“This is insane,” a woman says as a butler takes her on a tour of The Chancery Rosewood’s spa.

We are in the basement levels of what used to be the US embassy in London. Now, there are state-of-the-art hydrotherapy pools and a cold-plunge tank, along with a sauna and steam room. I am stretched like a cat on a heated marble lounger that is far more comfortable than any angular stone slab has the right to be.

“This is kind of insane,” I think, as a member of staff uses an instant-read thermometer to check that the pool water has not cooled.

The Chancery started to take shape even before US diplomats moved their British headquarters from Mayfair’s Grosvenor Square to Nine Elms, near Battersea, in 2018.

The hotel now occupies the mid-century modern building designed by Eero Saarinen. A giant gilded eagle, once perched over the building’s edge, now crowns the rooftop bar, a spot that has already established itself as London’s poshest after-work hang-out. The hotel has been accepting guests since early September, and its grand opening was October 14.

The renovation, by David Chipperfield Architects, was extensive but faithful to Saarinen’s original designs. Ground-floor office walls were taken down, opening the space into a grand lobby and expanding its diagrid ceiling. The windows and Portland stone facade got a careful restoration, but everything on the upper floors was removed and turned into 144 rooms – all suites.