President Biden wants to get local governments to unlock their strict zoning laws. Minneapolis scraps strict single family zoning. The California Assembly repeatedly tries to force local governments to allow transit oriented development.
All good things to relieve the underbuilding of housing in the country, but all are aimed just one part of the housing market: apartments and condominiums. The problem is, most Americans don’t live in multi-family housing, most don’t want to, and the supply of single family houses is not keeping up with demand in large parts of the country.
And in the Seattle area, where single family housing prices are marching upward in double-digit territory again, apartment construction has been furious. We have an affordable apartment problem, but we do not have an overall apartment supply problem. We do have a single family supply problem, but it does not get discussed much. Housing policy solutions always seem to steer back to multi-family.
Figure 1 shows the occupied housing stock.

Combining single family detached and its close cousins, townhouses and duplexes, about 70 percent of the nation and state live in these housing forms, with 30 percent in all other forms. Within the 3-county region, the picture is telling. In King County, 60 percent of households live in single family/townhouse settings, while over 70 percent do in Pierce and Snohomish. King County has emphasized apartment construction, and the adjacent counties have absorbed demand for detached housing. Also of note is the share of King County residents in the largest multi-family complexes.
Figure 2 shows the propensity of various types of households to live in single family housing (in figures 2, 3 and 4, single family includes townhouses, but not duplexes).

Among married couple households of all ages, with and without children, over 80 percent live in single family homes in the region, with slightly less than 80 percent in King County. Single parents are more likely to live in apartments. Non-family households, which consist of either single person households or groups of unrelated people, are also much more likely to live in apartments.
Figure 3 shows the rate of single family living by age group.

This seems to cast doubt on the notion that large numbers of seniors would give up their single family homes to live in convenient, exciting urban centers (the data do not include people living in institutional settings such as nursing homes). In the 3-county region, 67 percent of those over 65 still live in single family homes, and that only dips to 64 percent in King County.
Figure 4 shows the propensity of the smallest households to live in single family housing.

There are a lot of empty bedrooms out there. 40 percent of one-person households live in single family housing, and two-person households live in single family homes at almost the same rate as the population as a whole.
This picture has not changed much over time. In the past decade, apartment living has increased quite a bit as a share of the total in King County, but actually decreased in the two adjacent counties. The slight growth in the apartment share region-wide is likely tied to the rapid growth in the population of younger people who, as seen in Figure 3, tend to live in apartments.
Looking Ahead
As we have noted, construction of single family homes in the region is not keeping up with population growth. Yet, those priced out of single family housing in the areas they would like to live in will not tend to settle for multi-family living. Multi-family is seen as a poor substitute for single family, so households will tend to substitute neighborhoods, instead, engaging in the “drive to qualify” pattern that has come to be the norm for middle income households. But drive-to-qualify might be endangered as prices are increasing rapidly in the more affordable parts of the region.
You have pointed to the problem. Compare the projected growth of this region over the next 20 years to the available space for housing. Where will they all go? Is there an intersection between demand and supply, or are we doomed to be Phoenix without the sunshine?
For tetra has done work on this.