What is considered an impact to a historic property?
The State Office of Historic Preservation answers this question as follows:
Substantial adverse change includes demolition, destruction, relocation, or alteration such that the significance of an historical resource would be impaired (PRC Section 5020.1(q)).
While demolition and destruction are fairly obvious significant impacts, it is more difficult to assess when change, alteration, or relocation crosses the threshold of substantial adverse change. The State CEQA Guidelines provide that a project that demolishes or alters those physical characteristics of an historical resource that convey its historical significance (i.e., its character-defining features) can be considered to materially impair the resource’s significance. [State CEQA Guidelines Section 15064.5]
These are sometimes judgment calls. If you disagree with the conclusion reached by the agency, contact a professional or other expert on historical resources to review and comment on that conclusion. If there is litigation over the project, non-expert opinion or statements will not be considered substantial evidence to support an argument that the impact is significant.