Because resources are normally eligible for the National Register only if they are at least fifty (50) years old, it is extremely important to provide an accurate date of construction on the Historic Preservation Resource Identification Form. This date should be accurate within five (5) years of the actual date of construction.

There are a number of fairly quick methods to estimate the date:

  1. Sanborn Fire Insurance Maps. The Sanborn Fire Insurance Company mapped most incorporated towns in Oklahoma periodically between 1890 and the 1940s. Within the mapped area (which may not include all of the town), a plan view of each building was drawn. It is possible to interpolate the date of construction by between the map on which the building first appears and the previous map (if that map showed a vacant lot or a different building). The Sanborn maps are available at the Western History Collections, University of Oklahoma. Photocopies of the entire map series for a given community may be purchased.

  2. City Directories. Beginning about 1920, the city directories for many of the larger communities in Oklahoma included a "criss-cross directory" which listed owners in the order of the street address. One can generally interpolate the date of construction between the year of the directory in which a building is first listed and the previous directory. City directories are often available at local libraries or historical societies.

  3. Plat Maps. If all of the buildings within a platted addition appear to be approximately the same age, one can often assume that the buildings were constructed within 5 years of the date of the plat. Make sure that the addition was not replatted at a later date. Plat maps are available at county courthouses.

  4. Tax Assessor Records. This method should be used with caution, since the tax assessor records for older properties are often inaccurate.

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