What is an Accessory Dwelling Unit?

August 25, 2025
3:22 am

What is an ADU? It is a small, independent residential unit built on the same lot as a main house, also called an Additional Dwelling Unit, granny flat, or secondary dwelling. While most popular in the U.S., ADUs are gaining attention worldwide as an affordable housing solution, offering a private, fully equipped living space as an alternative to shared rooms or full-sized homes. So, now we know what is an accessory dwelling unit, and we can dive deeper into the different types and other important information about an ADU

What is a JADU?

A “junior accessory dwelling unit” or JADU is a unit that is no more than 500 square feet in size
and contained entirely within a single-family residence. A JADU may include separate sanitation
facilities or may share sanitation facilities with the existing structure. (Gov. Code, § 66333.)

Instead of living in shared rooms or studios, people strongly prefer their own independent unit with a private entrance, bedroom, full bathroom, and a complete sense of home—often at half the cost of a full house! But let’s look at the main benefits and challenges of living in an ADU home.

Who can build an ADU house?

If you’re unsure whether you can build an accessory dwelling unit on your property, the best way is to contact your local county or city building department to learn their specific ADU regulations for ADU construction or building a granny flat. States like California, Oregon, and Washington encourage building ADU houses as part of their solution to housing shortages. If you live in these states, this could be a great opportunity as a landowner to build an Accessory Dwelling Unit and benefit from the steady income it can generate.

In total, 13 states across the U.S. have legalized ADU house construction to help reduce housing costs. California is the leading state, with the highest number of ADU homes. California is the leading state, with the highest number of ADUs, often referred to as backyard cottages or secondary units. Among all California cities, Los Angeles, San Francisco, and San Diego have the largest numbers, mainly because housing costs in these areas are extremely high, making ADU in los angeles a more affordable option for residents.

So, according to California Government Code (§§ 66313(a); 66314(d)(3)), what is an ADU? an ADU is accessory to a primary residence and must include complete independent living facilities for one or more persons. An ADU can be detached, attached, or located within the proposed or existing primary dwelling. This includes attached garages, storage areas, or similar spaces, as well as detached garages or other accessory structures, provided everything is located on the same lot as the main residence.

How much does it cost to build an ADU?

The cost of building an ADU (also called ADU building expenses) varies depending on the state and the type of unit you want to build. There are different types of ADUs you can choose from based on your needs and budget. We’ll cover those in the next section, but first let’s look at the major steps that affect the overall ADU construction cost:

Checking ADU regulations with your local city or county
Evaluating the best space on your property for an ADU
Developing a concept design
Preparing ADU permit documents
Securing ADU permits
ADU Construction
Supervision and project management

Building an ADU house can cost anywhere between $100k to $200k, but there are many strategies to help reduce construction expenses and make your ADU project more affordable.

Types of ADUs

To better understand your options, let’s explore the main types of ADUs and how each can fit your property. So, what is an ADU in practical terms? It can be detached, attached, or converted from existing spaces like garages and attics.

Accessory Dwelling Units (ADUs) can be built in different ways depending on the space you have, your budget, and your goals. The main types of ADUs include

Detached ADUs

A fully separate unit built on the same lot as the main house. It has its own entrance and complete living facilities. Examples include:

A small backyard cottage
A standalone unit near the main house

(Gov. Code, §§ 66314(d)(3); 66323(a)(1), (a)(2), (a)(4)(A))

Attached ADU

An ADU that shares at least one wall with the primary home. These are often built as an extension of the existing house. Examples include:

An addition to a duplex
A unit built above an attached garage
A new second story on a single-family home
A newly constructed basement unit

Converted ADUs

Also called garage conversions or interior ADUs, these units are created by transforming existing spaces into livable areas. Examples include:

Converting a garage into a small apartment
Turning an attic, basement, or unused room into an ADU
In multifamily buildings, converting storage rooms or other non-livable areas into residential units

👉 In summary:

Detached ADUs = standalone backyard units.
Attached ADU = built as an extension of the main house.
Converted ADUs = made from existing spaces like garages, attics, or basements.

This flexibility makes ADUs a popular housing option, since homeowners can choose the type that best fits their lot size, lifestyle, and budget.

Essential Facts About Building an ADU: ADU Regulations, and More

Before starting construction, it’s important to understand what is an ADU and the regulations that govern it. Knowing the rules around size, permits, parking, and placement will help ensure your project meets legal requirements and avoids costly delays.

Height Requirements for an ADU house: How Tall Can Your Unit Be?

California law sets different maximum heights for accessory dwelling units (ADUs) depending on the type of main dwelling, whether the lot is near transit, and the building’s design (Gov. Code, § 66321, subd. (b)(4)):

16-foot height limit (subd. (b)(4)(A)) – This is the standard height for ADUs on lots with an existing or proposed single-family or multifamily dwelling. It applies to most locations without special conditions.
18-foot height limit with an extra 2 feet for roof pitch (subd. (b)(4)(B)) – On lots with an existing or proposed single-family or multifamily dwelling that is within half a mile of a major transit stop or a high-quality transit corridor, ADUs can be slightly taller: up to 18 feet, plus an extra 2 feet if needed to match the roof slope of the main building. This rule encourages ADUs near transit while allowing them to blend with the main structure’s roofline.
18-foot height limit for multifamily, multistory dwellings (subd. (b)(4)(C)) – For lots with an existing or proposed multifamily, multistory dwelling, ADUs can reach 18 feet regardless of transit proximity. This accommodates larger, multistory buildings.

Key differences:
- Type of dwelling: (A) and (B) apply to single-family or multifamily; (C) applies specifically to multistory multifamily.
- Proximity to transit: Only (B) allows extra height based on being near major transit or high-quality transit corridors.
- Roof alignment adjustment: Only (B) explicitly allows an additional 2 feet to match the main building’s roof slope.
These rules are intended to balance ADU development with neighborhood character and transit accessibility (Gov. Code, § 66321, subd. (b)(4)).

Parking Requirements for an ADU house:

Parking requirements for Accessory Dwelling Units (ADUs) are very limited. By law, a city cannot require more than one parking space per ADU or per bedroom, whichever number is smaller. These spaces can also be provided as tandem parking on a driveway.

Homeowners do not need to provide guest parking for ADUs, and if a garage or carport is converted into an ADU, the city cannot force you to replace the lost parking spots. Understanding what is an accessory dwelling unit and the true need that cities are incentivizing—building Accessory Dwelling Units—helps clarify why these parking rules apply. In many areas, local agencies are even reducing or eliminating parking requirements altogether to make ADU construction easier and to encourage walking, biking, and public transit.

ADU Size Requirements and Restrictions: What’s Allowed

Minimum Lot Size:
There is no minimum lot size required to building an ADU. A city or county cannot stop you from building just because your lot is considered “too small.”
Minimum Unit Size:
Local governments cannot set a minimum size that blocks very small “efficiency units.” By law, an efficiency unit can be as small as 150 square feet.
Maximum Unit Size:

For attached ADUs: Local agencies can use a percentage of the main house’s size (for example, up to 50%). But in all cases, the ADU must be allowed to be at least 850 sq. ft. (or 1,000 sq. ft. if it has more than one bedroom).

For detached ADUs: At least 850 sq. ft. (or 1,000 sq. ft. with more than one bedroom) must be allowed. If a city does not have its own ADU ordinance, the default state law allows up to 1,200 sq. ft. for detached ADUs.

Cities can adopt less restrictive rules and allow even larger ADUs if they choose.

Conversions:
If you convert an existing space (like a garage or part of your home) into an ADU, those unit size limits do not apply.

ADU Permit Applications: Timeline and Agencies Involved

The ADU permitting process usually takes about 90 days total (30 days to check your application + 60 days for a decision) as long as your paperwork is complete and fees are paid.

Application Review (30 days)

After you submit your ADU application, the city (or local agency) has 30 calendar days to check if it’s complete.

If anything is missing, they must give you a full list of the missing items.

Approval or Denial (60 days)

Once your application is complete, the city must either approve or deny it within 60 days.

This decision is “ministerial,” meaning it’s based on clear rules and standards, not personal judgment.

Fees

You’re responsible for paying the processing and permitting fees when you apply.

Agencies Involved

The review may involve different departments like planning, building, utilities, or local districts—any official group that must check your ADU project.

Quoted from the PW website:
These Pre-approved ADU Standard Plans cannot be revised and must be permitted as designed. The ADU’s may be oriented in any direction and floor plans may be mirrored (flipped). Any changes to these pre-approved plans are not allowed.

In short:

No minimum lot size.
ADUs must be allowed to be at least 850–1,000 sq. ft., depending on bedrooms.
Detached ADUs can go up to 1,200 sq. ft. (or more if local rules are looser).
The smallest legal ADU (an efficiency unit) can be as small as 150 sq. ft.
Conversions of existing structures don’t have to follow size limits.

Final words:

Now you know what is an Accessory Dwelling Unit,(actually what is an ADU) and why it has become one of the most practical housing solutions today. By offering affordable, flexible living spaces—whether detached, attached, or converted—ADUs add value to your property while addressing modern housing needs. If you’re considering building one, understanding local regulations, costs, and design options is the key to making your ADU project a success

What is an Accessory Dwelling Unit?

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