"I'm a proper Londoner. I lived in Spitalfields for 20 years, for goodness sake," says Harriet, who now finds herself in rural splendour. "The idea of moving to the country was alien."
The barn is split in two by a central play area for Harriet's sons, Jackson, 13, and Freddie, 11, and Mark's boys, Archie, nine, and Toby, four.
Mark says: "I love the fact that the newer glass structures sit so unapologetically within the barn while respecting its age and beauty."
The barn's breeze-block walls have been left unrendered while the floor is polished concrete, providing a contemporary counterpoint to the 700-year-old beams.
Three years ago, Harriet, Mark and their respective children were bursting out of their London home when they heard about the barn, with 10,000 square feet of living space and acres of surrounding meadows.
It sounded ideal but the couple had a huge row as they drove to see the house over whether they could commit to remote Essex life.
"Then we saw the barn and fell in love," says Harriet. "Nothing prepares you for walking inside. It's like a cathedral.
"It was my mum who told us about it. David Pocknell, the architect who owned it, is an old friend of my dad. I actually have pictures of me in the bath with his youngest son when we were kids," she says.
"David is a genius and has impeccable taste, so buying the barn was an easy decision. It's the best move we could have made."
Harriet has renovated houses in the past, but admits it was a relief to find a house that felt as if it had been made to measure for her, without going through the hoopla of planning permission and endless design decisions. "We just fit here," she says.
The barn is a Grade II* listed building that had a tin roof and a dirt floor when David bought it.
"The planners would have been happiest if it had been restored as a monument to the past," says Harriet. "Fortunately, David came up with a solution that preserved its structural integrity while slotting a contemporary home into the space."
A floor-to-rafters glass wall separates one end of the barn. Originally four cart bays, it's now a spacious two-storey structure, housing the boys' block, with a cabin-style bedroom each and a locker room-inspired bathroom.
Mirroring it at the other end of the barn is an open-plan family space with kitchen, dining and living rooms, where design-classic furniture sits happily alongside ancient beams. The central space has two internal structures that resemble Monopoly houses and which function as print and packing rooms for Woop, the design studio that Mark and Harriet set up with two friends.
Harriet, a photographer who has worked in the war zones of Somalia, Afghanistan and Iraq, has bought iconic prints such as Don McCullin's Shell-shocked Soldier and Nick Ut's image of a girl fleeing a napalm attack in Vietnam.
"These images are a part of our history," she says.
* Check out Woop's designs at woopstudios.com
* See architect David Pocknell's work at pocknellstudio.com
Photographs by James Merrell
Styling by Mary Weaver
* The full version of this article appears in the August issue of Livingetc, out now