BLENHEIM PALACE HERITAGE FOUNDATION is based out of THE ESTATE OFFICE BLENHEIM PALACE, WOODSTOCK, United Kingdom. Blenheim Palace is the seat of the Dukes of Marlborough. Originally called Blenheim Castle, it has been known as Blenheim Palace since the 19th century. One of England's largest houses, it was built between 1705 and 1722, and designated a UNESCO World Heritage Site in 1987.