The principal requirement of the Growth Management Act is that cities and counties must prepare 20-year plans (see Seattle's Comprehensive Plan) to accommodate the population forecasts provided by the state Office of Financial Management (OFM). OFM first provided population forecasts for planning purposes in 1992. OFM provided new forecasts in 2002 that extend an additional 10 years into the future.

The new OFM forecasts showed that the state is expecting a much slower rate of growth in King County for the next 20 years than the forecasts prepared in the 1990s. The 2002 forecasts suggest that the county's population will grow only half as fast as projected previously.

The group recommended a method for distributing population growth that recognized the importance of locating employment and housing near each other. The method also recognized the differences in the types of households found in different regions of the county.

The first step in the method divided the urban portions of the county into four regions:

  • Seashore - where Seattle is located, also includes the cities and unincorporated portions of King County between Lake Washington and Puget Sound.
  • South County -cities and unincorporated portions of King County south of Seashore, all the way to the county line.
  • East County - cities and unincorporated portions of King County within the urban growth area, east of Lake Washington and north of Renton.
  • Rural Cities - six cities that are islands within the rural area.

These areas are shown on King County's Urban Sub-areas Map (file in PDF format).

The group then determined the proportion of future employment that the Puget Sound Regional Council (PSRC) has forecast for each of these regions and assigned the same proportion of the population forecast to each region. For instance, the PSRC forecast expects the Seashore region to take 33 percent of the county's new jobs over the next 20 years, and so was assigned 33 percent of the OFM population forecast for the county.

Because jurisdictions regulate growth by housing units and because it is easier to monitor growth by counting units than people (between Census years), the group converted the population forecasts for each region to housing units by using estimates of future household sizes. Different household sizes were used for each of the four regions, based on data observed in the 2000 U.S. Census, which showed two significant facts about household size:

  1. the average household size in the county remained essentially the same as it was in 1990, defying a decades-old trend of continuous decreases;
  2. average household sizes were quite different in the four different regions - smallest in Seashore and largest in the rural cities.

Finally, planning directors from each region met to allocate the households to each jurisdiction within the region, using factors such as the location of urban centers and other employment locations and the presence of existing zoning capacity.

In September 2002 the GMPC approved the household growth targets and employment targets developed by the staff group. These targets represent the amount of growth each city must be able to accommodate by the year 2022. This means that cities must have zoning and at least plans for infrastructure (roads, water, sewers, etc.) that would allow the targeted growth to occur. The adopted targets were incorporated into the ten-year comprehensive plan updates that all the jurisdictions were required to perform by December 2004.