In West Marin, where vacation rentals have doubled in recent years and roughly two thirds of dwellings are second homes, new rules for short-term rentals will soon be on the books. 

After more than a year of exhaustive debate, Marin County’s polarizing proposal to regulate Airbnbs and other S.T.R.s gained unanimous approval from the California Coastal Commission last week. The vote allows the county just enough time to enact the rules before a moratorium…