What is an Accessory Dwelling Unit (ADU)?


An ADU is an additional dwelling unit that can be attached or detached from the primary dwelling.  These units can be rented, year-round, and add a lot of value to a property.


The ADU could provide affordable housing options for family members, friends, students, the elderly, in-home health care providers, the disabled, and others.


You may want to consider taking advantage of the change in California State law which made it easier to add a second house, called an Accessory Dwelling Unit (ADU), to your residential zoned property.


ADU’s are what many know as Granny flat, in-law suite, converted garage, backyard cottage, basement apartment, companion units.  Regardless of the name, ADU’s are self-contained homes legally part of the same property. ADU’s can take many forms and vary in size, but always contain everything someone needs to live, including a kitchen, bathroom, and place to sleep. ADU’s typically range from 400 sqft 1,200 sqft.


Where is an ADU allowed?


ADUs are allowed on a residentially zoned property that has an existing single-family residence.  Fill out the contact us form above and we’ll let you know if you’re in a zone that allows ADUs.


Is a Building Permit Required for an ADU?


Yes, you will be required to obtain a Building Permit to ensure that the new house meets all the zoning, building, health & safety codes.


What Size ADU is Allowed? How Big?


An ADU is intended to be secondary in size to the single-family residence.  ADUs can be attached to or detached from the primary dwelling.  If the ADU is attached to the main home then the size if the ADU may be as large as 50% of the size of the main home, up to a maximum of 1,200 square feet. Check local ADU regulations as some cities within San Diego County have adopted other requirements.


Why Install an Accessory Dwelling Unit?


The most popular reasons to build an ADU are rental income and/or to house relatives. Building second units (ADUs) are a relatively affordable type of construction because they do not require purchasing land or major new infrastructure. 


With rents for a 1 bedroom 1 bathroom unit averaging $1400 / month in many areas of Southern California, many people install an ADU to help pay their primary mortgage or earn extra income.


Can you use extra income or do you have a family member /friend that needs a place to live?  Fill out the contact us form above and we’ll help you determine if an ADU is right for you.

How Much Does it Cost to Install an ADU?


Cost depends on several factors:


-1st or 2nd floor

-Accessibility to ADU location

-Local fees

-Permit fees





ADUs must be placed on 1) single-family residentially zoned lots and 2) must already have a single-family dwelling.  ADUs are allowed in all residential zones.


In San Diego, companion units can also be permitted in multi-family lots under specific circumstances. They are allowed on lots where the maximum number of dwelling units is two. So, if you have, for example, a lot zoned RM1-1 with two units or less you may be able to add a companion unit.  Multi-family can get complicated, so we recommend booking a free appointment with a San Diego ADU expert by calling 619-588-4346 to find out if your lot would qualify.


If your area is in a Coastal Overlay or Historical Zone, there may be additional regulations in order to protect our coasts.


Lot size


There is no minimum lot size requirement for an ADU in San Diego. Only one ADU and a Junior ADU is allowed per lot.


Min/Max Building Sizes


Your maximum build size differs depending on whether your accessory unit is attached or detached from the primary home.


Attached ADU: The floor area of the ADU should not be more than 50% of the existing residence up to 1,200 square feet. For example, if your existing house in 2,500 sqft. then the attached ADU can only be 1,200 sqft (no ADU can exceed 1,200 sqft. in size.)


Detached ADU: For a detached ADU, the total floor space area of the ADU as large as 1,200 square feet, regardless of the size of the primary residence.  For example, if the existing house is only 600 sqft the detached ADU can still be 1,200 sqft. 


The floor area of the ADU should be included when calculating the total floor area of the lot.


Height Restrictions


The maximum height of an ADU is 30 feet.




Companion units should follow the same parking regulations as the main house. If there is parking access from an alley before, there should still be accessible after building an ADU, unless there is a garage.

There are many options for parking spaces. They may be put within setback areas, covered or uncovered, or use mechanical lifts. The important thing is that they provide safe and efficient vehicle access. The minimum parking requirement is one parking space per ADU, with 0.5 parking spots per bedroom.


Parking is not required if the ADU is:

  • 500 square feet or less
  • Located within half a mile of transit

  • Located in a historical resource area

  • Already part of the existing building

  • Located in a residential permit parking district

  • Located within one block of a car share station

  • Located within one block of a bike share station




A setback is a minimum distance a structure must be away from the street, buildings, or other features.


ADU’s may extend into the setbacks if the structure is one story and less than 30 feet in length.


Owner Occupancy


An ADU can’t be sold separately from the main house. The owner does not need to live in the main unit or the ADU. Both structures can be rented.




California passed Senate Bill 1069 in an effort to increase the supply of housing to the state. For this reason, any accessory dwelling unit may be used for long-term rental. ADU’s cannot be rented out for less than 30 consecutive days.


How to Evaluate ROI


Building an accessory dwelling unit, especially if you intend to keep the property for a few years, represents an enormously beneficial opportunity to maximize your property.


By constructing an accessory dwelling unit you are adding additional square footage to the property. So if you can build a unit for $275 a square foot, but the average value of homes in your neighborhood is worth $400 a square foot you are gaining instant equity through the project. Plus, you also now have the added benefit of a flexible space that can be rented or used for family.


Additional notes

  • Construction of a passageway is not required when building an ADU.

  • A tree must be planted in the front yard or parkway unless a tree that is at least 15 feet high and wide already exists.

  • A junior unit is an accessory structure that is contained entirely within the main house. They are exempt from parking regulations, and should have a separate outdoor entry, a connection to the main living area, and should include an efficiency kitchen. They may have their own bathroom or share a bathroom with the main house. The junior unit and the main house must be sold together and the owner must live in either the junior unit or the main house.

(619) 588-4346

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