In 1711, John Holles, 1st Duke of Newcastle, bought the Manor of Tyburn from its owner John Austen. Within months, Holles was dead and Tyburn’s ownership passed to his daughter, Lady Henrietta Cavendish-Holles. Two years later, she married Edward Harley, son of Queen Anne’s chief minister who had become the 1st Earl of Oxford and Earl Mortimer. In 1717 Francis Seale became steward of the estate.

The younger Lord Harley in due course inherited his father’s title. His uncle – also called Edward – was much more interested in the value of the land than the 2nd Earl and started to look for investors.

In December 1717 the elder Edward Harley wrote to his nephew to tell him that six peers planned to build houses on their estate. He in June 1718 secured the first building agreement for the square, with Lord Carnarvon (soon to become Duke of Chandos).

Harley asked Francis Seale to suggest a road layout and plan with a Mr John Prince, a builder from Covent Garden. The two men started to negotiate building leases. What was to become Cavendish Square’s garden was enclosed in 1718 and sewers were laid. It was described as a ‘very spacious and noble Square, and many streets that are to form avenues to it’. Cavendish Square was planned to be an area where aristocrats would live – large villas with ample gardens.

In 1719, John Prince published the plan which would urbanise the fields of Marylebone. It was a novel design which would be much copied by other upper class landowners looking to cash in on their agricultural estates in what would become the West End. It was a grid system based around Cavendish Square.

Such was the social cache of the square that rental values in the area were calculated as a function of distance from Cavendish Square – the further away one chose to live, the further down the pecking order one was.

Streets leading from the square were named after family connections – Holles Street and Harley Street for example. Oxford Street on the southern boundary of the new plan was not named as it led to Oxford (which it ultimately did) but for the Earl.

Another innovation of the estate was the 99 year lease to “attract better-quality building”. The South Sea Bubble burst at the end of 1720 and collapse ruined many - the decline in the Cavendish Square’s prospects was rapid.

In 1730, nearby Hanover Square – the main upmarket rival to Cavendish Square - had been completed. Cavendish Square was nowhere near as advanced with some vacant land being used as a rubbish dump. Smaller houses began to be built on the south side with completion not achieved until 1741.