When the clerks of Camden’s highways department were issuing parking fines from their gloomy office in the 1970s, little can they have imagined that jet-setting hipsters would one day be supping cocktails in the public library below them before taking an al fresco dip up on the roof. Maligned for years as the concrete “egg box” of Euston Road, the old council headquarters have been reborn as the glamorous Standard Hotel, the first outpost of the risque boutique chain outside the US.

“People thought we were crazy to suggest open-air bathtubs in London,” says Shawn Hausman, the Los Angeles-based designer behind the Standard’s flamboyant interiors, who started out creating film sets for Saturday Night Fever. “But I think it’s always nice to have a bath outside, even in the rain.”

Naked bodies bobbing above the rooftops is par for the course for a chain with a reputation for racy touches. Its first hotel, which opened on Sunset Strip in West Hollywood in 1999, features live models in a vitrine behind the reception desk, part of an art installation called The Box. Its New York flagship, which straddles the High Line park, takes voyeurism to the next level via the celebrity-frequented Boom Boom Room, an exclusive club that boasts four sitdown toilets in a single cubicle.

Facebook Twitter Pinterest Transformation … how the former ‘Egg-box of Euston Road’ now looks. Photograph: Garry Maclennan

Then there are its full-height bedroom windows, which have given the hotel a reputation as a site of sexual exhibitionism (encouraged by raunchy scenes in Steve McQueen’s film Shame being shot there). The price of a room is listed on the restaurant’s dessert menu, and where other hotels put the Gideon Bible, here you’ll find condoms and ibuprofen.

Is this what’s in store for what was, until recently, a bastion of council bureaucracy? Built in 1977 by Camden’s esteemed in-house architects’ department, the town hall annexe had long been seen as a concrete carbuncle. It was listed in the local conservation area plan as making a “negative contribution”, its rough pre-cast facade looming across the road from George Gilbert Scott’s gothic fantasy of St Pancras station like a cheese-grater.

When Scott’s creation was reborn as luxury apartments and a swanky hotel in 2011, it was only a matter of time before Camden cashed in on the location and architectural heritage of its own offices. Such muscular works of brutalism were rapidly becoming the subject of coffee-table books and tea towels – and being eyed up by trendy hotel chains looking to make a statement.

Facebook Twitter Pinterest Recalling Paolozzi … the reception desk. Photograph: Tim Charles

Facing hefty repair bills for lifts and services, the council sold the leaky hulk in 2014 for almost £60m, transferring its offices to a new block just to the north, saving an estimated £500,000 a year. For the Standard, which had been scouting in London for a decade, it was the perfect fit. “We thought all we needed to do was put a logo on it,” says managing director Bruce Robertson.

The reality wasn’t so simple. To make the project viable, a three-storey “crown” has been added, supported by threading new steel columns through the existing structure, whose facade has been scrubbed a gleaming white, while the brown-tinted windows have been replaced with clear curved glazing specially manufactured by a boatbuilding company. Orms architects have made it look as good as new, finishing it off with a jaunty red bubble lift so guests can feel like they’re being shuttled up the facade in a flying phone box.

This flash of glossy red is a hint of what lies within. Anyone who remembers queueing up inside the council’s concrete colossus to sort out their housing benefit won’t recognise the place. Working with interior architects Archer Humphryes, Hausman’s team has conjured a surreal vision of swinging London: a souped-up Austin Powers world of enamel optical-art murals and moulded ceramic wall features, with a hoard of vintage furniture sourced from the jazziest corners of eBay.

“We were inspired by a range of movies, from Fahrenheit 451 to Silent Running and Logan’s Run,” says Hausman, “as well as the iconic aesthetics of the London Underground. Mid-century Italian design played a big part too.”

Facebook Twitter Pinterest Bon voyage … the ferry-style rooms. Photograph: Tim Charles

A reception desk striped with exotic hardwoods stands in front of a bulging blue and red ceramic wall by Lubna Chowdhary, recalling the Paolozzi mosaics at Tottenham Court Road station, while a chunky pink terrazzo floor leads visitors through to what was once St Pancras public library. It is now a lounge bar themed around an imagined 1970s library, complete with an in-house librarian who will curate a selection of “radical” books.

“The original library was not so aesthetically pleasing,” says Hausman, “so we’ve invented the kind we would have liked to find here.” It is evidently the fictional library of a design aficionado: alongside retro veneered bookshelves are collectible pieces of Memphis furniture, baggy leather Saporiti sofas, and a ceiling dotted with red globe fixtures. To one side stands a “sounds lounge” recording booth, where musicians and DJs will make podcasts and record interviews, while around the corner lies the first of several restaurants, complete with a “fermentation bar”.

The interiors exude the retro charm of a Wes Anderson film set, but there is something unsavoury about a place once designed for public services being co-opted as a luxury lifestyle destination. It’s like Hackney’s former Asian Women’s Advisory Service being crassly reborn as The Advisory hipster burger bar. At least in this case, St Pancras library has a bright new home in the Kings Cross development, where a new public swimming pool and leisure centre have also been provided beneath the new council offices – an amenity the hotel itself lacks.

Facebook Twitter Pinterest Recline and read … inside the Library Lounge, which was once a public library. Photograph: David Cleveland

The Standard claims its ground floor will be as “democratic” as possible, with a dive bar-style venue spilling on to a passage, and a new public garden to the south where a closed route has been opened up. It was the only bidder for the site that proposed keeping the building – others planned to raze it in favour of a tower. When brutalist gems like the nearby Welbeck Street car park are being demolished to make way for lumpen new hotels, the vision to reuse such a massive concrete edifice should be applauded.

The bedrooms – which range from £150 to over £1,000 per night – continue the transport theme, with the curved windows recalling those of a ferry, along with tightly pleated train-style curtains and bold geometric bedspreads by Wallace & Sewell (fabric designers for Transport for London) complementing window seats in burgundy Harris tweed. Suspended ceilings have been ripped out to reveal the concrete waffle-slabs, while the open-plan bathrooms are an on-trend Pinterest board come alive, with riotous symphonies of bubblegum pink and navy blue tiles punctuated by mint taps.

Others aren’t as appealing. Across the boldly-carpeted corridors – designed as “an archaeological dig of 1970s patterns” – lie what are being marketed as “Cosy Core” rooms, a euphemism for windowless bunkers. It is here that the challenges of converting a deep-plan office into a functioning hotel are most sorely felt – a substantial block of rooms in the centre won’t enjoy a chink of daylight.

Facebook Twitter Pinterest Sssshhhh … blue and red styling for the Library Lounge. Photograph: David Cleveland

“It would be illegal in the US,” Hausman admits, “but here it was a great opportunity to exceed expectations of what a windowless room could be.” Aimed at jet-lagged travellers and nocturnal party animals, the rooms have a loungey hungover vibe, with low corner-sofas that meld into the beds, and long wood-floored showers filled with plastic plants in the hope of distracting guests from their light-starved existence.

The heady climax, which won’t open until later in the year, is found at the top of the crown, where one side will be a Latin-themed restaurant run by a double-Michelin-starred chef, imagined as a 1970s tropical plant-strewn lair of macramé curtains, rattan ceiling panels and raised terraces of crazy paving.

On the other side will be an opulent 24-hour-licensed venue with faceted mirror ceilings, leather banquettes and what the designers describe as a “big, boisterous, spectacular curtain”, drawn back to reveal five-metre high curved windows looking out over King’s Cross – London’s very own version of New York’s Boom Boom Room.

Facebook Twitter Pinterest Like a flying phone box … the Standard’s lift. Photograph: Garry Maclennan

Longtime local residents will be astonished that such a thing could ever appear on the Euston Road, but it completes the rapid mutation of this once-seedy district into London’s gilded “gateway to Europe”, where the likes of Google, LVMH and Universal Music rub shoulders with one of the country’s best art schools, Central Saint Martin’s – and where rooftop bathers will now be able to wave across to hedge fund managers living in the former railway offices of St Pancras. Standard, really.