AN environmental scientist has designed and built an innovative garden office that maintains a comfortable temperature with the minimum of heating.

Neil Johnston, who has built a prototype in the back garden of his house in Granby Avenue, Harpenden, says it needs no electricity at all on clear winter days.

But unlike rival designs which maximise sun capture by large amounts of glass, his office, through carefully positioned south-facing windows, stays cool in summer.

He said: "Garden offices are a growing market, but I thought there was scope for something high quality, with much better insulation."

The walls, floor and ceiling have ten centimetres of sawdust for insulation, bolstered by two-inch air gaps under the floor.

The 52-year-old father of two said: "Most of these offices have half that."

Although he sets the thermostat at only 18C, the office feels far warmer as the heating comes from strips under the floor. He added: "It warms the body from the bottom up. A radiator has to be almost untouchable to push heat across a room."

The office, constructed with Douglas fir from a sawmill in Ayot Green and a special plasterboard made from recycled plastic, provides about ten square metres of space - ideal for a one-man business. The roof is only 2.25 metres high, reducing the space that has to be heated.

Mr Johnston, who intends to market his design, says it could also be used as a musician's rehearsal room -or a therapy space.