 as soaring fuel bills force homebuyers to act green, many movers are opting for the lower running costs of a new-build property which, on average, are six times more energy-efficient than older homes. The message coming through is that you do not have to compromise on comfort, style and luxury, or even pay a premium, for a low-energy new home that offers a better, cheaper lifestyle.  Green design has entered the mainstream housing market, with most developers leaning towards contemporary architecture and utilising hi-tech factory production.  all new-builds are covered by the Government's Code for Sustainable Homes, a green standard that works on a points system and grades properties on a scale of one to six, using criteria such as water-saving features and on-site power generation, from solar panels to mini wind turbines and ground source heat pumps. Level six is the top rating and requires all energy supplied to come from zero-carbon sources.  Hanham Hall, being built by Barratt in Bristol, is Britian's first large-scale level six development - 185 homes around a listed former hospital. Properties will be connected to an on-site combined heat and power plant, have rainwater harvesting systems, greenhouses, allotments and "smart meters", which enable householders to monitor their energy consumption as they use it.  Four modern town houses at aubert Park, Highbury, north London, are so energy-efficient that virtually no heating is required. Triple-glazed, south-facing and thoroughly insulated, they promise an astonishing 90 per cent saving in energy. The 3,000sq ft homes use a German construction method dubbed Passivhaus, which has ground-source heat pumps and an air-cooling and ventilation system. The timber frame structure has a thermal skin wrapped in white render, wood and zinc cladding. Internally, every room has a full-width wall of glass allowing light in. Bedrooms have terraces, and the galleried lounge looks out over the garden. Hot water and heating costs are around ?30 a month. Prices from ?2.25m. Call Kinleigh Folkard &amp; Hayward on 020 8222 7200. While in Notting Hill architect Seth Stein has created a ?9m luxury eco-home in the space behind a traditional terrace fa?ade (www.aylesford.com).  London architect Justin Bere specialises in Passivhaus new-builds and retrofits, and has completed one-off projects in Camden and Stoke Newington. Because developers have to meet eco-ratings based on everything from how close a home is to public transport to how thermally efficient it is, how well it tackles recycling and whether it uses sustainable materials, green housing is not just about technology it is about how it fits into the local environment.  Seven luxury town houses at Crabtree Fields in Fitzrovia conform to code level four by enhancing wildlife habitats through nesting boxes and sedum roofs. Biomass boilers use woodchips, resulting in cheaper fuel consumption. Prices from ?2.99m. For details call Ea Shaw on 020 7240 2255.  architect Luke Tozer ingeniously squeezed a low-energy house on to an 8ft-wide plot between two listed houses in a Bayswater conservation area. Called Gap House, its traditional, white-rendered fa?ade hides a Tardis of contemporary living - 