At this time of year, a man’s thoughts turn to compost – and why he has forgotten to buy any. Many of us resort to the garden centre in such times of crisis; organised souls use their own compost or buy in a ton of well-rotted horse manure.

But human poo? Who’d put that on their garden? Anyone who has a composting toilet. These lucky souls use their large intestine to create a supply of rich, sweet-smelling fibre (and it’s fine, I have smelt it) that, to all intents and purposes, might be the finest prepacked Arthur Bowyer’s: wholesome, healthy and about as recycled as you can get.

Emptying the earth closet onto the veg patch was common until the 20th century, and on Wednesday, on Grand Designs, I visit Kelly Neville and his wife, Masako, who have installed a reed-bed sewerage system by the oak-framed hexagonal house they have built on their six-acre smallholding in the Cambridgeshire fens. The garden is fed by roof-harvested rainwater and the enriched “grey” and “brown” water is filtered and cleaned by the reed bed. An underground microbiotic “digester” feeds on the waste solids, and the accumulated sludge makes great fertiliser. This isn’t “fork to fork” gardening, it’s “pan to pan”.

A wind turbine generates some electricity, and Kelly, 48, harvests the coppice willow that they grow and burns it in a low-tech furnace. This provides hot water and underfloor heating for their superinsulated house.

The building itself, which cost less than £110,000, is a graceful four-bedroom polygon with a pitched roof and a glazed lantern (basically, a large skylight). It looks not unlike a tiny Globe theatre, or a half-timbered medieval spaceship, as it “floats” more than 2ft off the ground on a timber-jettied plinth, supported by thin steel columns. The lantern draws vast amounts of light into the centre of the building, which is basically circular on the inside; it illuminates a landing, the bedrooms hidden in the roof and the double-height living room.

Everyone who sees the place, from local farmers to academics at Cambridge’s architecture faculty, is captivated. The oak structure was drawn up by an architectural technician, and there has been engineering input, but the design and construction is really entirely down to Kelly, a carpenter by trade, and Masako. What little help they did have was bartered for return favours. This was a project that was always meant to be built by one man and an old grey tractor, with help from his wife.

“I always end up building on my own, almost by default, because that’s what I’ve always done,” says Kelly, who has taken just one day off during the 20-month project. The shape is inspired partly by nature – leaf cells and honeycomb – and partly by the Japanese teahouses and temples that the couple have visited together. And there is a philosophical rigour. Ben Platts-Mills, a furniture-maker friend of Kelly’s, carved the hobbit-style staircase from the base of an ancient oak tree that forms the home’s central structural spine; where possible, each component is natural and locally sourced. Timber abounds; walls are clay or lime plaster over straw-bale insulation; floors are terracotta; wall tiles and a bath were made by artist friends.

“I thought, I do have a choice,” Kelly says. “Wouldn’t it be great to make a chapter of my life building a house out of natural materials that the earth yields, and using them in their natural form?”

Kelly and Masako – who have a son, Acer, 7 – describe what they do as a form of permaculture, a movement that started in 1978. Its roots were in Australia, and one of its founders (with Bill Mollison) is David Holmgren. “It came out of awareness about the limits of resources, especially the energy crises of the 1970s,” Holmgren says. “It began when I was a student of environmental design in Tasmania, with the premise of looking at the redesign of agriculture using ecological principles, but it extended from there to the redesign of the whole of society.” Today, there are 4,000 permaculture projects in 120 countries.

In the 21st century, permaculture seems to mean a self-sustaining lifestyle that’s a mix of ecological construction and organic gardening. There’s a strong emphasis on design, and an avoidance of chemical fertilisers and harmful products. At least, I think that’s what it is. Not that Kelly and Masako are hardline dropouts. Far from it. They still drive cars. Their warm, comfortable home is a beautiful space. Kelly’s hair looks as though he has been testing his wiring with his fingers, but Masako, 52, is all soignée elegance. She is also poetic about the Good Life: “It’s nice to be free from things like energy crises and uncertain food products. I want to live in harmony with nature.”

In fact, the Nevilles are a little unsure as to whether they’ve joined the permaculturalist ranks. The term is an abbreviation of “permanent agriculture”, suggesting the kind of sustainable operation that the couple run. Yet the Permaculture Association also talks of “permanent culture”, and in much more general sustainable terms: “By thinking carefully about the way we use our resources, it is possible to get much more out of life by using less.”

Design is a simple series of smaller processes; it’s something many of us do intuitively every day. As Mollison says: “Permaculture is about thinking before you act.” In which case, I’m happy to use the term. We could all use a little more permaculture.

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