To: Board of Supervisors
From: John Kopchik, Director, Conservation and Development
Report Title: APPROVE and AUTHORIZE the Chair of the Board of Supervisors to execute a Historical Preservation Agreement (Mills Act Contract) for 1850 Alameda Diablo in the Diablo area.
☒Recommendation of the County Administrator ☐ Recommendation of Board Committee
RECOMMENDATIONS:
APPROVE and AUTHORIZE the Chair to execute a Mills Act Contract for 1850 Alameda Diablo, Diablo.
DIRECT the Department of Conservation and Development to record the Mills Act Contract with the County Clerk-Recorder, and to forward copies to the California Office of Historic Preservation and the County Assessor.
DETERMINE the proposed action is categorically exempt from the California Environmental Quality Act (CEQA Regs, Section 15331, Class 31).
DIRECT the Department of Conservation and Development to file a Notice of Exemption under CEQA with the County Clerk-Recorder.
FISCAL IMPACT:
The applicant is responsible for application and processing costs for the preparation of the Mills Act Contract. Approval of the Mills Act Contract for 1850 Alameda Diablo will result in a reduction of property taxes for the subject property based on the calculations used by the State Board of Equalization for use of assessing properties under the Mills Act.
BACKGROUND:
Mills Act Contract: The Mills Act Program (Government Code, Sections 50280-50290) is a state law that grants participating local governments authority to enter into contracts with owners of qualified historic properties who actively participate in the restoration, rehabilitation, and maintenance of their historic properties while receiving property tax relief. The law enables the County to enter into 10-year agreements with owners of historic properties, which are automatically renewed each year for an additional year, unless the County or owner files a notice of non-renewal. The Mills Act Program can be utilized with historic properties listed in any state, city, or county official register of historical or architecturally significant sites, places, or landmarks. Property taxes are recalculated using a formula in the Mills Act and Revenue and Taxation Code. The Mills Act requires that historic property owners spend the property tax money that is saved through the Mills Act on preserving and/or restoring their property.
Only a “qualified historical property” may enter into a Mills Act contract. The Mills Act defines a “qualified historical property” as privately owned property that is listed in the National Register of Historic Places, an official California register (the California Register of Historical Resources, the California Historical Landmarks, and the California Points of Historical Interest), or an official local register like the County’s Historic Resources Inventory.
1850 Alameda Diablo, Diablo: The County Board of Supervisors added the Diablo Historic District to the County Historic Resources Inventory on December 3, 2002. The subject property is a recognized contributor property to the Diablo Historic District and thus is a “qualified historical property”.
The property has also been evaluated for sustainability for inclusion on the California Register of Historical Resources (CR). Eligibility criteria include the following: Criterion 1: It is associated with events that have made a significant contribution to the broad patterns of local or regional history, or the cultural heritage of California or the United States; Criterion 2: It is associated with the lives of persons important to local, California, or national history; Criterion 3: It embodies the distinctive characteristics of a type, period, region, or method or construction, or represents the work of a master, or possesses high artistic values; and/or Criterion 4: It has yielded, or has the potential to yield, information important to the prehistory or history of the local area, California, or the nation. In addition to meeting one of the CR criteria, an Integrity Analysis using the seven aspects of integrity (location, design, setting, materials, workmanship, feeling, and association) as established by the National Parks Service must be included to show the resource(s) have maintained their historic integrity. Lastly, a property must present a 10-year plan of proposed preservation, maintenance and/or improvements to the building(s) and/or structure(s) to protect or enhance its/their historical value and integrity. These proposed improvements must also show compliancy with the U.S Secretary of Interior’s Standards for Rehabilitation.
As part of the application submittal, the property owners commissioned a Historic Resource Evaluation report dated July 22, 2025, prepared by Preservation Architect Mark Hulbert of the firm Preservation Architecture. According to the report, in the context of community planning and development, the primary residence, its deck, and the bridge that goes over the creek for 1850 Alameda Diablo, strongly relate, as this residential property is one of fifty-two residential properties that contribute to the Diablo Historic District. Moreover, this property and house is prominently sited adjacent to the central club house and on former club house land. Given such direct associations to the origins of the historic community of Diablo, the report concludes 1850 Alameda Diablo is historically significant under CR criterion 1.
Further explained in the report, the property’s resources - the primary residence, its deck, and bridge - will have their historic integrity maintained through the proposed 10-year rehabilitation plan. The proposed rehab improvements are necessary to preserve the resources, and will transpire over a 10-year span, resulting in an estimated cost that should equate to any savings in property taxes assessed under Mills Act. The proposed rehabilitation work relative to the U.S. Secretary of the Interior’s Standards for the Treatment of Historic Properties (Standards) and, specifically, the Standards for Rehabilitation, retains all identified characteristic forms and individual features of the house and its property. The proposed work respects the overall form and individual characteristics of the historic house and its associated deck and bridge. The rehabilitation program proposes to repair identified deteriorated, exterior features and materials and to selectively replace, as required, to match existing features and materials.
Historical Landmarks Advisory Committee (HLAC): At the HLAC meeting held on May 8, 2025, the HLAC reviewed this application and supplemental materials and reports, and had no objection to 1850 Alameda Diablo, Diablo receiving approval of a Mills Act Contract.
Conclusion: Approval of a Mills Act Contract for 1850 Alameda Diablo, Diablo would secure the restoration, rehabilitation, and maintenance of the historical and architectural integrity to the property’s historical building and structures - primary residence, its deck, and the bridge, by current and future owners of the property.
CONSEQUENCE OF NEGATIVE ACTION:
If the subject property, 1850 Alameda Diablo in Diablo is not approved for a Mills Act Contract the restoration, rehabilitation, and maintenance of the property’s historical integrity for the residence, deck, and bridge would not be monitored and may not be maintained by current or future owners of the property.