The basement — long an amenity under homes in Eastern states — is relatively new to Portola Valley and its tradition of single-story ranch-style houses without below-ground facilities.
That tradition is changing. Of the 71 new homes built in town between 1998 and 2005, 31 have had basements, Town Planner George Mader said in a Dec. 3 memo summarizing the trend for the Town Council.
Many of the basements are the size of a small house. The average is listed at 1,650 square feet. The largest, at 3,450 square feet, is 54 percent as big as the house it sits under, while the smallest is 200 square feet, about 11 percent as large as the house above.
The basement trend may well continue, but the days of building something half as big as the house above are numbered. Under new regulations introduced Dec. 12 by a 4-0 vote (with Councilman Richard Merk absent), such a basement would mandate a smaller house. A council vote to enact the ordinance is set for Jan. 9.
The new rules generally give a pass to basements less than or equal to 20 percent of the house’s above-ground floor area. Basements larger than that trigger a formula that trades 2 square feet of basement for 1 square foot of the house above it.
Thus, if a homeowner insisted on a 3,000-square-foot basement under a planned 5,000-square-foot home, the ordinance would ratchet back the size of the house to 3,500 square feet.
One exception to the 20 percent rule concerns high ceilings. A basement area with a ceiling higher than 12 feet would double its square footage calculation. For example, a 100-square-foot section of basement with a 15-foot ceiling would be counted as 200 square feet of floor space.
The new ordinance reflects much deliberation within town government about basements, including 11 discussions by the Planning Commission since 2003, three by the Architecture & Site Control Commission, and 12 staff reports, Mr. Mader said.
Among the main concerns: the impact on vegetation from the extra grading and on traffic from off-hauling of dirt, the higher intensity of use of homes that have basements, and excavation outside the basement’s footprint to allow light to enter during the day but also leak out at night. Portola Valley safeguards its darkness for activities such as stargazing.
“There ought to be some limit,” Councilman Ted Driscoll said. “The question is, what is the right limit? I believe this permits a very substantial basement to be built on every lot in town.”
Crossing the line?
As it deliberated on where to set the basement size limit, the Planning Commission was guided by the idea that “a person ought to be able to have a reasonable-sized basement with no penalty,” Mr. Mader told the council.Resident SallyAnn Reiss was not convinced. The commission has departed from its focus on minimizing a homeowner’s impact on neighbors, she told the council. “I feel like this ordinance is starting to cross the line into personal property rights,” she said.
“All I’m hearing is the need to decrease the size of houses. Below ground, at the cost of all these walls and various things, who cares? It’s their property. … It almost seems like it’s a religion that members of the Planning Commission belong to, but not everyone in town belongs to that religion.”
Resident Paul Wick sounded a similar note. “The proposal from the Planning Commission strikes me as regulation run amok,” he said. “So what if Portola Valley had lots of ranch-style houses. We’re not living in 1960 anymore.”
Deputy Town Planner Tom Vlasic noted, with agreement from council members, that Portola Valley is acting in anticipation of tough new building codes coming from the state.
Former mayor Bill Lane linked the issue to global warming. “I think people are going to find out that dealing with this problem is going to complicate our lives,” he said. “We’ve got to think differently. … I support what the town is trying to achieve.”
“The rest of the county and the world should be so lucky as to be governed the way Portola Valley is,” added former mayor Jon Silver.
Redwood City attorney sues ‘First Amendment auditors’ after Menlo Park disputeApril 2, 2026 11:31 am
