Peter Bullen has spent about $800,000 building a home that's underground. The garden's on the roof and he lives down below.

From the street, you would hardly know it's there (unless he's digging bok choy on the roof or picking broad beans). It looks like an empty plot - but go down the slope of the drive and knock on the door below street level and you're ushered into a different world.

Bedrooms, bathroom, workshop and kitchen are all encased in concrete walls sunken into a hole, covered with a concrete roof on which sits a garden for 230 plants.

It feels like a man cave but he says it's very comfortable - and very practical.

And interesting, so interesting that the Grand Designs programme will feature it later in the year.

"I wanted maintenance-free. I wanted it to be energy efficient. I wanted a big garden but I didn't have a big block," he said.

He also wanted a workshop where he could operate an angle-grinder in the middle of the night without disturbing the neighbours - and a buried room provides that.

So he designed exactly the right place for his needs and desires. It's the last house he'll ever have. "They'll take me out in a box on this one."

He had been thinking about it for for five to eight years.

The whole idea is simple. The walls were cast as concrete and brought in slabs on a truck and lowered into the pit by a crane.

The concrete roof was then put on and 350 tonnes of earth put on top for the garden. The earthen garden was surrounded by stone slabs from a quarry in Wee Jasper.

He's now planted broad beans in the overhead garden (which he is dismayed to find have been attacked by bowerbirds - "vandalised" he says).

In general, though, the plants are meant to be bird-friendly. There are smaller, thorny shrubs, for example, meant to provide a refuge for smaller birds hiding from larger thorn-averse ones.

"I like trimming hedges. It will be a haven for birds."

Ultimately, he wants to turn the roof-top garden into a kind of heath, largely with native species of plant. He is not religious about what kind of plants, native or imported - lavender will have a place.

In the yard at the back, he has planted 800 small pots hanging on the wall. He and a friend are experimenting with plant types with the aim of getting them to cascade down the wall and fill the yard with colour.

Indoors, there is no heating or electric cooler because the home-owner doesn't feel it needs it. The earth above insulates it from the cold of winter and the heat of summer.

There's a sunken yard at the back, and the sun shines through the big window and heats the floor.

Is there a danger of flooding? On his calculation, his pump can cope with seven inches of rain in 45 minutes - though he is thinking about a small generator to keep everything, including the pump, going if the power fails. "Once the power goes off, I'll be in trouble," he admits.

He is a satisfied man - in his man cave. Every man's dream.