Chandos House is the high point of the Adam Brothers’ Portland Estate Development. Though named after its first resident in 1774, the Whig politician and courtier James Brydges, 3rd Duke of Chandos, it was not designed and built for him, as was previously thought, but was an unusually lavish building speculation on the part of the Adams.
The full story surrounding its construction is not entirely clear but recent research has brought more facts to light. Like the southern half of Portland Place and Mansfield Street, the ground here was included in James Adam’s building agreement with the Duke of Portland in 1767 and was part of Robert’s and James’ master-plan for developing the area. It was constructed around 1769-72 by the Adam contracting and builders’ supplies firm, William Adam & Company, in which all four brothers held equal shares.
Its purpose was twofold: as a showcase for the brothers’ design talents, and as a lure to entice a big-name aristocratic purchaser, who in turn might help draw fashionable metropolitan society to the new housing they were planning in the surrounding streets. But there was a third reason why it was thought worth William Adam & Company undertaking the construction of this and several of the best houses in Mansfield Street and Portland Place as a ‘company’ speculation – and that was in order to promote the company’s business interests. In a later letter to his brother John in Scotland, William Adam explained that when builders first began taking ground at Marylebone, it was decided ‘a very eligible Speculation to build some Houses there on our own Accot… especially as it helped greatly to extend our Connections in the Brick & Timber business’. 
The Adams, as was their wont, expected great things of the house. A company financial statement of January 1772 noted that by then they had received but refused an offer of 11,000 guineas for it. However, their increasing financial problems, which were exacerbated by the Scottish banking crash in the summer of 1772, the cessation of work at the Adelphi and a general downturn in the building trade, made it difficult for them to find a purchaser, and the house was added by the brothers, along with two properties in Mansfield Street, to the top prize valued at £50,000 in the Adelphi lottery sale held a year later.
By then it had been mortgaged two or three times over to the banker and international financier Sir George Colebrooke. It is not clear what happened to the house at or immediately after the Adelphi lottery sale but it was back in the Adams’ ownership by June 1774 when the Duke of Chandos agreed to buy it from them for £11,000. This was less than they had already turned down, and less than they owed Colebrooke, but in their situation was too good an offer to refuse. As part of the deal they were required to pay interest on Chandos’s loan, as well as ground rents and all other taxes and charges until all outstanding building work was done to his satisfaction – indicating that even then the house was still unfinished. 
In terms of its architecture, Chandos House is notable for several reasons. Most immediately obvious is its unusual use of Craigleith stone as the facing material for its rather austere, stripped-down façade. The plainness is relieved by a crisply carved porch of Portland stone and elegant wrought-iron railings and lampholders.
Chandos House was the first of Robert Adam’s four great masterpieces of central London townhouse design – the others being 20 St James’s Square; Derby House, Grosvenor Square; and Home House, Portman Square – and it prefigures their achievement in bringing together sophisticated sequences of rooms of varying size and shape, for both public and private use, as well as services, on a central London house plot. Although not as lavish in its interiors as the other three, which were commissioned by very wealthy private clients, its rooms nonetheless form a very important surviving example of Adam’s decorative style – in this case designed with enough flexibility to appeal to a wide range of potential purchasers.
As part of a major restoration for the Royal Society of Medicine in the 2000s, stolen chimneypieces were substituted with replicas and the Adam ceilings were restored and carefully repainted in tones based on the Adam office drawings at Sir John Soane’s Museum. New carpets were made based on a variety of Adam designs.
 Blair Adam Muniments
 Middlesex Deeds Register 1770/1/381; 1770/2/40; 1772/6/378: Blair Adam Muniments, NRAS1454/4/16/18: Public Advertiser, 23 May 1772; 13 March 1773; 26 July 1773: Journals of the House of Commons, 13th Parliament of GB: 6th Session (26 Nov 1772 – 1 July 1773), p. 339