THE first thing you notice when you walk into “Tranquillity”, Mike Hillard’s house in the Cotswolds, is how warm it is. On a chilly, windswept day in May, it feels positively tropical inside — and it is no thanks to central heating. This house is warmed by the sun. It also has a highly efficient water-conservation system. Welcome to what Hillard says is the most eco-friendly house on earth.

“My objective was to design and build the most energy-efficient and environmentally friendly house I could,” he says. But Hillard, who was architect, designer, engineer and project manager for this building, had grander ambitions. He wanted to show that energy-efficient did not mean ugly and uncomfortable. This is a luxury home, with four bedrooms, two offices, an enormous kitchen, a sauna, a hot tub and a vast solar room that not only provides warm, fresh air to all the house but gives wonderful views of the valley below and will eventually act as a mini-biosphere in the house, containing a host of exotic plants constantly breathing out oxygen.

The house features everything that you might expect an environmentally sound development to include: solar panels on the roof, good insulation, dual-flush lavatories and even a mini-urinal to conserve water. But it contains a host of more unusual features, too. “The house is all original thinking,” says Hillard, who invented the solar water heater, a “hypocaust”, that is installed beneath the hall floor. He also developed the key to the house’s success: its ventilation system.

“Houses lose heat in two ways,” Hillard says. “Through the fabric of the house, through conduction, convection and radiation, and through ventilation.” Improving energy efficiency by reducing heat loss via the fabric of the house — the walls, windows and roof — is fairly common. This house had very thick walls containing 200mm of fibreglass, as well as top-range windows that conduct very little heat. But insulation is the easy part. As Hillard explains, until now “no one had cracked the ventilation problem”. He believes that he has.

Under standard building regulations, houses must be designed so that the air changes at least once an hour. But this is very inefficient because it entails a loss of energy. Tranquillity works differently. A sophisticated computer constantly monitors air quality for temperature, humidity, carbon monoxide, carbon dioxide, methane and particulates. “This is the first intelligent house, which only changes the air when it needs to,” Hillard says. As a result, the house loses very little heat through ventilation.

Heating in the house comes from various sources, including the solar room, which traps the sun’s energy and stores it, so that warmed fresh air taken from that room can be distributed around the house, from under-floor heating and from the hypocaust.

The results are very effective. Hillard says that once he is living in the house (he expects it to be completed in October) the cost of heating the rooms should be about £50 a year — not bad when you consider that the house is 5,000 sq ft, arranged over three floors. His water costs are even lower, at £11, the cost of running the filtration system. Domestic hot water will cost a mere £15 a year.

The filtration system, like many of the house’s features, has been donated by one of its many sponsors — in this case, Culligan. The roof windows come from Velux, the computer system that manages the home’s environment from Honeywell (although Hillard has tailored it to meet his specifications), the hot tub from Jacuzzi and the glass from Saint-Gobain. Hillard estimates that, even without sponsorship, building an environmentally friendly home need not break the bank. He says that eco-builders should expect to pay a premium of about 5 per cent above normal costs.

To raise money to build Tranquillity, Hillard borrowed against his present home, taking out a standard mortgage through Savills Private Finance. Raising money to build a home if you do not already own one is less straightforward. Lenders will usually offer you up to 75 per cent of the purchase price of the land. After this, money is released in stages as the building progresses. You can usually borrow up to 75 per cent of the estimated final value of the house. Some lenders offer more. Ecology Building Society, for example, will lend up to 90 per cent of the land valuation to builders planning to construct new green homes, and will then lend a further 90 per cent of the estimated final value of the property. Ecology also gives mortgages for the rescue of derelict buildings and to convert disused buildings into homes. If you wish to buy an eco-friendly property, it is possible to find a green home loan to match it. Norwich and Peterborough Building Society (NPBS) will lend against properties that have a standard assessment procedure (SAP) rating of more than 100. The SAP rating is an energy rating given to homes based on annual energy costs for space and water heating. The highest rating is 120.

The Tranquillity house is unlikely to come up for sale any time soon, and, although Hillard hopes that it can be a prototype for future housing developments, it may be some time before his intelligent housing system is widely avail- able. But there are other green developments in the UK. Lowry Renaissance, a firm of property developers, has recently converted an enormous Edwardian Grade II listed textile mill near Huddersfield in West Yorkshire. Titanic Mill, as the property is known, not only because of its size but also because it was completed in 1911, the same year as the ill-fated liner, houses 130 apartments, as well as restaurants and bars, a gym and a spa. It is also an energy-efficient development, with super-insulation and 400 sq m of solar electric panels on the roof. There are also plans to build a power-generation plant that will run on locally available sustainable wood chippings. Prices for apartments in the development start at just over £130,000.

If you are interested in buying an environmentally friendly house, a good place to start is Greenmoves, a website that advertises energy-efficient homes for sale.

Savills Private Finance, 0870 9007762 Titanic Mill, www.lowryhomes.com Greenmoves, www.greenmoves.com

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