This estate was to be his summer retreat — all 1,200 acres of it. The equivalent cost to build it today would be something in the region of £20 million.
Castle Coole is a massive porticoed building, startling white and constructed with perfect symmetry, as was the fashion in the late 18th Century. The style is described as neoclassical Georgian, its exterior reminiscent of a splendid Greek temple, with both ionic and doric features.
Inside, the décor is ornate and opulent, with surviving flock wallpaper from the era and an especially decadent room prepared for a proposed visit by King George IV, who then cancelled. There are other rooms furnished in French, Greek and Chinese styles to show off the family's knowledge of the world at large.
And what of the servants? Their quarters were hidden underground in cellars, with the only natural light coming through grated pits, accessed through a tunnel with a concealed entrance. This section of the house has recently been restored and gives a fascinating insight into another way of life.
The grounds are magnificent, with landscaped woodland and grassy slopes sweeping down to the shores of Lough Coole, after which the mansion was named. What better place for a leisurely stroll after you have been mesmerised by the upstairs-downstairs contrasts of the house?
Castle Coole is managed by the National Trust.