A proposed home construction project on the last undeveloped seafront lot on Stinson Beach has raised challenging questions about how the county should manage coastal development in the age of climate change and sea-level rise.

The Planning Commission approved the project on Aug. 28, but neighbors have appealed the decision to the Board of Supervisors, arguing that it violates the California Coastal Act and Marin’s coastal development regulations, which prohibit building in environmentally sensitive habitat areas.

Opponents of the project argue that it makes no sense to build in a flood zone where the storms that batter the shore are expected to become more frequent and powerful in coming years. Last winter’s storms flooded the neighborhood, inundated septic systems and damaged several homes. 

County staff agree that the project would violate the county’s coastal regulations, but they are encouraging the supervisors to approve it anyway to avoid a government taking of private property. The United States Constitution requires governments to compensate property owners when their regulations prevent them from realizing the economic value of their land. 

The parcel in question, at 21 Calle del Onda, was recently appraised at $3.5 million.

When property takings are at issue, the county conducts an analysis to determine the minimum amount of development required to avoid depriving a property owner of realizing their reasonable economic expectations. In this case, planning commissioners approved the project only after the owners agreed to trim the size of their proposed home from 1,500 square feet to 1,300 square feet, eliminate a proposed 288-square-foot garage and limit the house to one story instead of two.

The property is owned by Lake Tahoe resident Brian Johnson and several of his relatives. They have been fighting for years to obtain the necessary approvals and have spent more than $350,000 on permits and environmental assessments.

In addition to agreeing to a smaller footprint, the Johnsons agreed to set the home 100 feet behind the shorefront property line and construct a 4-foot retaining wall around the septic tank.

They also agreed to remove the building if it is destroyed in a fire or storm. They would not be permitted to rebuild. 

Opponents of the project point out that Marin’s coastal regulations prohibit development on coastal dunes, which are considered environmentally sensitive habitat areas. And last year’s series of atmospheric rivers, which caused flooding in Stinson Beach, underscore the increasing dangers of climate change, they argue.

“The reality is, this lot is in a hazard zone,” said Scott Tye, vice chairman of the Marin chapter of the Surfrider Foundation, a conservation group. “And it’s going to get worse; it’s not going to get better. We’re not going to have beautiful Mexican beachfront weather. We’re going to have extreme weather. It’s not rocket science.”

But Steve Kinsey, a consultant to the Johnsons, said allowing them to build the home is a simple matter of fairness. Their property is no different from all the others along the Stinson Beach shore.

“Why does this last 80 feet of frontage warrant being treated differently?” he asked.

Mr. Kinsey said the property does not meet the statutory definition of an environmentally sensitive area because the soil on the property has already been disturbed. It was previously the site of a 540-square-foot cottage purchased by Mr. Johnson’s grandparents.

The original house burned down in 1983, but the non-native plants the Johnsons planted still survive, along with the cottage’s chimney.

“This is really an environmentally sensitive habitat area in name only,” Mr. Kinsey said, adding that the Johnsons agreed to restore dunes on the parts of the 15,200-square-foot property unaffected by construction.

The Planning Commission voted 3-2 to approve the project. Don Dickenson, a commissioner who voted against it, said he believed the project should undergo a more comprehensive environmental assessment that would require considering alternatives to the plan proposed by the Johnsons. 

“There may be more houses that get destroyed in storms,” he said. “Are we going to just let them rebuild? Or do we consider an alternative of planned retreat?”

The neighbors who filed the appeal to the Board of Supervisors include the late Robert Friedman, who owned a home at 17 Calle del Onda, and Marisa Atamian-Sarafian and Stephen Sarafian, who own a home at 24 Calle del Sierra.

The supervisors had planned to consider the case last week, but they postponed the hearing after Mr. Friedman passed away. They now plan to take the matter up next Tuesday morning.

If the supervisors approve the project, the Sarafians intend to appeal to the California Coastal Commission, according to their attorney, Elizabeth Brekhus.

“The neighbors have come in to prevent the Johnsons from having what they have on their own properties,” Mr. Kinsey said. “The hypocrisy rolls like the tide out there.”