Napa County has maintained a rural agricultural environment in a large part of the valley floor while neighboring Sonoma, Solano, and Yolo counties have allowed large tracts of former farmland to be rezoned for commercial and residential development.
In 1968 vintners and civic leaders in the county seized an opportunity to preserve farmland by taking advantage of the Williamson Act enacted by the California Legislature to give landowners property tax relief for designating their land for agricultural purposes.
This agricultural preserve on the floor of the valley in unincorporated areas between Napa and Calistoga was the first of its kind in the state.
 Initially, the preserve encompassed 23,000 acres (93.1 km²),but it has grown to more than 30,000 acres (121.4 km²).
In 2010, legislation was passed by the California State Senate and State Assembly and sent to the Governor for signing in the form of Senate Bill 1142.
This bill was created to provide relief stream of funding to augment the Williamson Act.

The county has resisted encroachment on the preserve since it was created with voters reaffirming their desire to keep it intact on several occasions.
In 1990 voters passed Measure J  adopting an initiative freezing all county zoning changes until the year 2020 unless there is a ⅔ majority vote to adopt such changes.
 Measure J was reaffirmed by a 5–2 vote of the California Supreme Court in 1995 in the case of Devita v. County of Napa.

The Land Trust of Napa County was founded in 1976 by a group of local citizens with a mission to protect the natural diversity, scenic open space and agricultural vitality of the county.
The trust acquires conservation easements, facilitates land transfers to local, state and federal agencies along with accepting outright donations of land within and outside the boundary of the agricultural preserve.
The trust now covers over 50,000 acres (202.3 km²).

While establishment of the agricultural preserve and the land trust has slowed residential development in much of the county, residential growth within the incorporated cities has continued at a moderate pace.
Several substantial homes have been built on the hills surrounding the valley in areas not covered by the preserve or the land trust.
A large portion of the land south of the City of Napa remained undeveloped for many decades until the 1980s.
Several wine bottling facilities and wine storage warehouses now stand on what was once vacant land.
A number of light industries have also sprung up in this region as new business parks have been built.
The growth of American Canyon, Napa County's southernmost and newest city, incorporated in 1992, has prompted the establishment of several new retail outlets in the southern end of the county in recent years.
American Canyon has also established a green belt preserve of over 1,000 acres (4.0 km²) on the western and eastern sides of the city.

In November 2009, the Napa Valley Vintners Association, the Napa Farm Bureau, the Napa Valley Grapegrowers and an advocacy group called Preserving the Integrity of Napa's Agriculture completed a two-year study of Genetically Engineered Organisms (GEOs), and released a joint letter recommending no GEO usage in Napa County until the risks and benefits of GEOs are reevaluated and a "satisfactory" regulatory framework is put in place.
Napa County has maintained a rural agricultural environment in a large part of the valley floor while neighboring Sonoma, Solano, and Yolo counties have allowed large tracts of former farmland to be rezoned for commercial and residential development. In 1968 vintners and civic leaders in the county seized an opportunity to preserve farmland by taking advantage of the Williamson Act enacted by the California Legislature to give landowners property tax relief for designating their land for agricultural purposes. This agricultural preserve on the floor of the valley in unincorporated areas between Napa and Calistoga was the first of its kind in the state.  Initially, the preserve encompassed 23,000 acres (93.1 km²),but it has grown to more than 30,000 acres (121.4 km²). In 2010, legislation was passed by the California State Senate and State Assembly and sent to the Governor for signing in the form of Senate Bill 1142. This bill was created to provide relief stream of funding to augment the Williamson Act.
The county has resisted encroachment on the preserve since it was created with voters reaffirming their desire to keep it intact on several occasions. In 1990 voters passed Measure J  adopting an initiative freezing all county zoning changes until the year 2020 unless there is a ⅔ majority vote to adopt such changes.  Measure J was reaffirmed by a 5–2 vote of the California Supreme Court in 1995 in the case of Devita v. County of Napa.
The Land Trust of Napa County was founded in 1976 by a group of local citizens with a mission to protect the natural diversity, scenic open space and agricultural vitality of the county. The trust acquires conservation easements, facilitates land transfers to local, state and federal agencies along with accepting outright donations of land within and outside the boundary of the agricultural preserve. The trust now covers over 50,000 acres (202.3 km²).
While establishment of the agricultural preserve and the land trust has slowed residential development in much of the county, residential growth within the incorporated cities has continued at a moderate pace. Several substantial homes have been built on the hills surrounding the valley in areas not covered by the preserve or the land trust. A large portion of the land south of the City of Napa remained undeveloped for many decades until the 1980s. Several wine bottling facilities and wine storage warehouses now stand on what was once vacant land. A number of light industries have also sprung up in this region as new business parks have been built. The growth of American Canyon, Napa County's southernmost and newest city, incorporated in 1992, has prompted the establishment of several new retail outlets in the southern end of the county in recent years. American Canyon has also established a green belt preserve of over 1,000 acres (4.0 km²) on the western and eastern sides of the city.
In November 2009, the Napa Valley Vintners Association, the Napa Farm Bureau, the Napa Valley Grapegrowers and an advocacy group called Preserving the Integrity of Napa's Agriculture completed a two-year study of Genetically Engineered Organisms (GEOs), and released a joint letter recommending no GEO usage in Napa County until the risks and benefits of GEOs are reevaluated and a "satisfactory" regulatory framework is put in place.
Napa County, California