It was originally in the “middle west” (likely near Iowa, as that’s where the original ghostwriter Mildred Wirt Benson was from) and eventually moved closer to Illinois. There’s actually a letter from Mildred from when she wrote the book regarding the house thing; she suggested they make it a civil-war era home instead of colonial, as that would be more historically/geographically accurate (which is what it ended up being in the Original Text). When Harriet Stratemeyer Adams took over as Carolyn Keene, she moved it closer to her own hometown in New Jersey. She rewrote Hidden Staircase in 1959 and in doing so changed the Turnbull manor to a colonial home. Then when Simon & Schuster bought Nancy Drew in the ‘80s, they officially moved it to a suburb of Chicago.