The design for the units would be in keeping with local zoning laws and could be preapproved as a group by city and town planning departments and councils, although all the designs might not work in every city, Vatkin said.
The program is known as the Green and Liveable Accessory Dwelling Unit Resource program, or GLADUR.
"We released our (request for proposals) to find a firm to help us design the plans, which will be reviewed by certain cities in San Mateo County," Vatkin said. "We would put out the designs for free, and homeowners could take the designs and customize or finish them as they wish."
Homeowners could download the plans from HEART's website. The agency is trying to complete its first designs by the middle of next year, Vatkin said.
"Once we have the initial designs, we'll have a couple of public meetings to get feedback, then proceed to construction drawing and prereviews by cities," he said.
After plans for detached units are created and distributed, HEART plans to introduce designs for units attached to the main home or built within the existing walls of the home, he said.
The plans could be especially valuable for communities such as Portola Valley and Woodside that have sky-high housing prices and many parcels with a surplus of open space, if a number of other conditions fall into place, he said.
Both communities have recognized the opportunity for creating affordable housing via ADUs by passing accessory dwelling unit ordinances regulating their construction.
"Portola Valley and Woodside have plenty of space for ADUs," Vatkin said. "They could be rented out as an extra source of income. We've had a lot of response that the units could be utilized by family members."
Portola Valley leaders have been keeping track of the evolution of the program, which could help residents save money building ADUs, according to Councilman John Richards.
Richards wrote in an email that the town is "waiting to see what comes out of the (HEART) endeavor."
However, he cautioned that Portola Valley has "such varied and challenging site conditions here that a one-size-fits-all approach may be difficult."
"The program will probably start in a few communities with smaller lots and more consistent topography," Richards wrote.
Portola Valley's ADU ordinance, passed by the Town Council in March, allows accessory units on 1 acre or larger parcels that have a house and are in a neighborhood zoned for residential construction.
The homeowner is responsible for making sure that the main house and potential ADU do not exceed the total amount of allowable floor area for the parcel.
The unit must also have the same address as the main house and cannot be sold separately. Either the unit or the main dwelling must be owner-occupied, according to the ordinance.
Sage Schaan, principal planner with the Woodside building and planning department, echoed some of Richards' concerns.
Schaan said that Woodside lots come in a wider variety of shapes, sizes and elevations compared with a more urbanized city, which might limit the benefits that could be derived from a standardized approach.
Under Woodside's ADU ordinance, adopted in 2017, a homeowner could build two ADUs on a property equal to or greater than 1.5 acres, and a maximum of one detached and one attached unit on a property greater than 1 acre, but less than 1.5 acres.
Any property in Woodside less than 1 acre is limited to one ADU.
HEART has been active on several other housing fronts. The agency, founded in 2003, has invested more than $19 million in developing or rehabilitating 1,297 affordable homes in San Mateo County, provided loans for 90 homebuyers and worked with the Pacifica School District to provide workforce housing, Vatkin said.
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