Frogner Manor ("Frogner Hovedgård") is a manor house and former estate in today's borough of Frogner in Oslo, Norway.
 The estate comprised most of the modern borough of Frogner, which has been named after the estate, and Frognerseteren with parts of the Nordmarka forest (Frognerseterskogen).
 The remaining part of the estate is now the site of the Frogner Park, with the manor house found in the south of the park and the Vigeland installation in the park's centre.
 The 18th century buildings on the grounds are now occupied by the Oslo Museum.
Sandbjerg is a former estate and manor house 5 km north of Sønderborg in the southeast of Jutland, Denmark.
 The estate dates from 1571, but today's house was built in 1788.
 Since 1959, the estate has been used by Aarhus University for teaching and research.
 Today it is the university's conference centre.
Old Westbury Gardens is the former estate of John Shaffer Phipps (1874–1958), heir to a U.S. Steel fortune, in Nassau County, New York.
 Located at 71 Old Westbury Road in Old Westbury, the property was converted into a museum home in 1959 and is open for tours April through October for a fee.
George Abraham Crawley (1864–1926) was a British artist, designer and purveyor of English taste.
 He is best known for the design of John Shaffer Phipps' (the heir to a U.S. Steel Magnate fortune) home, Westbury House (built 1904–10; additions constructed in 1915–16), at Old Westbury Gardens, in Long Island, New York.
Potheridge ("alias" Poderigge, Poderidge, Powdrich, etc.), in the parish of Merton, in the historic hundred of Shebbear, 3 miles south-east of Great Torrington, Devon, England, is a former Domesday Book estate and the site of the former grand mansion house re-built by George Monck, 1st Duke of Albemarle (1608–1700) "circa" 1660 on the site of the former manor house occupied by his family since at the latest 1287.
 It was mostly demolished in 1734 after the death of the widow of his son Christopher Monck, 2nd Duke of Albemarle (died 1688) and the surviving section forms the present Great Potheridge farmhouse, a Grade I listed building, inside which however some remnants of the former mansion remain, including a grand staircase, two massive 17th-century classical-style doorcases and a colossal relief-sculpted wooden overmantel.
 The latter depicts within a wreath of flowers, against a background of an elaborate antique trophy of arms, five putti, two of which, in flight, hold between them a crown, an allusion to Monck's central role in the Restoration of the Monarchy.
 The chapel "of Grecian architecture", i.e. classical, was in ruins in 1770 and was almost entirely demolished before 1822, with only the west wall left standing.
 In 1879 the stables, however, were still standing and were said to "give the visitor some idea of the magnificence of the ancient building".
 In 2014 Great Potheridge with 6 acres of land remaining of the former estate is used as an outdoor activity centre for young people operated by Encompass Training.
 It is today known as "Great Potheridge" to distinguish it from the nearby house, formerly on the estate, known as "Little Potheridge".
Golf House is a former estate house that was constructed in the early 1900s by John D. Rockefeller in Lakewood Township in Ocean County, New Jersey, United States.
The Rafael Guastavino Sr.
 Estate, also known as Rhododendron, is a historic estate and a national historic district located near Black Mountain, Buncombe County, North Carolina.
 The district encompasses three contributing sites and three contributing structures associated with the former estate of the noted Spanish-born architectural engineer Rafael Guastavino.
 Although the house was razed
The Duluth Works was an industrial steel and cement manufacturing complex located in Duluth, Minnesota, in operation 1915 to 1987.
 The complex was operated by the United States Steel Corporation.
 Officially, the plant's purpose was to supply the growing Midwest with steel finished products.
 Unofficially, they were built as part of a "gentleman's agreement" between U.S. Steel and the State of Minnesota to not impose hefty iron ore taxes on U.S. Steel in exchange for a fully integrated steel plant within Minnesota, whose mines furnished 80% of the ore to U.S. Steel.
 The combined works of the steel and cement plant were the largest employers in the city of Duluth and the fourth largest industrial complex in the state of Minnesota.
Milky Way Farm in Giles County, Tennessee, is the former estate of Franklin C. Mars, founder of Mars Candies.
 The property is named for the company's Milky Way candy bar.
 During the Great Depression, the estate was the largest employer in the county.
 The estate is listed on the National Register of Historic Places as an historic district; its manor house is now a venue for special events.
Maplelawn is an historic house and former estate located in Ottawa, Canada.
 The house was built between 1831 an 1834 as the centre of a farming estate by the Thomson family.
 In 1877 the Cole family bought the estate and lived there until 1989.
 The house is now owned by the National Capital Commission and it is a designated national historic site.
 It is particularly noted for the well preserved and rare walled garden next to the house, The Maplelawn Garden.
 Since 1999 the Maplelawn has been the location of the "Keg Manor" restaurant.
 Today the house is located in Westboro at 529 Richmond Road.

Question: In which county is this former estate of the heir to a U.S. Steel fortune located where George A. Crawley constructed a house?
Answer: Nassau County