It was while driving back from one of their frequent trips to Totnes, a medieval market town in one of South Devon’s prettiest corners, that Hianta Cassam Chenaï and her husband Matthias Peters first spotted the Regency townhouse they now call home. “We kept coming down to Devon for weekends to see his friends and family, then dreading going back to London,” she says. “Then we saw the ‘For Sale’ sign.” Following their Provencal wedding in 2014, the couple traded their 1970s Hoxton flat for the handsome four-storey property first built for the Duke of Somerset (owner of the nearby Berry Pomeroy Castle) in 1830. Even on dull days the neighbouring River Dart beams in its watery light, lending the Grade II-listed house an airy, seaside quality. “It still feels like a holiday home,” she says, to the sound of seagulls.

Though the site had lain empty for a few years following its previous incarnation as offices, the couple were instantly attracted to its grand proportions and closeness to town. Smitten, they didn’t realise how bad a state it was in. “We naively thought we would just give it a lick of paint,” she says. Little did they know that they were embarking on an epic renovation that entailed replacing the stairwell, the roof and windows as well as reconfiguring the layout at the top and bottom.

‘It’s like a laboratory where I get to experiment with ideas’: Hianta Cassam Chenaï. Photograph: Simon Upton

Today, the entrance hall gives way to a large living room dotted with midcentury design, and an office with the double-sided desk she shares with Peters, a motion graphic designer. On the lower ground floor a sequence of dingy, poky rooms is now a serene open-plan kitchen and dining room overlooking the garden, with a utility and Lilliputian corridor leading to a home recording studio fashioned from the original cellar (“It was full of stalactites”). On the first floor are two comfy guest rooms and a dark-hued bathroom electrified by blue tumbling- block floor tiles. The couple knocked into the eaves to create a cavernous master bedroom, with acres of custom storage that doubles as a room divide, cordoning off their clean-lined Ashton & Bentley roll-top bath – and enjoying exceptional views on to the river and rolling hills.

So all-consuming was the project, which took more than 18 months in planning alone, that it prompted a fully fledged career change for Chenaï. The former digital strategist has now fulfilled her long-held ambition as an interior decorator. Since she retrained, and launched HCC Interiors four years ago, the house has become a test-site for her whimsical design: “It’s like a laboratory where I get to experiment with ideas,” she says. Her latest projects include a family cottage near Exeter and a custom furniture line made in collaboration with the architectural and design firm Woco, where she’s a part-time interior design consultant. Its first fruit is the elegantly proportioned ghost console (“no legs means less cleaning”) that is sleekly positioned in the hallway.

Though the couple’s past life of partying has happily been usurped by decorating and gardening, neither would have imagined that their social lives would actually enjoy an uptick in Totnes. “It’s way more sociable here than it was in London, where people would be so exhausted by the hustle they’d need to go home and rest,” she says. “Here, we see friends all the time and it’s often very spontaneous.”

‘Everything has been designed to face outwards, so we can cook and chat’: the kitchen. Photograph: Simon Upton

It’s not unusual to congregate for post-work river swims or impromptu dinners at their nearby allotment to share the spoils of the latest crop (right now, it’s courgettes, artichoke flowers and white currants). Central, in interiors terms, to this communal mentality is their west-facing kitchen. “We wanted it to be a very sociable kitchen,” says Chenaï of the space which, like the rest of the house, is decked with vintage artworks and treasures gathered on sourcing trips to Copenhagen, Brussels and the south of France. “Everything has been designed to face outwards, so that when friends come over we can cook and chat.”

Much of the artwork is inherited from the portfolio of Peters’s late mother, an amateur artist and keen potter, who taught her craft at a local school. Everything is inventively hung using cleverly upgraded charity-shop frames. Self-portraits sit alongside an eclectic gathering that includes a painterly seaside canvas picked up for €150 near Montpellier, and an otherworldly tapestry by the Danish artist Naja Salto. It’s these touches, together with Chenaï’s evangelical approach to lighting (“It’s decorative and sculptural”) that lends warmth to the cool, monochromatic palette, punctuated by the occasional dash of pink and purple.

Window on the world: the bathroom. Photograph: Simon Upton

On the furniture front, Chenaï operated a strict swivel-door policy for everything aside from the vintage twin beds inherited from her French grandmother: “Nothing from our London flat really worked proportionally,” she says. “So we slowly sold to replace on eBay.” The couple’s shared love of sci-fi lends another unexpected sartorial layer. “It’s Tron-meets-Regency,” she posits, gesturing to the spherical shelving that looms over the living room. Stacked with books on everything from English herbs to Thomas Heatherwick, it’s modelled after the futuristic Tyko bookshelf by Italian artist and architect Manfredo Massironi.

With the house complete, you get the sense that they’re now ready to fully immerse themselves in the local community. “Our lives have been dominated by DIY for the last few years,” she says. “I’m always tinkering – but there is an end point.”