Why did the Adelphi development become a financial disaster for Robert and James Adam?

The Adelphi development became a financial disaster for Robert and James Adam primarily because it was a large-scale speculative venture that required significant capital investment. In 1768, the brothers leased a site on the River Thames and spent a substantial sum of money embanking the land and constructing several terraces of houses between 1768 and 1772.1

While the project showcased their signature architectural style—bringing interior design elements like slim pilasters and shallow friezes to the building exteriors—the high costs associated with the site development and construction were not recouped, leading to the project's failure. Their subsequent attempt at speculation in 1773 with a group of stuccoed terraces in Portland Place, London, was also unsuccessful.1