Complex sentence: Poplar Forest was Thomas Jefferson's plantation and plantation house in what is now Forest, Virginia, near Lynchburg, which he treated as a private retreat and upon which he lavished attention from 1806 until his death 20 years later.
Split into simpler sentences: Poplar Forest was Thomas Jefferson's plantation and plantation house in what is now Forest, Virginia, near Lynchburg. He designed it and treated it as a private retreat, working on it from 1806 until his death 20 years later.