Environment review streamlining refers to the level of environmental analysis a development project is required to perform as part of the California Environmental Quality Act (CEQA).
CEQA requires that all development projects in California conduct some level of environmental analysis. In many cases, a city will conduct a comprehensive environmental analysis for a neighborhood as part of a community-driven planning process. This comprehensive analysis will be based on the community vision of a neighborhood (ex. Number of housing units, traffic, sea level rise, etc.) and look at the environmental impacts and the ways to address these impacts.
In areas where a comprehensive environment analysis has already been conducted, development projects that meet this community vision are able to utilize the comprehensive analysis to meet their CEQA requirements. If a project does not meet the community vision, they would have to conduct a new environmental analysis.
Now, this process is not always the case. CEQA is a very complex law with lots of caveats.
The Plan Bay Area website includes a helpful webpage listing and explaining the various types of projects that are eligible for streamlining and other exemptions from environmental review. Click here for more background on CEQA.