131,000 net acres of undeveloped land were urbanized between 1990 and 2020, representing a 20% increase in urban area
In the 2010s, Santa Clara County had the highest share of regional greenfield development, at 22% (5,200 out of 23,900 acres)
Since 2003, deurbanization efforts have returned 2,748 urban acres to wildland status
Introduction
Where is open space being converted to development?
From the days of the first Native American settlements, human activity in the Bay Area clustered around the San Francisco Bay. Over time, development of the region's 7,000 square miles of land spread inland from the Bay, and this trend accelerated markedly during the rapid economic expansion that followed World War II.
As this process of development claimed lands formerly reserved for other uses, Bay Area residents have come to regard undeveloped lands as a precious resource – and not only for their scenic value. Construction on undeveloped land – referred to as greenfield – decreases the area of natural habitat and agricultural land and increases the area covered by impervious surfaces. This can worsen flooding during rainstorms and reduce natural groundwater recharge. The loss of greenfield land across the Bay Area tends to correlate with an increase in the overall number of vehicle-miles driven, as people who settle on the newly developed lands often have to travel long distances to work.
Regional Performance
The pace of Bay Area greenfield development slowed by over half from the 2000s to the 2010s.
Between 2000 and 2010, the Bay Area’s urban footprint expanded by around 5,700 acres per year. In the following decade, that rate fell by 58%, to 2,400 acres per year. The latest data – between 2018 and 2020 – show 3,975 acres developed over two years (a 0.5% increase in total developed land), continuing recent trends of slower greenfield development.
Development across the Bay Area increased the size of the region’s developed footprint to 792,200 acres in 2020, making it 20% larger than 1990s’ footprint. Between 1990 and 2020, greenfield development was most rapid between 1994 and 2004, when approximately 73,100 acres were added to the region’s developed area. Nevertheless, the Bay Area has grown denser over time, with the rate of population growth outpacing the rate of greenfield development since 1990.
were developed between 1994 and 2004, the most of any 10-year period since 1990
were developed between 2010 and 2020, which was 18% of all greenfield development between 1990 and 2020
Historical Trend for Greenfield Development
Regional Distribution
Regional greenfield development since 2010 has been concentrated in Santa Clara, Alameda and Contra Costa counties.
These counties, along with Solano County, accounted for 73% of the Bay Area’s greenfield development between 2010 and 2020. Development in these three counties largely has consisted of nonagricultural land converting to housing or retail. Sonoma County significantly reduced its share of greenfield development in the 2010s, accounting for just 9% of the region’s greenfield development between 2010 and 2020 – down from 20% in the 1990s.
At the same time, returning land back to nature has become an important priority across the region. Restoration initiatives in Solano and Marin counties have led to the creation of the Mare Island Shoreline Heritage Preserve and Hamilton Wetlands, together contributing over 1,500 acres of rewilded land to the region.
22.9% of the region's greenfield development from 1990 to 2020 occurred in Contra Costa County, the most of any Bay Area county
Marin County accounted for just 4% of Bay Area greenfield development from 1990 to 2020, the least of any Bay Area county
Historical Trend for Greenfield Development by County
Local Focus
The Bay Area’s urban footprint grew in large part due to activity in eastern Contra Costa and southern Santa Clara counties.
Powered by development activity in Brentwood, Antioch and Oakley, a significant chunk of the region’s greenfield development from 1990 to 2020 occurred in eastern Contra Costa County. The 29,982 acres of greenfield development in the county represented 23% of all such development within the region during this period. Combined with similar activity around South San Jose, Gilroy and Morgan Hill, Contra Costa and Santa Clara counties together account for more than 40% of the region’s greenfield development in this 30-year timeframe.
Sonoma and Solano counties have accounted for the majority of greenfield development in the North Bay. In Sonoma County – where nearly 20,600 acres (16% of total regional growth) have been developed between 1990 and 2020 – nearly all development can be attributed to the conversion of small parcels in unincorporated areas along the Interstate 101 corridor. This contrasts with the development pattern in Solano County, where new, often larger exurban residential subdivisions are much more common.
Between 1990 and 2020, Contra Costa County's urbanized area increased by 25% or 29,982 acres, the largest absolute change of any Bay Area county
Between 1990 and 2020, Napa County's urbanized area increased by 46% or 7,619 acres, the largest percent change of any Bay Area county
Greenfield Development Areas
Sources & Methodology
For regional and local data, the Department of Conservation Farmland Mapping and Monitoring Program (FMMP) maps the extent of "urban and built-up" lands, which generally reflect the developed urban footprint of the region. The footprint is defined as land occupied by structures with building density of at least 1 unit to 1.5 acres. Uses include residential, industrial, commercial, construction, institutional, public administration, railroad and other transportation yards, cemeteries, airports, golf courses, sanitary landfills, sewage treatment, water control structures, and other developed purposes. Note that San Francisco is deliberately excluded from the county-level source datasets for all years. Also note that this dataset is derived from aerial imagery and thus does not perfectly capture ground-level conditions. For more information on how FMMP creates their datasets, see their Making Important Farmland Maps page.
To determine the amount of greenfield development (in acres) occurring in a given two-year period, the differences in urban footprint are computed on a county level. FMMP makes slight refinements to urban boundaries over time, so changes in urban footprint +/- 100 acres are not regionally significant.