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Environmental Review

Initial Study

Notice of Preparation

The Diamond Bar General Plan 2040 established a new vision for a pedestrian-oriented “Town Center” that provided housing opportunities, food-oriented retail, restaurants, and entertainment. The proposed Specific Plan would guide the development of the Diamond Bar Town Center, implementing the goals and policies related to reestablishing the Town Center as a mixed-use civic area promoting walkability through the planning horizon of the Diamond Bar General Plan 2040. The proposed Specific Plan involves the phased redevelopment of approximately 45 acres of existing suburban-style retail shopping centers to create a pedestrian-oriented “Town Center” that would provide housing opportunities, food-oriented retail, restaurants, and entertainment and cultural uses.


A General Plan Amendment and a Zoning Amendment would be required to increase the maximum density and floor area ratio (FAR) to:

  1. allow the Specific Plan’s proposed density,

  2. establish Objective Design Standards for development within the Planning Area, and

  3. rezone the Planning Area parcels for the Specific Plan’s proposed uses.


What is a Supplemental Environmental Impact Report (SIR)?


An SIR is an Environmental Impact Report (EIR) that evaluates potential new or substantially more adverse effects to the environment from minor changes to a previously certified (i.e., approved) EIR to make the previous EIR adequate for the project as revised. Pursuant to the California Environmental Quality Act (CEQA), an SIR shall be given the same kind of notice and public review is given to a Draft EIR, and the decision-making body (Lead Agency) shall consider the previous EIR as revised by the SIR when deciding whether to approve the project.


The proposed adoption and implementation of the Specific Plan constitutes a “project” for the purposes of CEQA. Thus, an Initial Study has been prepared to address the potential impacts associated with this project. This Initial Study and supporting environmental analysis will support the decision-making process to be undertaken by the City of Diamond Bar (City), in its role as the Lead Agency pursuant to CEQA, in considering the proposed project for approval.


The 2019 certified EIR for the Diamond Bar General Plan 2040 and Climate Action Plan (certified EIR; included evaluation of a Town Center Mixed Use Focus Area with a maximum FAR of 1.5 and maximum residential density of 20.0 dwelling units per acre (du/ac). In April of 2021, the Urban Land Use Institute-Los Angeles (ULI-LA) convened a technical assistance panel (TAP) to report on the market possibilities, implementation strategies, and design frameworks for implementing the frameworks for implementing the Town Center defined in the General Plan (; a Town Center Specific Plan was recommended to provide detailed development standards, infrastructure requirements, and implementation measures for the Town Center. As part of the TAP’s analysis, the real estate economics firm RCLCO Real Estate Consulting prepared a marketing feasibility study for the Town Center that concluded that (1) market demand exists for over 2,000 housing units (an average of 44 du/ac), including more than 1,500 rental units of varying product types, based on market conditions, demographics, and locational advantages in Diamond Bar; (2) a strong opportunity is presented for redevelopment and repositioning of existing retail space, which would benefit from synergies created by new housing; and (3) there is low demand for office development in the area. An Addendum to the certified EIR was adopted on November 2, 2021, with adoption of the 2021-2029 Housing Element Update, that determined there would be no new or substantially more adverse effects from additional housing unit capacity for the Town Center Focus Area identified in the Housing Element Site Inventory completed in the City’s Regional Housing Needs Assessment (RHNA) California Department of Housing and Community Development certified Housing Element Update 2021-2029. The proposed Specific Plan under consideration, under maximum buildout, would result in construction of additional housing units, hotel rooms, and open space and less commercial space than evaluated in the certified EIR in response to the TAP’s analysis and the additional housing unit capacity identified in the Housing Element Site Inventory.


This environmental analysis acts as a “supplement” to the certified EIR completed for the General Plan. An SIR is needed to address changes to the Town Center not discussed in the certified EIR to evaluate the degree to which there would be any new significant impacts or substantially more adverse impacts caused by adoption of the Diamond Bar Town Center Specific Plan than have already been evaluated in the certified EIR, identify possible ways to mitigate or avoid the significant effects, identify any significant and unavoidable adverse effects that cannot be mitigated, and identify reasonable and feasible alternatives to the proposed Specific Plan that would eliminate any significant adverse environmental impacts or reduce the impacts to a less-than-significant level. The Initial Study found that nine environmental issue areas would have the potential to result in significant impacts requiring further analysis in the Draft SIR: 1.) Aesthetics, 2.) Air Quality, 3.) Greenhouse Gas Emissions, 4.) Hydrology/Water Quality, 5.) Noise, 6.) Public Services, 7.) Recreation, 8.) Transportation, and 9.) Utilities/Service Systems.


Public Scoping Period


The 30-day public scoping period for the Draft SIR for the Diamond Bar Town Center Specific Plan starts June 5, 2023, and ends July 5, 2023.  Under CEQA, this period is an early and open process for determining the scope of issues to be addressed in the environmental document, intended to identify the range of issues pertinent to the proposed project and feasible alternatives or mitigation measures to avoid potentially significant environmental effects.

What is the process for the SIR?

The SIR process takes approximately nine months to complete; and began with the notice of preparation (NOP) on June 5, 2023. The steps to complete the SIR are identified below. There will be opportunities for the public to provide comments during the 45-day public review period for the Draft SIR and during public hearings before the City Council acts as decision-making body on the Final SIR. The graphic below highlights when the community will have an opportunity to review environmental information and provide comments.


  • Initial Study and Notice of Preparation (NOP)

  • Public Environmental Scoping Meeting

  • 30-Day Scoping/Comment Period on Initial Study and NOP

  • Prepare technical studies, Draft SIR, and Notice of Completion/Availability

  • 45-Day Public Review Period on Draft SIR

  • Prepare Responses to Comments on Draft SIR

  • Prepare Final SIR

  • Prepare Mitigation Monitoring Program

  • Prepare Statement of Facts and Findings, and Overriding Considerations

  • Hold Planning Commission Hearings

  • Hold City Council Hearing(s)

  • Council Action on Final SIR

  • File Notice of Determination (NOD)

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