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 (either single-family residence or multi-family).  Call us or send us an email and we’ll let you know if you’re in a zone that allows ADUs.  We have more ADU information on our FAQ page.


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.  ADU Geeks can help you get the required building permits for your ADU.  We have more ADU information on our FAQ page.  


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 limited to 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.  We have more ADU information on our FAQ page.  


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 $1,400 / 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?  Call us or send us an email and we’ll help you determine if an ADU is right for you.

How Much Does it Cost to Build an ADU?


Cost depends on several factors:


-Single vs. two-story ADU

-Accessibility to ADU location

-Local fees

-Permit fees




ADUs are allowed in all residential zones.  ADUs must be placed on 1) single-family or multi-family residentially zoned lots and 2) must already have an existing dwelling unit. ADUs can be proposed on plans for vacant lots as long as it's concurrent with a proposed main home as well.  


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 and already have more than 1 unit then you may qualify for up to two detached ADUs and also convert existing non-habitable space into ADUs (up to 25% of the existing units).  Note: there are many 'reliefs' that are not afforded to ADU projects when on a multi-family property (like parking relief, if in the city of San Diego).  Multi-family ADU projects can get complicated, so we recommend booking a free appointment with a San Diego ADU expert by calling us to find out if your lot would qualify.


If your area is in a Coastal Overlay Zone, there may be additional regulations in order to protect our coasts.  Right now city of San Diego is subjecting all detached ADUs (even conversion of a detached garage) to coastal development review.  The impact on you is an increased timeline during the 'plan review' phase and extra costs ($10,000 - $15,000) that is paid directly to the city.


Lot size


There is no minimum lot size requirement for an ADU in San Diego. Only one ADU unit is allowed per lot.  We have more ADU information on our FAQ page


Min/Max Building Sizes


Your maximum build size differs depending on whether your accessory dwelling 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.


Detached ADU:  The total floor space area of the ADU can be 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.  There are certain cities within San Diego County, however, that have gone above and beyond allowing up to 1,800 sqft ADUs.


The gross floor area of the ADU should be included when calculating the total floor area ratio, even the square footage of the second-floor.  We have more ADU information on our FAQ page


Height Restrictions


The maximum height of an ADU is 24 feet.  Each municipality has different height restrictions so double-check.  We have more ADU information on our FAQ page




There are many options when it comes to parking.  The general rule is that parking space is not required if public transportation is within 1/2 mile (walking distance) of public transportation.  City of San Diego, however, requires parking for ADUs regardless of the proximity to public transportation if on a multi-family zoned property.


Parking is not required if the ADU is:

  • 500 square feet or less
  • Located within half a mile of public transportation (eg, bus stop)

  • 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

  • Located within a transit priority area




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

Typically, you can expect 4' rear setback and 4'  side setbacks applied to ADUs.  If you're on a corner lot then the front setback and exterior side yard setbacks apply as well.  *Exterior sideyard setbacks are usually 35' measured from the centerline of the adjacent road into your side yard.  

If you're considering a detached ADU then you'll want to consider the distance required between your ADU and the existing structure - usually 6' is required but as with everything every municipality is different.

Attached ADUs must follow the same setback requirements for the existing structure.  *This is a rule that is often overlooked and can severely limit the amount of space you have to build an ADU.

We have more ADU information on our FAQ page


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.

If you're considering a Junior ADU (JADU) then this would be the only time owner occupancy is required.




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 $250 a square foot, but the average retail value of homes in your neighborhood is worth $400 per 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.

Another factor impacting your ROI is cash flow.  Your net monthly cash flow (rental income less expenses paid) compared against the cost of the ADU project can also yield an ROI ratio.  For example, if your monthly cash flow is $1,000 then that's $12,000 per year; over 30 years that's $360,000!  Let's assume the total cost of the project was $200,000 (project cost + financing costs over 30 years) then your ROI is approximately 180% which is an ROI ADU Geeks is seeing a lot on our projects!

Call us or send us an email and we’ll help you calculate your potential ROI and determine if an ADU is right for you.


Additional notes

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

  • A junior unit is an accessory structure that is contained entirely within the footprint of the existing home. 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.