In the face of strong concerns from activists, neighbors and Fish and Wildlife officials, the Marin County Planning Commission on Monday unanimously upheld plans for a new three-story home on a lot prone to landslides above Woodacre Creek. 

The approval followed hints of a lawsuit from the Salmon Protection and Watershed Network, which said the county should complete a court-ordered cumulative environmental impact study on development in the San Geronimo Valley before green-lighting the home; the group also called for more analysis on the project’s chances of exacerbating erosion.

Filed by Marin real estate agent Jim Murray back in 2004, plans for the proposed 1,899-square-foot home and 462-square-foot garage were put on ice for over a decade after the county found severe cracks in Redwood Drive, which fronts the property, in 2006. The project was further stalled in 2010, when an 80-foot-wide landslide caused by winter rains forced the road’s closure; the county repaired the slide in 2012.

The project hit another snag that year, when a superior court judge issued a moratorium on building in the valley until the county drafted an ordinance on streamside development. A state appellate court lifted the moratorium in 2014, but ordered the county to produce a cumulative-impacts study that would establish mitigation measures to reduce negative impacts. (SPAWN has sued the county three times since 2005 over creekside development in the valley.)

At a hearing on Monday, a spokesman for SPAWN implored the commission to halt the project until the county completed its impact study—or risk possible litigation. “We believe this plan could lead to unnecessary litigation,” said Preston Brown, a watershed biologist for SPAWN. “We believe the county should follow the law and complete the cumulative impact review.”

Conceding that his proposed home lay in a landslide trouble spot, Mr. Murray on Monday said two geotechnical studies—one from a county-hired firm, the other from a firm a neighbor, Eric Morey, hired—showed that the project would not trigger another landslide. He said he felt eager to move forward after years of costly studies and plans.

“We have studied and re-studied the property’s stability,” said Mr. Murray, who put up story poles last week. “I still have the intention to build and live in this home, to make it my residence.”

But neighbors at Monday’s commission hearing feared the construction could erode the slope and damage the creek. They pleaded with the county to focus on shoring up the slope beneath the proposed house.

“We’re very frightened about this cataclysm that’s about to happen,” said William Rock, a 25-year Woodacre resident. “We can’t quite get any help from anybody, and it’s going to be quite a catastrophe. Please, help us.”

Staff with SPAWN and the California Department of Fish and Wildlife pointed out the project would enlarge the valley’s heaviest section of impervious surfaces, which increase runoff and erosion in the creek. (Mr. Preston referenced data on impervious surfaces from a salmon enhancement plan drafted by the county in 2010.) Both groups requested more geological review of the property.

“I really hope that you will do more investigation, not only for the salmon but also for Mr. Murray and for the neighbors who may be affected,” said Gail Seymour, a senior environmental scientist and supervisor of restoration with Fish and Wildlife.

Some commissioners took issue with the differing geotechnical analyses of the slope. Commissioner Wade Holland, of Inverness, expressed frustration that the consultant hired by Mr. Morey found the county’s road work had contributed to the slope instability, while a county-hired consultant reported that geological conditions in 2006—prior to the road repair—were the same as they are today. 

“I don’t know whose side to believe,” Commissioner Holland said. “I’ve got two reputable engineers telling me different stories.”

County engineers, however, expressed confidence that the road work done in 2012 will hold. Ernest Klock, the Department of Public Work’s principal civil engineer, noted that the county in 2012 buttressed the road’s foundations into the bedrock and that Mr. Murray’s house might even help shore up the weak slope. 

“We have fixed the stability of that roadway,” he said. “I am confident that it will not slip further.”

Ultimately, commissioners said that questioning the engineering reports was beyond their regulatory duties. They unanimously approved staff’s recommendations with conditions, including the submittal of a stormwater runoff pollution control plan. 

“We somehow pretend that we’re licensed engineers, and we’re not,” Commissioner Katherine Crecelius said. “I don’t think we have any choice but to trust what staff is telling us.”