There are no known design plans remaining of the policies or grounds at Tulliallan. The Countess de Flahault is known to have introduced the Italian Garden and there are old photographs of the estate in the 1900 period.
The traditional owners of Tulliallan were the Blackadders but the lands were purchased in 1798 by the distinguished Admiral, the Hon. Sir George Keith Elphinstone KB. He was the 3rd son of the 10th Baron Elphinstone of Drumkilbo and was created, in 1797, Baron Keith in the peerage of Ireland, with special remainder to his daughter Margaret. In 1803 he was created Baron Keith in the peerage of Great Britain and, in 1814, Viscount Keith. The modern Castle was built between 1818-20, and Viscount Keith died in 1823. He was succeeded by his elder daughter Margaret, Baroness Keith in her own right, who married Auguste Charles Joseph, Count de Flahault de la Billardrie, and later became Baroness of Nairne in 1837. The Countess had a large role in developing the gardens, particularly the formal Terraced or 'Italian' Garden. She died in 1867 leaving five daughters and on her death the Barony of Keith became extinct, but the Barony of Nairne devolved on her eldest daughter Emily who had married the 4th Marquess of Lansdowne. The estate of Tulliallan passed first to her half-sister Georgiana who had married Lord William Godolphin Osborne, nephew of the 6th Duke of Leeds (whose elder brother succeeded his cousin, as 8th Duke), and on her death in 1892, passed to Emily, 4th Marchionness of Landsdowne. She died in 1895, and was succeeded by her son, the 5th Marquess of Landsowne, a distinguished politician.
The estate was acquired in 1901 by Sir James Sivewright who carried out much new building on the estate. He died in 1923 when it was purchased by Colonel Alexander Mitchell, TD DL JP, who was succeeded in 1934 by his son Harold Paton Mitchell MP, author and lecturer, who was created Baronet of Tulliallan and of Luscar in the Province of Alberta in 1945. As Liaison Officer to the Polish Forces in North-West Europe, Harold Mitchell allowed Tulliallan Castle to become the Headquarters for the Polish Forces in Scotland and the Castle was occupied by the President and the Prime Minister of Poland along with the Polish Forces. Memorials of this occupation are in evidence both within the building and the Estate. After the war, in 1949, the Castle and 88 acres of the surrounding policies were purchased by the Scottish Home Department and became part of the Crown Estates, for use as a police training establishment for the whole of Scotland, now known as the Scottish Police College. The Castle and offices were modernised and the first course was run in 1954. New college buildings have been added and the first new block was opened by HM the Queen in 1960. A new heather garden was added in 1972 and training pitches have gradually been constructed since 1954. The Skid Pan was constructed, in the north- eastern corner of the present College Estate, in 1965/66 for Traffic training. The Traffic Division was housed at the same time in what had been former farm and stable buildings at Blackhall.