Complex sentence: The house was originally built by his father-in-law, Judge Elijah Miller, in 1816, then substantially modified by the Sewards in 1840 and 1866 to accommodate the diplomatic entertainments expected of his offices.
Split into simpler sentences: The house was originally built in 1816 by Judge Elijah Miller, Seward's the father-in-law. The home was substantially modified by the Sewards in 1840 and again 1866 to accommodate the diplomatic entertainments expected of his offices.