Marin County covers about 520 square miles of land, and more than 800 square miles when the surrounding bay and ocean waters are included. That works out to about 504 residents per square mile, far less dense than San Francisco or the Peninsula.

Those numbers overstate how much of Marin is actually buildable. A large portion of the county is permanently protected:

Federal parkland: the Point Reyes National Seashore and the Golden Gate National Recreation Area.
State and water district land: Mount Tamalpais State Park and the Marin Municipal Water District watershed.
County land: county open space preserves.
Ranchland: much of West Marin is under agricultural easements and A-60 zoning, which requires a 60-acre minimum parcel size.

Development is concentrated in the eastern Highway 101 corridor. Combined with tight local zoning, this keeps housing supply fixed while demand stays high. It’s one of the main reasons Marin home prices sit among the highest in the country.