The U.S. Secretary of the Interior has developed a set of standards to guide the treatment of historic properties. The standards address the technical and philosophical aspects of historic rehabilitation, the tenets of preservation and restoration, and issues related to compatibility on a district-wide level.
10 Principles of the Secretary's Standards for Rehabilitation
Illustrated version of the Secretary's Standards for Rehabilitation
How to avoid incompatible work on historic storefronts - Illustrated Guidelines
Guidelines for Preserving, Rehabilitating, Restoring, and Reconstructing Historic Buildings
Technical Preservation Briefs - How-to publications on specific restoration topics.
Information found at the links above is maintained by the National Park Service.