California will have to build about 3.5 million homes over the next eight years, more than triple its current pace of construction, simply to keep up with expected population growth and hold down housing costs to affordable levels. But how could the state actually do it?

That question is the subject of a new report by the McKinsey Global Institute that lays out a long list of policy ideas — including many that would be contentious and politically difficult — to help the state step up housing production. Those ideas include streamlining grants of local permits, and using tax policy to withhold money from anti-growth cities that drag their feet on new housing.

Vacant Lots in Los Angeles

Something clearly needs to happen. Already, the McKinsey report estimates, onerous housing costs rob California, with its current population of 39 million, of $140 billion in economic output a year, or about $3,500 per person.

For all the controversy surrounding land development policy, one idea stands out from the rest: Build 225,000 or so homes and apartments on plots of urban land that are currently sitting vacant. Most of those lots are in either San Francisco or Los Angeles. These Google maps provide a block-by-block view of where these units might be built and some of the challenges they could face.

First, a note of caution about the data, which is publicly available and came from city and county planning departments: Some of it could be wrong, either because of a data entry error or because the data is out of date. Also, a vacant lot can have a building that is unoccupied or even partly occupied.

If you click around the maps of San Francisco and Los Angeles County, one of the first things you notice is a lot of surface parking lots that could be used for housing instead. There are plots of city land that could theoretically be sold off and used for affordable housing or the like, and houses that are on double lots.

But building on these lots is easier said than done, and some plots have already been subject to controversial development plans.

Vacant Lots in San Francisco

In San Francisco, for example, there is a triangular lot with parking and tennis courts near Washington Street and the Embarcadero.

That was the site for the highly controversial 8 Washington development, a proposal for a towering condominium that set off a voter referendum that handicapped the project and led to its demise this year.

And just because vacant lots exist, that does not mean the owners want to use them or a particular city would allow new construction. Some are small and oddly shaped. Others have open fields or school playgrounds, which many local residents want to preserve.