The next proposal, "low rise" apartment block:
> Let's assume a 6000sf building footprint. This would contain twenty 250sf units per floor, with some shared space. Four stories means eighty units. With 80 units, we have 80 x $10,000=$800,000 budget for land. Our 6000sf building footprint sits on a quarter-acre (10,500sf) of land, so the land costs could be as much as $800,000 X 4=$3.2 million per acre.
And next, the "medium-rise" plan:
> Next, we have a six-story structure, which is about the upper end of the Traditional City or "low-rise" format. We will step up here to a better standard of construction, such as brick or steel-frame, with a cost of $200/sf. This reduces our unit size to a fairly tiny 120 square feet, plus some common areas. Our footprint is still 6000sf, but since the unit size is smaller, we can get 40 units per floor, or 240 units for a six-story building. At $10,000/unit for land, that gives us a budget of $2.4 million for land. We will use more of an "attached" format here, with a little space in the back, or a 7000sf (sixth of an acre) plot. This translates into $2.4m x 6=$14.4 million per acre land and other costs.
The author is saying something about the density required to support affordable housing in San Francisco.