No stone unturned: Joanna Berryman has injected a rock'n'roll vibe into her Cotswolds cottage

It may be her Cotswold bolthole, but interior designer Joanna Berryman hasn't exactly gone all Country Casuals when it came to decorating her cottage. "I wanted it to feel rustic but I didn't want it to be that classic Cotswold aesthetic of insipid shades of green and blue. I'm not dissing these lovely ladies that have these gorgeous…" here she breaks off, warily, before sparkling "hey – I'm a girl from London, let me just bring my urban thing to the country! I wanted to be sympathetic to the building, just inject a bit of fun and whimsy and a tiny bit of rock'n'roll."

Berryman bought The Folly, a large stone 17th-century cottage, with Victorian add-ons, in 2010 – and proceeded to add to it herself. "On the cusp" of a little village called Didmarton in Gloucestershire, it took a year of refurbishment before the property was livable. Being a self-proclaimed city girl, though, she was hardly homeless – Berryman spends most of her time living and working in London, where she has an interiors and product design company called Matrushka. She has a house in the capital, too, but points out that even in "lovely, luscious family-friendly Hampstead", you can't let the kids run wild; but at The Folly, her six-year-old daughter (whom she had with ex-husband, Coldplay bassist Guy Berryman) is free to explore.

Now, after gutting the kitchen and adding an extension, completely re-doing the garden, and taking up all the floors, she's got The Folly just the way she wants for a perfect weekend retreat. "I was resolute about having a local flagstone through the entire ground floor, because I felt it was so in keeping – it's like a little Hobbit house, especially when it's well-lit in the winter and you see all the warm lights from within. It's so pretty in the k evenings." The décor is probably not what Bilbo Baggins would choose, however; while Berryman has made features of those traditional flagstones, wooden floors and old beams, she's also splashed about eye-popping colours and modern designs.

And it's a constantly changeable space; being a natural magpie, Berryman is always finding new treasures. "I opted for the stony palate, the French grey, on the walls, to create a great canvas so that stuff could move around," she explains. "I want little crazy vignettes and tableaux everywhere – poetic chaos."

Her interiors advice is to be brave. "People are so often afraid in mismatching things or being bold with colour… it's so great to mismatch." So in the living-room (see previous page), she's happy to mix luxurious George Smith velvet-upholstered furniture in deep blue with a neon-yellow French chair and bright cushions in Ethiopian fabric. Then throw an equine theme into the mix: that painting was picked up at Brick Lane Market, while the horsey lamp is an antique from Howie & Belle in west London.

The local area is good for antiques hunting, too. "It's very near Tetbury, [which is] such a great antiques mecca – and it's turned into a sort of gastronomic destination because there are loads of great delis and markets." That gilt-framed picture of a sailor (previous page) came from Sharland & Lewis, the sunflower mirror (above) from Merlin Antiques and the old trunk (also above) from Brownrigg – Tetbury emporia one and all.

Berryman loves to spend weekends at The Folly, "just to romp around the countryside", and in the school holidays she decamps for weeks at a time with her partner, plus extended family and friends. But although she loves her hideaway – "There's a part of me that's so connected to nature" – Berryman soon realised she didn't use the property as much as it deserved. So since the end of 2010, she's also been occasionally renting The Folly out as a holiday cottage. "It's great, as I really enjoy my time there and others can too. Because it's such a lovely house, it is a certain type of clientele and they are very respectful," she says, adding that now, the sharing has become "part of its character – I feel like custodian of this incredible space. It's nice to share."

For more: matrushka.co.uk