Marin County’s strategy for affordable housing hinges on encouraging homeowners to build accessory dwelling units. But in West Marin, where most homes rely on septic systems, the necessary sewage upgrades can be cost prohibitive. Now, the county’s Community Development Agency is taking steps to ease that burden. A new pilot program will provide a small number of homeowners who want to build ADUs in unincorporated parts of the county with wastewater engineer consultants who will…