The family name “Garrett” was changed in as a result of David senior’s construction of a Greek Revival burial vault in 1835, which was the first of its kind in the area. Located at the top of a hill, the vault was used as a landmark for passing riverboats as it could be seen for two miles in either direction. The sides feature native stone, while the front entrance feature marble quarried in Vermont. The massive slab was transported via the Mississippi River from New Orleans, then via the Ohio and Big Sandy rivers to Louisa, where it was then hauled by an ox team to the site.

But once the marble slab had arrived, it had become apparent that the family name had been misspelled as “Garred” instead of “Garrett.” The mistake remained intact and the name change was made permanent. (Excerpt from the following webpage:)
http://www.abandonedonline.net/residences/garred-house/