Grand Designs, series 17, episode 7: see inside this stunning timber-clad home in the shape of a ploughshare in Devon

A couple set themselves the challenge of building a home with a swooping roof on this week's Grand Designs, as well as constructing a barn from which they can run their own business.

The star of the design of both buildings is a complex, curved roof, covered with turf and in a ploughshare shape to highlight the couple's relationship with the land.

Architect Edd Medlicott said the roof reflects "the whole ethos of what happens on the site… growing things from the ground".

Both roofs are made from straight girders or timbers, but cleverly joined to create the illusion of curves by being built, as Grand Designs presenter Kevin McCloud says, like a splayed "stack of playing cards".

For 12 years, food writer Mark and design co-ordinator Candida Diacono have run a 17-acre smallholding near Tiverton in Devon, growing peppers, pecans, quince and other fruits rarely produced in England.

They also grow apples for cider and grapes for sparkling wine - bizarrely, both proved vital in funding construction, which they estimated would cost £560,000, but ended up in a final bill of £650,000.

Deciding to expand their business and finally live on the land, they sold their village house to help fund not only a home for them and daughter Nell, but also a barn in which they can host cooking lessons and run a farm shop.

Despite selling their house and raising a mortgage, the couple are £100,000 short but are not deterred. Mark reveals he lives by the optimistic, but somewhat cryptic adage: "Have faith, but tie up your camel."

His contractor Phill Ireland, who struck a fixed price with the couple on just a handshake, says: "I don't think you'd be on this site if you weren't optimistic."

While the barn is steel-framed, the four-bedroom house is made from eco-timber frames built off site and clad in larch, and both buildings are set hard against a vast wall of cob.

This mixture of clay, sand and straw has been used as a building material in Devon for thousands of years, but work on the wall was held up for months because its needs dry weather to stop it freezing and fracturing and, of course, the couple built it over a cold, wet winter.

They set themselves a five-month deadline to enable them to start running courses in the spring and carry on the planting of crops that provide their income.

However, the fierce winter slowed work and the couple began to run out of money. Undeterred, Mark decided to try to raise £60,000 via a most unusual method - a crowdfunding drive.

While normal crowdfunding ventures reward investors with cash or products, Mark says: "Our rewards might be a signed book, a launch party, or a special day for a group of friends."

Not surprisingly, many people took up the offer of free cider in exchange for their investment.

During construction, Grand Designs presenter Kevin McCloud visits the site and says of the then bare, cob-walled house: "It looks like some mud hut in New Mexico or Benin."

Inside, however, he discovers "a cathedral of cob, a steel-roofed building held up with a 5,000-year-old wall. It really has pushed the material to the limit."

Despite the five-month delay, the house is finally finished, with a vast, 500 sq ft double-height kitchen overlooked by a mezzanine being the centrepiece downstairs and a cosy living room offering stunning views of their smallholding.

Upstairs, the master bedroom is built into the prow of the ploughshare, from where it offers even better views. Mark says: "From here, every day, you can see something change, you can see what you've done."

Abandoning his initial scepticism about the viability of the project, Kevin says the couple have created "a Shangri-La of sustainable smallholdings" and concludes: "Mark and Candida are an irresistible force of nature… the prospect is wonderful."

Grand Designs is on Wednesdays at 9pm on Channel 4