"It's like being in the middle of the game and they change the rules," 20-year open space property owner Cathy Cartmell said.
Planning and Transportation Commission Chair Karen Holman said proposed zoning changes are actually a way for the city to get back to development restrictions that existed before the advent of special permeable paving surfaces.
"It has been difficult to express to the property owners that we're not really changing the code; we're trying to restore it," Holman said.
Palo Alto restricts the size of homes in the city's open-space zones by limiting the amount of "impervious coverage": the house, driveways, paths, decks and garage all count as square footage that does not allow water to pass through to the ground.
Property owners in the open space may only take up 3.5 percent of their land with impervious surfaces -- which Cartmell called "very restrictive."
However, new permeable paving materials for walkways and driveways have allowed property owners an unintended privilege, Holman said.
Since these permeable materials are exempt from the impervious square footage tally, residents can then construct larger homes with the square footage they save on driveways.
"It's kind of contrary to the overall intent of the open space zone to be transferring that square footage into houses," Assistant Planning Director Curtis Williams said.
Holman said a new zoning code is needed to "at least put a plug in the hole in the dyke" and prevent extremely large, visible houses from being built in the open space.
But residents in the open space argue that these zoning changes are unreasonable and that the city did not consult them first.
"They're applying a broad brush to a lot of properties without studying what the impact is," said Mark Conroe, who owns 3.6 acres of undeveloped land off of Los Trancos Road.
He has been working on plans to build a house there with a footprint of 3,500 square-feet, not counting a driveway, decks and walkways.
"It's not even a mega house. Normal out there is 5,000 square feet," he said.
But Conroe, who lives in San Francisco, said the city's proposed changes to open space regulations would "effectively render our family's land unbuildable."
"We couldn't build a shack out there if they count the driveway as impervious coverage," he said.
Further limiting development in the open space would affect property values, added Cartmell, who lives on Page Mill Road.
"It's an attempt to down-zone the area without actually calling it that," she said.
The open space district includes 4,258 acres, about a third of which is privately owned, according to a staff report. Of 85 private parcels of land in the open space, 33 have yet to be developed.
Since permeable paving materials became more popular, Holman said, "In the last three or so years, we've seen it impact the development in the foothills."
Brian Schmidt, legislative advocate for the Committee for Green Foothills, said limiting house sizes and ground coverage -- whether permeable or impervious -- helps the environment.
"If you previously had open space with biological value and you replace it with a driveway, that is a change," Schmidt said.
Any development in the open space, he continued, "tends to have disproportionate impacts on the environment, especially the size of homes that are built up there, which are often gigantic."
After Cartmell alerted her neighbors in the open space to proposed zoning changes, they sent e-mails to the city protesting the proposal.
The negative response from property owners pushed the planning commission to find a "temporary fix until they can address other issues of house size and overall building on the site," Williams said.
The Palo Alto City Council will review changes to open space development regulations on July 16.
Williams agreed to convene a group of planning commissioners, open space property owners and environmental advocates to discuss open space zoning regulations within a 90-day period after the council's July 16 vote.
The commission will then explore alternatives to limiting impervious ground coverage, such as setting a maximum house size or a floor-area ratio.
Page Mill Road resident Daniel Dulitz hopes that the commission will avoid restricting home floor area and focus more on visibility of each home from beyond the property lines.
"If you can't see my house -- and if its impact on runoff, drainage, foliage and wild animals is small -- why do you care how much floor area I have? Even now, we are allowed far less floor area per acre than anywhere else in the city," Dulitz wrote in an e-mail.