This document presents ten clear and brief standards that now are commonly accepted practice for preservation design. Supplemental to these standards is an extensive set of principles that provides more specific guidance on exterior surfaces, roofs, windows, interiors-even sites and districts. These standards and guidelines, revised a number of times since their first publication in 1979, are adopted by historic district commissions to assist in determining whether or not to approve petitions for proposed changes to historic structures. The standards and guidelines are nationally accepted, and represent the best thinking on appropriate methods of intervention.

The ten standards for rehabilitation, as stated in the 1995 revisions, are:

1. “A property shall be used for its historic purpose or be placed in a new use that requires minimal change to the defining characteristics of the building and its site and environment.”
Commentary: As an example, consider a historic church, no longer needed by its congregation, that has been sold. What is compatible with its historic use? Appropriate uses might include a community center or religious bookstore. Less appropriate uses include conversion to a boutique clothing store or a gym.

2. “The historic character of a property shall be retained and preserved. The removal of historic materials or alteration of features and spaces that characterize a property shall be avoided.”
Commentary: As part of a building's historic designation, the significant historic characteristics should be clearly identified. These historic features, whether arched windows, steeply sloped roof, or terra-cotta details, should be kept even if the structure is modified for a new use.

3. “Each property shall be recognized as a physical record of its time, place, and use. Changes that create a false sense of historical development, such as adding conjectural features or architectural elements from other buildings, shall not be undertaken.”
Commentary: Additions and alterations should not try to look original. To maintain the integrity of the original elements, the new should be clearly differentiated from the historic.

4. “Most properties change over time; those changes that have acquired historic significance in their own right shall be retained and preserved.”
Commentary: As an example, a black Carrara glass front was put on an 1880s Italianate commercial building when it was converted to a jewelry store in the 1930s. This glass front had been in place long enough to have developed historic significance of its own; hence, the building façade may be most appropriately restored its 1930s period, leaving the glass in place.

5. “Distinctive features, finishes, and construction techniques or examples of craftsmanship that characterize a property shall be preserved.”
Commentary: Perhaps a designated building has a magnificently crafted staircase in its foyer. Although the stair opening may not satisfy current fire safety codes for egress, and other stairs will need to be built, plans should retain the staircase as an elegant example of the building's original craftsmanship.

6. “Deteriorated historic features shall be repaired rather than replaced. Where the severity of deterioration requires replacement of a distinctive feature, the new feature shall match the old in design, color, texture, and other visual qualities and, where possible, materials. Replacement of missing features shall be substantiated by documentary, physical, or pictorial evidence.”
Commentary: This standard often applies to wood windows. It is commonly assumed old windows should be replaced with new, thermally efficient insulated units made of modern materials, such as vinyl. However, this standard urges owners to repair their original windows if possible and, if the windows are beyond repair, to replace them with similar painted wood sash to retain as much as possible the original appearance and proportion of muntins, whether using single or insulated panes.

7. “Chemical or physical treatments, such as sandblasting, that cause damage to historic materials shall not be used. The surface cleaning of structures, if appropriate, shall be undertaken using the gentlest means possible.”
Commentary: This guideline directly reflects on early attempts to restore painted brick buildings by sandblasting their exteriors. The integrity of soft bricks can be destroyed when sandblasting removed the bricks' outer crust, resulting in the exposure of the soft inner area. Great care should be taken to use the gentlest means possible when restoring original materials.

8. “Significant archeological resources affected by a project shall be protected and preserved. If such resources must be disturbed, mitigation measures shall be undertaken.”
Commentary: Preservationists tend to limit their involvement to existing structures, but it is important to recognize the importance of preserving archeological artifacts as well.

9. “New additions, exterior alterations, or related new construction shall not destroy historic materials that characterize the property. The new work shall be differentiated from the old and shall be compatible with the massing, size, scale, and architectural features to protect the historic integrity of the property and its environment.”
Commentary: Contemporary design in a historic district can be perfectly appropriate, as long as the new is designed with recognition for the old and is compatible with it. There is a danger in insisting on nostalgic design. As Garrison Keillor observed, “The past was copied, quoted, and constantly looked at until one day, the country looked more like it used to than it ever had before.”

10. “New additions and adjacent or related new construction shall be undertaken in such a manner that if removed in the future, the essential form and integrity of the historic property and its environment would be unimpaired. [Reversibility].”
Commentary: New construction, often designed for an economic lifespan of thirty to forty years, rarely lasts as long as original historic architecture. Therefore, new construction should be built adjacent to historic with the assumption that if it is eventually removed, the old should still retain its original integrity.