City of Chelsea issued the following announcement on Sept. 11.
An overlay district is a zoning tool used to establish land development standards that are placed ‘over’ the current base zoning district.
It allows for standards that are stricter than the base zoning. While an overlay district establishes limits, its purpose is to ensure that design decisions complement each other and contribute to creating the quality of place envisioned in the plan.
Original source can be found here.