Department of Community Development
Reference: Intergovernmental Agreement (IGA) between the City of Brighton and City of Thornton
To: Mayor Gregory Mills and Members of City Council
Through: Michael P. Martinez, City Manager
Prepared By: Kim Bauer, Associate Planner
Date Prepared: September 17, 2026
PURPOSE
The purpose of this study session is to provide City Council with an overview of the previous Intergovernmental Agreements (IGAs) between the City of Brighton and the City of Thornton regarding the management of growth boundaries, which are now expired. As such, staff has been working with the City of Thornton on drafting a new IGA. The following background section outlines the proposed changes and revisions to facilitate Council feedback and confirm that the proposed direction aligns with Council’s expectations.
BACKGROUND
Brighton and Thornton share a common growth area and both cities agreed to adopt an Intergovernmental Agreement (IGA) in 2001 to address shared growth issues and establish a growth boundary. That IGA was originally set to expire on January 1, 2010, but was extended three times, ultimately expiring on April 30, 2012. On August 21, 2012, City Council approved an updated IGA that expired on December 31, 2022.
The previous IGA therefore has lapsed, but both parties are interested and eager to re-establish the growth boundary IGA.
The previous IGA has served both communities well, however, staff has started to develop an updated IGA, working with the City of Thornton, that will reflect new goals and circumstances. The proposed IGA includes several revisions intended to simplify the document, update references, and reflect current planning conditions that have changed since the adoption of the 2012 agreement.
While the overall purpose of the agreement remains largely the same, the proposed revisions are intended to improve clarity and remove provisions that are no longer applicable. The key revisions are outlined below with additional context provided.
STAFF ANALYSIS
One of the primary changes is the transition from the former “Corridor A” and “Corridor B” designations to a single annexation boundary line. Under the previous IGA, the designated corridors identified some areas in which either city could pursue annexation, subject to written approval from the other city.
Rather than continuing to define and administer separate corridor areas and their associated annexation provisions, this proposed IGA establishes a single annexation boundary that both cities can use to clearly identify the area within which each city may annex property. Accordingly, the separate descriptions associated with Corridors A and B are proposed to be removed and replaced with one description defining the annexation boundary line. The removal of the corridor designations is intended to simplify the agreement and provide a more straightforward framework for each City to work within.
In “Corridor Area A”, which is generally north of E-470 between Quebec Street and Havana Street, the proposed annexation boundary is identified along Yosemite Street and generally aligns with both city’s future land use map boundaries and the distinctions within the maps, with one area of exception that is discussed in greater detail later in this report.
A second change is the removal of references to the Platte River Trail. The Platte River Trail section that was discussed in the 2012 IGA has since been completed. Therefore, those provisions are no longer necessary
“Corridor Area B” is generally east of Riverdale Road and west of the South Platte River. It is adjacent to Ken Mitchell Lakes and runs south, encompassing Riverdale Regional Park. The 2012 IGA had specific provisions governing “Corridor Area B”, including limitations on the types of development Thornton could pursue within the area and provisions allowing either city to provide utility service to the Regional Park if requested by Adams County. The 2012 IGA limited the type of development in this area to low density residential uses of no more than two dwelling units per acre.
The proposed revision recognizes that Brighton’s Future Land Use Map does not identify a specific future land use designation for the southern portion of the former corridor and instead generally depicts the area as Parks and Open Space, which is consistent with Thornton’s future land use considerations for the area. The Future Land Use Map in the draft Comprehensive Plan does not include Riverdale Regional Park, further reinforcing the 2012 IGA provision that allowed only Thornton to annex the park. Given the existing and proposed future land use designations, the proposed IGA removes the specific development restrictions previously included in the 2012 IGA.
As referenced to in an earlier paragraph, there is one significant deviation from the future growth boundaries established under the 2012 Intergovernmental Agreement (IGA) and warrants additional discussion. Under the 2012 IGA, the area west of the centerline of Yosemite Street was identified as an area where Thornton would exercise annexation authority and provide municipal services upon annexation, while the area east of the centerline of Yosemite Street was identified for Brighton to exercise annexation authority and provide services. Since adoption of the 2012 IGA, both communities have further refined their long-term planning boundaries. Thornton’s 2024 Future Land Use Map identifies the area west of Yosemite Street as Urban Reserve and extends its future land use boundary north along the Yosemite Street alignment, rather than jogging west at Baseline to follow Weld County Road 19 as the exhibits in the 2012 IGA illustrate. As a result, Thornton’s current future land use boundary extends into an area that is generally reflected within Brighton’s long-term planning area as well. In contrast, Brighton’s proposed 2026 Future Land Use Map identifies the area east of Weld County Road 19 as Agriculture and establishes the future land use boundary along Weld County Road 19, consistent with Brighton’s current 2016 Comprehensive Plan and the 2012 IGA.
An image is provided in Exhibit A to visualize this boundary area. Staff is particularly interested in receiving Council’s feedback regarding the proposed overlap of the boundary line and the preferred approach for addressing this item with the City of Thornton. Potential approaches include revising Brighton’s boundary to shift east and eliminate the overlap, continuing negotiations with Thornton, or maintaining the overlap but then incorporating specific language into the IGA establishing parameters for annexation within the overlapping area, including which city may annex and under what circumstances. Direction on this will help guide staff’s continued discussions with the City of Thornton.
RECOMMENDATION
Staff recommends that City Council provide feedback on the proposed Intergovernmental Agreement and provide direction to staff in order to guide preparation of the IGA and a draft resolution for future Council consideration.
ATTACHMENTS
• Draft IGA
• Exhibit A
• Copy of the 2012 IGA between the Cities of Brighton and Thornton
• Staff Presentation