Considering Building a Secondary Unit in Palo Alto?


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Secondary units or homes, often referred to as “granny units” or “in-law units” can be an excellent way of allowing people to age in place. They can serve as a home for family or a caregiver or they can serve as an income stream during retirement, providing a relatively affordable rental option for students or young adults. Secondary homes can also be a terrific way of safeguarding against mobility issues in the future: they can be built as a single floor with no-step entries, extra wide hallways and doors, and switches and outlets reachable at any height. It can often be much cheaper to build a secondary home in this manner than to retrofit an existing home or to find another home to purchase, so many people choose to move into the secondary home themselves and let their children live in the main home.


If you’re considering living with family, you’re not alone. Twice as many people live in multigenerational households today as they did in 1980 and given that 1 in 5 people will be over 65 by 2030, we can only expect that number to go up. Most secondary homes today are built by people between 55 and 64 in preparations for their own retirement.

Secondary homes can be built as attached to the primary home or as detached. State law allows Palo Alto property owners to build secondary homes "by right," meaning that it requires no "discretionary review" from a city commission or the council. However, for the Planning Department to sign off on a project, the property must meet a laundry list of conditions, some of which might rule out a secondary home for your lot.


 In order to have a secondary home, your lot needs to be 35% larger than the minimum lot size for your type of zoning.

 Minimum Lot Sizes from Section 18.12.070 of the Palo Alto Municipal Code 


Minimum Lot Size

Minimum Lot Size to Add Secondary Unit (all lots except flag lots)

Minimum Lot Size (flag lots)


6,000 sf

8,100 square feet ("sf")

9,720 sf

R-1 (7,000)

7,000 sf

9,450 sf

11,340 sf

R-1 (8,000)

8,000 sf

10,800 sf

12,960 sf

R-1 (10,000)

10,000 sf

13,500 sf

16,200 sf

R-1 (20,000)

20,000 sf

27,000 sf

32,400 sf

Exclusive of any portion of the lot used for access to the street



Attached secondary homes can be a maximum of 450 sq ft and detached secondary homes can be a maximum of 900 sq ft. Detached homes have to be 12 ft away from the main house and have setbacks from the property line as well. Both require the addition of two parking spaces - one covered and one uncovered. This parking may not be added within the front setback, but must be at least 10 feet away from the property line, so driveways may not be considered additional parking space.

Secondary homes are also limited by the maximum built out square footage applicable to the lot as a whole and the floor area ratio requirements. The size of the secondary home and the square footage associated with the covered parking both count toward the total allowable square footage . So, if a home has already maxed out its square footage a secondary home cannot be added even if the minimum lot requirement and other requirements are met.

Secondary Home Requirements


Lot Size

35% greater than minimum for your district


Must add one covered and one uncovered parking space within your front setback and not closer than 10 ft from the street in a street side setback – your existing driveway does not count

Maximum Height of Secondary Unit

17 feet (must be one story)

Total Floor Area

The square footage of the unit and the square footage of the covered parking space both count against your maximum floor area. If your house is already at maximum floor area, you cannot add a secondary home

Max Square Footage of Unit

·Attached: maximum of 450 sq ft (a little larger than the size of a two car garage)

·Detached: Maximum of 900 sq ft


·12 ft minimum separation from house for detached units and all other setbacks as apply to the main home

 Big Picture

Restrictions on secondary homes in Palo Alto are quite stringent. The minimum lot size is very difficult to comply within a city that has so many smaller parcels, particularly in downtown areas that would otherwise be great candidates for secondary homes because of their walkability to retail and transit. We have about 14,000 single family homes in the city, but city staff estimates that only about 1600 parcels are big enough to be eligible for a secondary home.  Once other requirements are taken into account, even fewer homes are eligible. By comparison, in Menlo Park secondary homes can be built on 6,000 sq ft lots and Redwood City has no minimum.

Even properties that meet the minimum lot size may be excluded from adding a secondary home because of the parking requirement: two additional spaces (one covered).  This may not be realistic for secondary homes occupied by only one person or people (like many seniors) who don’t drive. There is also limited flexibility in where the parking can be placed. By contrast, Mountain View, Redwood City, Menlo Park, and Santa Cruz only require one space either per unit or per bedroom without requiring any covered parking.

The CIty of Palo Alto historically approves about four secondary homes each year. The proposed Housing Element anticipates only 32 new secondary homes will be built in the next eight years. That’s a very small contribution to the 1,998 homes that the Bay Area Association of Governments has required Palo Alto to zone for through 2030 and that’s unfortunate because secondary homes are a great benefit to our residents and one of the simplest ways to add housing without affecting neighborhood character.


If you’d like to know more about secondary homes, please come to Avenidas at 450 Bryant St, La Comida Room, on May 3, where we’ll be hosting a talk and a Q & A. Click here to RSVP.

Also, Palo Alto is currently updating its Comprehensive Plan and taking input at the Our Palo Alto Summit on May 30, 8:30am-6pm. Register here.


  • Dhruvd day99
    commented 2019-01-09 03:10:28 -0800
    thanks for all of this,
  • Anonymous
    commented 2017-08-01 04:44:28 -0700
    I have been following your blog for some time now and have found it quite informative and also interesting and you have very nice way of expressing the article.Looking forward to another article.


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