with the primary house.

A detached ADU is a freestanding structure that can be new or converted from an existing structure, such as a garage or barn.

A traditionally constructed site-built ADU is designed and built specifically to your preferences and property. This option allows for a lot of customization and smaller changes to be made throughout the construction process.

A manufactured or prefab ADU is built partially or mostly in a factory, then shipped to your site to be put together. This option requires a lot less of your builder since so much is done off-site, and usually includes laying the foundation, on-site assembly, utility hookups, and overseeing inspections. Manufactured and prefab ADUs often save time because the site can be prepared while the home is being constructed, but they also typically offer less customization.

It is often surprising for people to learn that manufactured or prefab ADUs often cost the same as traditionally constructed ADUs, although some companies do offer lower-cost options.

Types of prefab

Panelized: Homes constructed from two-dimensional, pre-built sections that are assembled on-site.

Manufactured (Mobile Homes): Built off-site like prefab but regulated by federal building code and usually more affordable. The construction standards are lower for these, and they usually do not hold up as well.

In almost all cases, the answer is yes! The best way to find out is to use the County’s Web GIS Tool to find out more information about your parcel. This online mapping tool provided by the county allows you to look up important information about your property using an address or APN, such as lot size, what you can build, and whether there are any additional restrictions. With some technical exceptions, ADU development is allowed wherever residential development exists or is permitted. Alternatively, you can contact the county’s building and planning department to speak with a planner.

ADUs and JADUs are allowed in any zone that allows single family or multifamily dwelling residential use and includes a proposed or existing dwelling.

Homeowners can build both an ADU and JADU on their property, and multifamily property owners can include two or more ADUs.

ADUs can range from 150 to up to 1,200 square feet except for lots zoned Agricultural Exclusive (AE) that are 40 acres or more where an ADU unrestricted in size may be allowed in addition to a main residence. You can also build an ADU larger than 1,200 square feet with a Special Permit. ADUs fall into three categories:

Detached ADUs can be up to 1,200 square feet if it fits on your property. They can be up to 16 feet tall (or 26 feet tall on larger residentially zoned lots) and if it’s a new structure, it must have at least 4-foot side and rear yard setbacks. Setbacks do not apply to an ADU converted from a freestanding structure.

Attached ADUs can be up to 50% the size of our existing primary house, OR 1,200 square feet – whichever is smaller. So, if your home is 2,000 square feet, you can build a 1,000 foot attached ADU. Even if your house is huge, your attached ADU cannot be larger than 1,200 square feet without a Special Permit. When doing calculations, do not include your garage, sheds, or storage areas and ask the county if they have an official record of your existing floor area or what spaces to include. An attached ADU can be new or converted from an interior space as long as it shares at least one wall with the primary house.

Junior ADUs (JADUs) can be up to 500 square feet of converted space in a single-family home and must include an efficiency kitchen (sink, stove, fridge, and counter). Some JADUs have their own bathroom, while others share one with the main house. State law now allows homeowners to have both a JADU and a regular ADU on their property. Construction costs for JADUs are typically much lower.

State law allows homeowners the right to convert many types of existing space into ADUs. The state law covers garages, other accessory buildings (like art studios), or even part of the main house, including attics and basements. To qualify, your property must meet the following criteria:

• The house must be located in a single-family zone.
• The structure must have been built legally initially.
• The ADU must have a door that provides direct access to the outside. (This door can be added during construction if it is not there in the existing structure.)
• The existing structure must be far enough away from the property line to not be a fire hazard. Discuss specifics with the county.

ADUs located within a county identified zone that restricts development may need to go through an additional Special Permit process, such as outside a Fire Protection District or within an Airport Overlay District. Use the Humboldt County Web GIS Tool or contact county staff to find out if there are additional rules that apply to your property.

Building an ADU is not only a financial investment, but an investment of time. Most projects take one to two years to complete. Typically, it takes homeowners one to three months to get started and assemble their team, then one to six months to develop plans, meet with the county, and submit the application. Depending on how many rounds of review are required and how quickly a homeowner and their project team can respond to comments, it will take one to six months to get permits. Construction usually takes six to twelve months. Interior conversions are often faster.

Humboldt County does not require homeowners to live on their property if it includes an ADU. However, both state and county laws require owner occupancy for JADUs.

Many homeowners use a mix of options to finance their ADU, including their own savings and assets, funds from family, and/or loans. Be sure to consider potential rental income when deciding how to finance your ADU. It will be a source for repaying any loans. It is strongly recommended that you do not begin any construction without your financing plan in place.

See our Guidebook and Interactive Exercises for more details on financing options and their related benefits and drawbacks

Adding an ADU will likely affect your property taxes and the resale value of your home. However, your primary house will not be reassessed, and your property taxes will only increase based on the added value of your ADU. For example, if you build an ADU that adds $150,000 to your property value, and your tax rate is 1%, your taxes will increase by 1% x $150,000, or $1,500 per year.

Building a JADU will have a significantly smaller impact on assessed value. In some cases, your taxes will not increase at all. Home sharing will also not increase the assessed value of your home. Generally, garage conversions will not raise your tax bill as much as new construction, but they will also not add as much value.

Each property will require a one-on-one analysis to determine the added value of an ADU, so contact the Humboldt County Assessor’s Office once you have an idea of your plan. Call (707) 445-7663 or send an email to assessor@co.humboldt.ca.us and they will be able to provide you with a rough estimate of tax implications.

Adding an ADU may impact your income taxes as well. This can be rather complicated, and it’s best to discuss these with a tax advisor. The rental income you receive for your ADU is taxable, but you can deduct the cost of allowable maintenance and depreciate the cost of construction, which can often reduce or eliminate any tax increase. You may also have capital gains tax related to the increased value when you sell your property.

Note that your unit could be assessed different ways for different purposes. If you are trying to get a loan, the bank might be more conservative and assume the value of your home will increase less.

Yes! The California Housing Finance Agency (CalHFA) launched an ADU Grant Program which may reimburse pre-development and non-recurring closing costs. To learn more about the program including who is eligible, please visit the CalHFA website.

The county owns sets of pre-approved ADU plans and is considering expanding their library which could save you time and money with the design and permitting process. Click here to view the plans or contact a county planner to learn more.

If your property is outside of a sewer service district, you will need to find out if your septic system will meet Humboldt County Health Department requirements when adding an ADU to your property. You may have to increase your septic capacity or establish a new water source, which can be an unexpected cost, so talk to the county early in the process to find out. For more information, view the Wastewater and Water Supply Guidance Memo or call the Environmental Health Division at (707) 445-6215.

New or separate utility connections may be required for ADUs, but not JADUs. Use the Web GIS Tool or call the Planning and Building Department to find out who provides water and sewer service to your property if you aren’t sure. Contact PG&E early in the process to learn if there are additional requirements for electricity and gas.

Adding parking is much less of a concern than it used to be. If your property does not meet any of the exceptions outlined below, you will need to add one spot for the ADU, and it can be tandem (directly behind another spot in the driveway). Also, if you convert a garage to an ADU (or demolish a garage and build an ADU), you do not need to make up for lost parking.

You will not need to add any parking for your ADU if it meets any of the following conditions:

• Within ½ mile of public transit
• In a designated architecturally and historic district
• Within an existing structure (e.g., garage)
• When on-street parking permits are required but not offered to the occupant of the ADU
• Within one block of car share locations
• If the ADU is less than 1,000 square feet and within a mapped Housing Opportunity Zone

An ADU in the Coastal Zone will follow a bit of a different process and may require a Special Permit, as well as notifying neighbors. To determine if your ADU falls within the Coastal Zone, visit Humboldt County’s Web GIS tool or talk with county staff.

The Safe Homes Program was established by the county to legalize ADUs that were built without first obtaining a building permit. County staff developed a Safe Home Program packet which outlines the programs in greater detail and the necessary steps to legalize your ADU. This program is open until 2025.

Construction costs for your second unit will vary significantly depending on personal preferences, site conditions, location, and many other factors.

Size: Despite what many think, smaller ADUs may cost almost the same as larger ones because construction costs are not related to size. Many costs like foundation, kitchen and bathroom work only increase slightly for larger ADUs. Kitchen costs will range from $25,000–$50,000 with each bathroom ranging from $15,000–$25,000.

Type: New construction, both detached and attached, tend to be the most expensive. Garage conversions are not much cheaper than new construction if at all. Manufactured or prefab, like garage conversions, are often not much less than new construction. Conversions of interior space (basement or otherwise) are often the cheapest.

Other factors: 

• Quality of interior finish work and amenities
• Architectural form and details
• Extent of utility, structural, mechanical, electrical, and plumbing upgrades required
• Required site upgrades (sidewalks, sewer and water)
• Whether sprinklers are required
• Whether doors and windows meet emergency exit standards
• Lot complexity (slope, trees, fault lines, etc.)

If the construction costs to build an ADU are too high, consider home sharing or converting interior space to an ADU. Home sharing requires no construction and brings in rental income. The cost to convert interior space into a JADU is often much lower than other ADUs and can be a great option.

Traditional construction will take 6-12 months, though this will vary depending on the specifics of the project.

Stages of construction include:

• Site preparation: 1-2 months
• Foundation: 1 month
• Walls, roof, doors: 1-2 months
• Plumbing & electrical: 1-2 months
• Insulation & drywall: ½-1 month
• Fixtures & finishes: 1-2 months
• Final touches: ½-2 months

The county’s ADU ordinance prohibits the use of ADUs and JADUs as short term rentals (rented for less than thirty days). This discourages the listing of ADUs on popular websites like Airbnb and VRBO and promotes them as a means to increase housing stock for the diverse needs of county residents.