A home - in a tank that used to hold 90,000 gallons of water

When Mark Lee, an architect, was asked by Yorkshire Water to assess a Victorian reservoir for redevelopment, he snapped it up - as a family home.

Mark Lee pushes open the wide wooden door. "We have been in this house for almost 18 months but we've only just moved into the bedroom," he says. "I wanted to design the bed," he adds, as if to explain the delay and lifts the duvet to reveal the simple, clean lines of the maple wood frame. "We are still waiting to put in place the canopy for the wardrobe which will be made of fabric but otherwise it's complete." We walk through to the study.

We are in the Round House, the Lees' home, which is built from a circular Victorian reservoir, 12m in diameter, that once held 90,000 gallons of water.

"It didn't seem to me to be an odd thing to do," says Mr Lee who is 34 and, as an architect, known for his "quirky" projects. "I thought it would make a pleasant, sunny family home," he says.

Mr Lee first came across the tank five years ago when Yorkshire Water asked him to visit it in his capacity as a partner with the Huddersfield- based Arthur Quarmby Partnership, to assess its potential for redevelopment. He decided he wanted to buy it for himself.

"We had long and serious discussions before leaving our cottage in Brockholes, which is nearby," says his wife Caroline, a physics teacher. "But I have faith in him and once the decision was made, I knew everything would turn out fine."

The Round House, a solid sandstone water tank with 3ft thick walls, which until the last war served the hamlet of Hall Bower, sits beneath the green slopes of Huddersfield's Castle Hill and its Victorian folly commemorating Queen Victoria's diamond jubilee. Its renovation was gruelling and involved moving 700 tonnes of clay and digging out the half-submerged reservoir to give a split level access at the front.

Step through the wide front door and a curved maple staircase leads up to the left. To the right are the doors to two children's bedrooms, a guest room, a utility room and a soon-to-be sauna. At the top of the stairs, the kitchen, living and dining areas, a play space with glass doors leading in to the garden, the newly completed bedroom and then the study sweep round in an open circle. The only solid barrier is the door to the bedroom.Twelve beams of Douglas Fir support the conical roof, radiating from a central frame work of six timber pillars.

"The Round House has made a difference to the way we live," says Mr Lee. "We are much friendlier, I think, because it's open plan and there is nowhere to hide except perhaps the pantry. We get on very well with each other and I think it's a very friendly house because it is so open and light. It's great for parties because everyone can see each other and mingle much more easily than in a conventional home."

He is eating lunch as we talk. The dining table stands within a circle of glass blocks through which light shines at night to form a subtle division between this area and the living space. It is directly beneath the centre of the conical roof and its glass lantern, bathing the us in bright summer sunshine. Long distance views of misty hills through the evenly spaced windows surround us.

The Lees believe in simple and subtle furnishing and decorating. Plain white walls, light wooden floors, carpeting only in the bedrooms to protect bare feet on cold mornings, but there are generous splashes of colour in soft furnishings and prints. Outside the reservoir's giant ball-cock mechanism which once controlled the water capacity stands in the garden and a giant stone plug makes a handy place to stand or sit and gaze into the distant country. Matthew and Katie, the Lees' children, prefer the swing.