Introduction to Accessory Dwelling Units

An accessory dwelling unit commonly called a granny flat or in-law suite must exist on a lot that’s zoned single-family. It has its own entrance, separate from the primary house, as well as a kitchen, bathroom and living space. These units can be rented, year-round, and add a lot of value to a property.

Accessory dwelling units take different structural forms. They can be garage conversions, stand-alone units, attic or basement conversions, or be attached to the main house.

ADU Regualtions



ADUs are allowed:

• In R-1, R-2, and R-M zoning districts on lots that have one single-family residence.

• On lots with a single-family residence In Planned Development (PD) districts that are subject to R-1 standards and uses OR the ADU conforms to the development and use standards of the PD district.

• On any lot with an existing single-family dwelling unit, where regardless of zoning, the lot has any of the following General Plan Land Use/Transportation designations:

– Residential Neighborhood

– Mixed-Use Neighborhood

– Urban Village or Urban Residential

– Transit Residential or Rural Residential

– Downtown

– Mixed-Use Commercial

• On two-family (duplex) lots and on multifamily properties

• In a low-density cluster development if it conforms to development standards of the R-1 zoning district, and the development and use standards of the low-density cluster permit.

Not sure what your zone is? Contact Maxable Space and we can help!

Lot size

Thanks to new ADU legislation there are no longer minimum lot size.

Min/Max Building Sizes

For lots 9,000 square feet or less homeowners can build up to 1000 square feet. For lots larger than 9,000 square feet homeowners may build an ADU up to 1,200 square feet.

Height Restrictions

"Height restrictions vary by zoning type.
Single-Family Zoned Property (Detached ADU):
Single story – 18 feet maximum
Second story – 24 feet maximum Multifamily properties - 16 feet maximum height for detached ADUs."


(MUNICIPAL CODE 20.30.150.I)
No parking is required for an ADU that meets any of the following criteria:

The ADU is located within .5 mile of a public bus stop or rail station, and the path of travel is publicly accessible.

The ADU is part of or within the primary residence or existing accessory building.

The ADU is on a street that requires on-street parking permits, but the permit has not been offered to the occupant of the ADU.

The ADU is located within one block of where a motor vehicle provides hourly and daily service as part of a regional fleet operated by a public agency or publicly-leased motor-vehicle-sharing organization.

The ADU is located within a historic district listed in the City’s Historic Resources Inventory.
If you convert your garage the parking spaces are no longer required to be replaced. This is in alignment with the new state laws. (MUNICIPAL CODE 20.90.220(B)(2)



4 feet minimum rear and side setbacks required for detached ADU in multifamily developments. There is no setback required for single-story detached ADUs built in a single-family zoned lot, although, building code must comply with fire and safety.

Owner Occupancy

Owner occupancy requires that the homeowner either live in the ADU or in the primary dwelling. Owner occupancy has been removed in all jurisdictions in California until the year 2025. Any homeowner who builds and ADU from 2020-2025 will forever be grandfathered into no owner occupancy requirement even if the legislation changes to require owner occupancy in the future.


An ADU can always be rented long-term. A long-term rental is defined as a rental period that is 30 days or greater. Often Airbnb, or other short-term rentals are restricted for ADUs. Check your local planning department to find out if a short-term rental in your ADU would be legal.

How to Evaluate ROI

People often underestimate the cost to build a small unit. There are a lot of fixed costs and the expensive square footage, like a kitchen and bathroom, are not offset by large cheaper square footage (for example, large living rooms, hallways, and multiple bedrooms).

With that being said, 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 $500 a square foot, but the average value of homes in your neighborhood are worth $636 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.

Property Value (avg value per sq ft)

$636 (Figure provided by Zillow)

Construction Costs (avg cost per sq ft)

$450-500 per square foot

Equity Gained through project

$636-500=$136 (Meaning you are gaining $136 in property value for every square foot built)

Additional notes

Ready to dive in? Check out our consultation services or start with a preliminary phone call.

Rear Yard Coverage: Not including pools, not more than 40% of the rear yard may be covered.

Front Yard Paving: No more than 50% of the front setback may be paved with an impervious or pervious surface.

Detached units do not need to meet any specific design standards. Attached ADUs and detached ADUs on properties listed on the Historic Resources Inventory must incorporate architectural style and elements of the primary dwelling.

ADU Fees for San Jose:

To get an estimate of permit fees associated with an ADU fill out and submit this worksheet:

In addition to a building permit fee, applicants for an ADU will be required to pay:

School Fees
Park Impact Fees. View the map that shows the Park Impact Fees that vary according to location.

In addition to a building permit fee, applicants for an ADU will be required to pay:

  • School Fees
  • Park Impact Fees

Mountainview ADU Regulations