Soane built additions to Bentley Priory from 1789 to 1799. A first addition was quickly followed by a second, and an entrance hall was added five years later. The first two additions, from 1789 to 1793, cost a total of £22,749 5s 1d and the third phase of works cost £4,970 9s 11d. Soane charged his usual fee of 5% for both.

Soane probably met Hamilton through Thomas Pitt, Lord Camelford, an early patron. Hamilton was living at Pitt's Petersham Lodge, Richmond, while Soane was making minor alterations to the property in 1781. In 1784, Hamilton commissioned Soane for alterations to his London townhouse at Park Street and in 1788 he asked Soane to find him a suitable house outside of London. Soane first visited Bentley Priory, Stanmore, on 9 August 1788 and by the end of the month Hamilton had purchased the property. Designs began immediately.

The existing house was a simple five-bay Georgian structure built on high ground overlooking Harrow-on-the-Hill and London beyond. Soane extended the house to the east, preserving and promoting these southern vistas. Years later, from 1798-99, an entrance hall was built on the north front. Over these ten years of alterations and additions to the house, Soane also erected new outbuildings on the property, including a dairy and lodge.

The first addition to the house included a library, breakfast room and a circular top-lit ‘tribune’ for displaying Hamilton's pictures and statues. Estimated at £4,625, the design was approved in April 1789 and begun immediately (drawings for the foundations had already been made in March). Before the addition was completed, however, another larger wing was commissioned in October 1789 when Hamilton inherited his family estates and the title of 9^th Earl of Abercorn. Designs for this second phase, estimated at £10,450 were approved in November 1798. This extension included a drawing room, eating room and music hall arranged around the recently-built circular tribune. The additions were completed in 1793.

In October 1790, the Earl became 1st Marquess of Abercorn and commissioned various building works at his other properties, including the Irish family estate at Baronscourt, the local church at Stanmore, and various London townhouses. Work at Bentley Priory recommenced in 1798 with designs for a new entrance hall, additional bedrooms over the offices, an altered kitchen and a repositioned principal staircase. A dressing room for the Marchioness is also mentioned in Soane's office day books for work at Bentley Priory. The new entrance hall was built to partly conceal the north front of the old house. The addition spanned the width of the old facade and was surrounded by plantings. It was designed in a Greek style. Visitors were led from a porte-cochère into the dimly-lit vaulted vestibule lined with Doric columns.

Although efforts were made to establish a symmetrical exterior, the final effect of so many additions was a somewhat cumbersome and incoherent building. Soane's drawings in both 1792 and 1798 include thick groves of plantings to disguise parts of the north front. Clearly he was not completely satisfied with his work, showing the building in his Royal Academy lectures: 'Will such an appearance as this lead any person to imagine it to be one of the principal fronts of a nobleman's residence? Does it not rather indicate the exterior of an hospital or an extensive Manufactory?' (Watkin). The interiors are also somewhat disjointed, failing to fully realize Soane’s principles of axes and enfilades.

Soane also made designs for the oubuildings at Bentley Priory from 1789 to 1798. Designs in 1789 show an office wing containing a brewery, laundry and washhouse designed. A rustic lodge was built in 1790, a hot wall and fruit sheds in 1792, and an ornamental dairy in 1798.

John Pullinger served as clerk of works at Bentley Priory for the last phase of building works, from 1798 to 1799. Soane's pupil and assistant Henry Hake Seward also oversaw much of the building work during this time, with assistance from John Caley and Matthew Lamburn. The earlier building works from 1789 to 93 were overseen by Soane, who visited the property often.

Bentley Priory was redecorated by John Crace in 1802 and altered by Robert Smirke from 1812 to 1813. After alterations by subsequent owners, in 1880 the house was converted to a hotel. It later served as a girls' school. The property was sold in 1926, the majority being purchased by the Royal Air Force, for whom it served as the headquarters of the Fighter Command in 1940. Fire severely damaged the house in 1947 and 1979. Under a new ownership, planning approval was received in September 2010 for building more than 90 flats on the grounds and converting the house into apartments and a museum dedicated to the RAF.

A model of the Greek entrance hall was made in 1798 (1367 M). The basic model has no base, allowing one to view the vaulted ceiling from below. A drawing of Bentley Priory by Soane's office is in the drawings collection at the Victoria and Albert Museum, London, showing a view of the interior of the entrance hall, as built in 1798.

Literature: D. Stroud, Sir John Soane, Architect, 1984, p. 138; P. du Prey, Sir John Soane, 'catalogue of architectural drawings in the Victoria and Albert Museum', cat.212, p.70; P.Dean, Sir John Soane and the Country Estate, 1999, pp.64-77

Madeleine Helmer, 2011.