They should apply to build a huge 5 story apartment complex on that lot, see how the community likes that. The county would have to approve that development according to that decision..
George Lucas once attempted to build low income housing on Grady Ranch after his studio expansion was successfully challenged by his neighbors, also in Marin County. They didn't like that plan very much[1].
That wouldn't work, but if this parcel is zoned for single-family homes they could subdivide it and build a duplex plus an ADU on each resulting parcel, without the interference of the local authorities, because of SB9.
That definition is the core problem the court is contorting itself to address in the way that it likes. Development has quite a few plain-English definitions. One of them seems plenty straightforward and applicable: A project consisting of one or more commercial or residential buildings.
Well so that's the problem, right? The legislature never defined it. So we're left to argue amongst ourselves and use dictionaries, leading to cherry picks. The did, however, state that the intent of the law was to promote housing regardless of economic segment.
[1] https://www.e-flux.com/architecture/housing/333702/standoff-...