Owners of National Historic Landmarks are not prohibited under Federal law from making changes to their property as long as they have not taken advantage of Federal funding, licensing or permits. However, owners should check with their State Historic Preservation Office (SHPO) to determine if there are any State laws or local ordinances that could impact such an action.

The four National Register criteria should outline a quality of significance possessed by a property within the areas of American history, architecture, archeology, engineering and culture. The quality of such properties is based upon whether it reflects one or more of seven categories of integrity and upon whether it is the best example within a local, state or national spectrum.

In comparison NHLs may be listed under one or more, of six criteria, which are more stringently examined for national significance than NR properties. NHLs are identified through theme studies, which provide a national historic context for a comparative analysis of properties associated with a specific area of American history. NHLs must be exceptional or outstanding thematic examples and should exhibit extraordinary importance compared to similar properties within that theme.

National Historic Landmark designation is a more lengthy process than that for National Register listing. Nominators can expect the listing procedure to typically take at least 18 months.

Once the nomination form is submitted, the NHL Survey will review and suggest revisions prior to placement on the agenda of the National Park System Advisory Board meeting, which usually meets twice a year.

If the property is determined by the NHL Survey to be eligible for the NPS Advisory Board meeting, all relevant parties will be notified of the date of the scheduled meeting.

The board will assess the documentation and make a determination as to whether it meets the criteria; if the property is eligible the board will recommend properties for NHL designation to the Secretary of the Interior who makes the final decision on listing.

National Register listing is accomplished through an initial review of the nomination by local SHPO staff, who presents the information at a scheduled meeting of the State Review Board (SRB), which currently meets three times a year. If the SRB agrees that it is eligible, the completed nomination form will be sent to the National Register office of the National Park Service for a determination on listing, which is usually within 45 days of date of receipt.

Both NHL and NR listings may have national significance, but most NR properties fall under state or local significance. NHLs are also included on the National Register, however they must exhibit exceptional significance to the legacy of the United States in the areas of history, architecture, archeology, technology and culture and must exhibit a high level of integrity of location, design, setting, materials, workmanship, feeling, and association.

NHL nominations also require more extensive research and are subject to more thorough examination by the National Park Service than National Register listings.

Limited federal grants through the Historic Preservation Fund are available to owners of NHLs. State Historic Preservation Offices can provide information about the availability of federal and state funds. In some instances NHLs receive higher priority in consideration for funding than do National Register (NR) properties.