Howell Mountain Vineyards: The 1,400-Foot Line, Hillside Rules, Wells and Fire

By Matthew Martinez, Founder, CEO & Managing Broker, Diamond Real Estate Group. California DRE #01869679.

Howell Mountain sits on the eastern rim of the Napa Valley above St. Helena, and its appellation is defined almost entirely by elevation: land counts only if it lies above the 1,400-foot contour. That makes a Howell Mountain vineyard a mountain property first. Owners deal with Napa County’s hillside conservation rules, wells drilled into upland rock rather than the valley aquifer, forested parcels, and real wildfire exposure, all in addition to the usual questions about vines and grape contracts.

Howell Mountain at a glance

Where the appellation runs

The federal rule for Howell Mountain, 27 CFR 9.94, is unusually spare. It does not trace roads or creeks. It starts where the 1,400-foot contour meets a line between survey sections on the Detert Reservoir quadrangle, follows that contour east and then southeast and back around to the northwest, and closes along a section line. In practice, the boundary is a contour ring around the top of the mountain, which takes in the community of Angwin.

That simplicity is useful for buyers. Whether a parcel is "in" Howell Mountain is mostly a question of where the 1,400-foot line falls on it, which a surveyor can plot. A parcel split by the contour can have one block inside the appellation and another outside, and the outside block can carry only the broader Napa Valley name. Ask for a map that shows the contour against the actual vine rows, not just against the parcel line.

Howell Mountain was established on December 30, 1983, under T.D. ATF-163, and amended in 1987, which makes it one of the earliest sub-appellations inside the Napa Valley AVA. The Napa Valley rule itself dates to 1981.

The newest neighbor arrived in 2024. When TTB established the Crystal Springs of Napa Valley AVA, it set that area’s northern edge on the same 1,400-foot contour that forms Howell Mountain’s southern edge. The two share a boundary and do not overlap. Below the mountain to the west lies the valley-floor St. Helena AVA, whose 1995 rule names Howell Mountain as its eastern neighbor. For the wider picture, see our Napa Valley appellation overview and our note on how Howell Mountain sits above the fog.

Hillside rules: erosion plans, slopes and canopy

Almost every new planting or replant on Howell Mountain will run into Napa County’s Conservation Regulations, Chapter 18.108 of the County Code. The County’s erosion control plan page says an agricultural erosion control plan is required for grading and earthmoving on slopes over 5 percent, and that replanting an existing vineyard on slopes above 5 percent also needs one. Approval is subject to CEQA review.

The County’s compliance guide for the Conservation Regulations adds the limits that matter most on a mountain:

• Earth disturbance on slopes of 30 to 50 percent requires a Use Permit Exception.
• Work inside stream, wetland or reservoir setbacks also requires an exception, and new stream crossings will likely need a CDFW streambed alteration permit.
• In the AW zoning district, 70 percent of the coniferous forest and oak woodland canopy that existed on June 16, 2016 must be kept, and preserved areas need a perpetual deed restriction.
• In the County’s sensitive domestic water supply drainages, the rule is 70 percent of tree canopy as of June 16, 1993 and 40 percent of shrub and herbaceous cover. The listed reservoirs include Lake Hennessey and Bell Canyon Reservoir.

Because Howell Mountain is wooded and steep in places, the canopy rule often decides how many plantable acres a parcel really has. A listing that describes "plantable acres" should be read against these rules, a slope map and a biological survey. Our vineyard development guide and environmental constraints guide go further.

Water on the mountain

Howell Mountain wells do not draw from the main valley aquifer. Napa County’s groundwater FAQ explains that the hillsides lie outside the state-defined Napa Valley Subbasin, though runoff and subsurface flow from them feed the subbasin’s water budget. The County’s baseline groundwater report treats Angwin as its own planning subarea, separate from the valley floor, and notes that these County subareas are not the same thing as state-defined basins.

For an owner, that means the subbasin’s Groundwater Sustainability Plan, which DWR approved on January 26, 2023, is not the main frame for a well on top of the mountain. The practical questions are well yield and history, storage, and whether a new or expanded use triggers a County water availability analysis. The County reports that updates to its groundwater ordinance and water availability analysis are in progress, so confirm the current standard before you rely on an old study. Many hillside vineyards also depend on ponds; see vineyard ponds and reservoirs, vineyard water rights and our Angwin rural water and wastewater guide.

Zoning, parcel size and Angwin

Most Howell Mountain land outside Angwin’s core is rural, and the General Plan sets its limits. The County’s Agricultural Preservation and Land Use Element sets a 160-acre minimum parcel size for land designated Agriculture, Watershed and Open Space (AWOS) and 40 acres for Agricultural Resource, with narrow exceptions. Policies that come from Measure J (1990) and Measure P (2008) cannot be amended or deleted without another vote until after December 31, 2058. A buyer hoping to split a mountain parcel should assume that path is closed unless a land-use attorney says otherwise.

The same element treats Angwin as a rural community with an "urbanized" area that is zoned Planned Development and meant mainly for institutional uses, and it asks the County to keep Angwin’s rural setting. Parcels near the village can face different rules from the surrounding vineyard land, so check the zoning map, not the mailing address. See Napa Ag Preserve and AW zoning, building a home on Napa ag land and Angwin parcel jurisdiction and services.

A winery is a separate permit question. According to the 2014–15 Napa County Civil Grand Jury report, the Winery Definition Ordinance set a 10-acre minimum parcel for new wineries and requires wineries approved or expanded after January 1990 to source at least 75 percent of their grapes from Napa County. Treat that as a summary and confirm current code before you plan; our Winery Definition Ordinance guide and winery use permit guide explain the process.

Napa County also runs a Williamson Act program with 40-acre (non-prime) and 10-acre (prime) minimums and 10-year contracts that renew each year. Ask whether a parcel is under contract; see our Williamson Act guide.

Fire, access and insurance

Fire is the defining risk on this ridge. CAL FIRE records that the Glass Fire began on September 27, 2020 near Crystal Springs Road, just below the mountain, and burned 67,484 acres in Napa and Sonoma counties, destroying 1,528 structures. The State Fire Marshal’s hazard maps for State Responsibility Area land took effect April 1, 2024, and Napa County is working through the 2025 maps for local responsibility areas. Look up the parcel’s zone, document defensible space, and get insurance quotes early. Long private roads and single access routes also matter for both fire and lenders; see private roads and site access in Angwin, wildfire smoke taint and vineyard and winery insurance.

A due diligence list for a Howell Mountain vineyard

Start with our full vineyard due diligence guide, then add these mountain-specific checks:

• Plot the 1,400-foot contour against each block to confirm which vines can carry the Howell Mountain name.
• Pull the County file for every erosion control plan on the parcel, including conditions and monitoring duties.
• Map slopes in 30 percent and 50 percent bands before counting future planting acres.
• Measure existing canopy against the 2016 AW baseline (or the 1993 baseline if the land drains to a listed reservoir).
• Confirm whether the parcel drains to Lake Hennessey, Bell Canyon Reservoir or another listed drainage.
• Get well logs, recent yield tests and water quality; check whether any planned use needs a water availability analysis.
• Inspect ponds, dams and diversion paperwork.
• Confirm General Plan designation and zoning (AWOS/AW, AR/AP or Angwin Planned Development).
• If a winery is planned or exists, review the use permit, production limit and visitation terms.
• Check the fire hazard zone, road access, water storage for fire and insurance availability.
• Review grape contracts, vine age, rootstock, clone and virus status.

Questions people ask

Sources & notes

1. eCFR: 27 CFR 9.94, Howell Mountain
2. eCFR: 27 CFR 9.23, Napa Valley
3. Federal Register: Establishment of the Crystal Springs of Napa Valley Viticultural Area; Modification of the Calistoga Viticultural Area (2024)
4. Federal Register: The St. Helena Viticultural Area (1995)
5. Napa County: Erosion Control Plan Review and Approval Process
6. Napa County: Compliance with Conservation Regulations
7. Napa County: General Plan, Agricultural Preservation and Land Use Element
8. Napa County: Groundwater FAQs and Groundwater Sustainability
9. Napa County: Baseline Data Report, Chapter 17, Groundwater Hydrology and WY2025 Annual Report Executive Summary
10. Napa County: Williamson Act and Agricultural Preserve Contracts
11. Napa County Civil Grand Jury: 2014–15 report on Napa County wineries (summary of the WDO; confirm against current code)
12. CAL FIRE: Glass Fire incident page
13. Office of the State Fire Marshal: Fire Hazard Severity Zones; Napa County: 2025 Fire Hazard Severity Zone Maps

The current CFR text does not state an acreage for Howell Mountain, so this page does not give one. General information, not legal or tax advice. Last reviewed November 2026 by Matthew Martinez.

Related: Napa Valley vineyards · Angwin real estate guide · St. Helena vineyards · Calistoga vineyards · Vineyard due diligence · Vineyard water rights · All vineyard and winery guides