A Muir Beach couple is suing the California Coastal Commission, alleging that the agency overstepped its bounds when it ordered them to scale back their planned seaside home. Graham Groneman and his wife, Brett Sibley, have been fighting for several years to build a house, a garage and a storage unit overlooking the Pacific Ocean, next door to Mr. Groneman’s parents. In February, the coastal commission said they could build—but only if they moved the house farther from the water, eliminated the garage and reduced the project’s footprint from 3,000 square feet to 2,000. The commission issued its ruling even though Marin County had already approved the project. The two agencies reached conflicting conclusions due to differing interpretations of Marin County’s Local Coastal Program, which establishes regulations to ensure that development is consistent with the California Coastal Act. The agencies disagreed about whether the house would have been located on a bluff face, which is prohibited, or a bluff top, which is not. They also disagreed about whether the project would require cutting into the bluff face to install retaining walls. The coastal commission instructed the couple to work with staff to try to resolve their differences, but they could not reach an agreement. “The coastal commission has redefined what constitutes a bluff without going through the proper process for doing so,” said Kasey Corbit, the couple’s attorney. “They used a definition that isn’t in the code.” Mr. Groneman is a firefighter and third-generation Muir Beach resident, and Ms. Sibley is a nurse. Their project enjoyed broad community support. Mr. Groneman said the commission had denied them due process. “We feel the only way to get a fair assessment of the project is to bring it into the courts,” he said. “The commission just seems to want to block coastal development projects, and they’ll look at any way to do that.” The couple filed their complaint at the Marin County Superior Court in April, but it wasn’t processed until Sept. 5 due to staffing shortages at the court clerk’s office, Mr. Groneman said. The coastal commission declined to comment on the case.