The Mountain View City Council voted unanimously Tuesday on a set of rules that will guide residential development under a controversial state law that allows up to four units on a single-family property.

Under Senate Bill 9 (SB 9), homeowners in so-called "R1" neighborhoods have the potential to subdivide their property into two lots and construct two units on each of those lots, all without requiring any discretionary approval by the city or county. Numerous Bay Area cities including Palo Alto publicly opposed the bill as a subversion of local control.

While SB 9 lays out requirements that cannot be subverted by local ordinances, cities still have some flexibility to set zoning standards that can make it easier -- or more difficult -- for homeowners to build additional units. Mountain View council members sought to strike a balance at the March 22 meeting, with regulations that would preserve the privacy of neighbors while still making it feasible to build additional units.

There are some things the city can't touch. SB 9 requires the city to allow homes that ride up to within 4 feet of the side and rear property line, and unit size can be as low as 800 square feet. Cities can't require more than one parking space and, in some cases, can't require a single parking space to be provided to additional units.

Still, Mountain View will be flexing some of its own regulatory power. Under the proposed ordinance, which the council supported on a 7-0 vote, the city will still require 25% of the front of SB 9 properties to be landscaped, and height limits will still be capped at 28 feet.

Up for debate Tuesday was whether those additional units, particularly ones built close to the property line, should be allowed to have second-story decks. City officials advocated for prohibiting second-story balconies for two-unit developments on a single lot, arguing that the minimal setback requirements would cause privacy concerns for neighbors. Last month, the Environmental Planning Commission said staff should give it another look.

Under a proposed alternative, balconies would be allowed if they are facing towards the street rather than adjacent homes, but council members still have misgivings about the potential invasion of privacy. Councilwoman Sally Lieber said she would not be able to enjoy sitting in her yard if a neighbor was able to look down on her, and that it would be asking for future conflicts between residents.

"I think what we're trying to get at with promoting ADUs is for more housing, not more conflict," Lieber said.

Council members ultimately declined to allow second-story decks in two-unit developments on R1 properties.

What proved to be the trickiest question of the night was how much newly created R1 lots would need to face the street. City planning officials suggested that at least 16 feet of any new so-called "flag lot" face the public right of way, giving plenty of space for street access and preventing any "maintenance issues or disputes" that could result from narrow access.

But could homeowners realistically split their property and meet that 16-foot requirement? Councilwoman Alison Hicks said that many homes in the city's older neighborhoods, particularly Old Mountain View and Shoreline West, are built in such a way that they could never accommodate 16 feet for a secondary unit. In other words, residents in those areas would either be unable to take advantage of SB 9 or would have to reconstruct the existing home to make more room.

"If we require 16 feet, we're telling everyone in those neighborhoods that if they want to do a flag lot and access the benefits of SB 9, they have to tear down their house," Hicks said.

Community Development Director Aarti Shrivastava said the 16-foot requirement was based on the idea that homes need 9 feet for a driveway and a buffer for fences and landscaping. There is no legal requirement that compels the city to have a 16-foot requirement, and Hicks made the case that a reduction to 12 feet would result in better housing and cheaper housing for these older neighborhoods.

Council members voted unanimously to reduce the requirement to 12 feet.

Mayor Lucas Ramirez raised concerns that the ordinance, as currently written, would prevent property owners from selling housing units on the same lot to separate owners. The so-called primary dwelling unit can be rented but cannot be sold separate from the accessory dwelling unit on the property.

Modifying the ordinance to allow multiple property owners to own units on the same parcel would require further study, according to Interim City Attorney Jannie Quinn, and would need to explore whether that would create a subdivision.

While council members agreed to add it to the future work plan for staff, Ramirez flagged the potential for sale of the SB 9 units as a chance to increase ownership housing and provide housing at a much lower cost relative to other homes in single-family neighborhoods.

"I feel like this is a tremendous lost opportunity if we don't remove that language (in the ordinance)," he said.