Complex sentence: Branch House lies within Richmond's Monument Avenue Historic District, which had entered onto the National Register of Historic Places in 1967 (extended in 1989) and was designated a National Historic Landmark in 1997.
Split into simpler sentences: Branch House lies within Richmond's Monument Avenue Historic District, which had entered onto the National Register of Historic Places in 1967 (extended in 1989). The district was designated a National Historic Landmark in 1997.