Southill Park contains the site of late medieval Gastlings or Gastlyns Manor House and is the name given to a country house in Southill, Bedfordshire and its adjoining privately owned gardens and separate public parkland; it includes a lake and woodland. Its focal point is an early Georgian house, for disambiguation known as Southill Park House which is a heritage-listed building in the highest category. The parkland has legal designations in heritage and plant or wildlife protection. Further structures in the grounds have heritage protection including the follies of a Tuscan architecture temple and a partially stone-faced bridge, both designed by Henry Holland.
Gastlyns, Gastlynbury or Gastlings Manor is first recorded by inference by tracing a holding of the Domesday Book of the same size and location to one perhaps early 13th century overlord "Albreda", the younger sister of Walter Espec, who married Geoffrey de Trailly and the manor falling within the Honour of Trailly until 1438. However many successive tenants or more accurately today called long lessees held it — for example the "tenant" in 1250 held it by a yearly rent of 6 and a pair of gilt spurs. He was succeeded by his daughter Joan who married Sir Geoffrey Gastlyn, from whom the predecessor house derived its distinctive name. In 1667 John Thurgood conveyed the manor to Sir John Keeling. The Keeling family held the property until about 1707 when sold to Sir George Byng, who bought largely in Southill during the second decade of the 18th century, and was created Baron Byng, of Southill in 1721.
;House A 1930-written article in Country Life by Christopher Hussey describes the house as "a complete work of art to an extent that is true of few other great English homes of any period... must be acknowledged the classic example of the most civilised decade in the whole range of English domestic architecture." Southill Park House was built 1720-1729 then remodelled in the 1790s; it is listed at grade I. It is towards the village and on a rise overlooking its northern lands and lake. It consists of a brick-built mass with outer pavilions and linking wings. The house was stone-clad, ashlar-laid, in 1796 by employment of Henry Holland who moved the entrance to the north, constructed colonnaded loggias to the south and built the eastern "service wing". A late 19th century brick-built orangery, with stone pilasters and a hipped glass roof is attached. The stable block designed by Henry Holland, separately listed adjoins constructed as part of the remodelling of the house in the late 1790s using white brick and slated mansard roof. It has a courtyard, with an entrance archway through the north range from the outer courtyard, which is entered through a Gothic archway off the main drive. ;Estate A raised mound marks the site of the earlier house 400m north-east of the replacement and close north-west to Southill parish church, within The Menagerie woodland; it was demolished 1720-1729. The focal point of the northern area is a lake. This has an island towards the south end, and at the north bank its discharging stream is covered by Smeaton's Bridge designed by Henry Holland — a red brick arch and abutments with a decorative stone-faced park-facing side. At the head of a broad lawn running down to the north shore of the lake, flanked by woodland, stands the Holland-designed Fishing Temple, a decorative, brick, Tuscan style temple flanked by screen walls to the west and east, with an imposing porte-cochère lakeside, and a small cottage attached to the west. Both structures are separately heritage-listed, at Grade II and II* respectively. The lake acts as a spawning ground for fish and is on a sub-tributary of the Great Ouse, via the Ivel. The list entry summarises the terrain:
