An overpriced, tiny guest house that may potentially double the use of the most inefficient housing format there is, is at best a tiny drop in the ocean.

Density is realized not by charitably lending out a few square feet in your hoarded greenspace at a premium cost. Density is realized by consolidating greenspace, building massively vertically, and rendering more sprawl back into the land pool for reassignment as housing and shared greenspace if close into urban centers, agri land otherwise. Over and over.

The age of hoarded greenspace is over. We need to tax that for the luxury it is.