By designating historic districts, communities can protect large numbers of historic resources and preserve the unique, authentic character of historic neighborhoods. 

In many cases, a local historic preservation ordinance includes language allowing for the designation of historic districts. In other cases, a community establishes a separate ordinance for this express purpose.

Historic districts may be referred to by a variety of names, depending on the jurisdiction, including Historic Preservation Overlay Zones (HPOZs) and Historic District Overlay Zones.

They are generally defined as physically contiguous groups of buildings, developed within a single period or in a similar style, that continue to express the design and patterns of the time in which they were developed. Though historic districts are primarily contiguous, non-contiguous resources may also be eligible for designation as a thematic grouping. 

Each historic district has geographic boundaries. Within those boundaries, most of the buildings must be considered “contributing,” meaning that they are historically significant to the neighborhood and have maintained the physical integrity of their original design.

A historic district will inevitably include some percentage of “non-contributing” structures as well—those built outside the district’s established “period of significance,” as well as those that have been greatly altered.

The strongest historic district ordinances enable a local Historic Preservation Commission to deny inappropriate alteration or demolition of historic structures within district boundaries. They also allow for design review of new construction within the district, to help ensure that new development is compatible with the neighborhood’s unique historic character and context.