Architecture: It's green, and you can't see it: Recycled bathwater, organic paint and a Tasmanian forest give environmental credibility to a new student residence, says David Nicholson-Lord

When the new hall of residence at Linacre College, Oxford, admits its first students this autumn, it will look discreet, handsome, harmonious - and rather ordinary. You will admire the Dutch gabling, Queen Anne facades, Georgian detail, and conclude, probably, that here is a competent but unambitious piece of pastiche. You would be right about the competence but wrong about the ambition.

Oxford has undergone a rash of building recently as colleges, under pressure from the city council, attempt to house more of their students in hall. Even with the new block, Linacre, one of the youngest and least pecunious Oxford colleges, scrunched on to a tiny one- and-a-half acre site on the edge of the Cherwell meadows, will be able to house only 60 per cent of its numbers. But Linacre residents will have the consolation of living in what may well be the greenest university building in Britain.

The greenery should be obvious as soon as you venture inside. The building will smell different, a result of organic paints, natural materials and finishes (beechwood floors or coconut-based carpets), the avoidance of the synthetic products that many people believe play a part in sick building syndrome. It should also look different: corridors will be unpredictably angled, widening into communal spaces, asymmetrically punctuated by pools of light, avoiding the 'rabbit-hutch' rectilinearity common in halls of residence (the design has been much influenced by the NMB Bank in Amsterdam, one of the landmarks of modern 'organic' architcture).

What you will not see, however, are the features that Linacre and ECD, the architects, believe make the project unique.

First, there is the 'greywater' recycling system: water from baths, basins and showers will be stored, filtered, mixed with rainwater collected from the roof, then fed back through pipes (recycled copper, not PVC) to flush the lavatories; this should cut consumption by 36 per cent.

Second, the plans were based on an exhaustive study of 'embodied energy' - not the energy the building will use, but the energy consumed in the production of its materials. This produced some fascinating results. For example, natural materials - slate, timber - do well, and hi-tech or man-made products badly, because of the energy costs of manufacture.

Plastic, non-ferrous metals and carpets perform poorly: in embodied energy terms, says Jes Mainwaring of ECD, carpet tiles are 'incredibly expensive'. Over-complex glazing systems never recover the energy lost in manufacture. And according to ECD, the same is true, in Britain's grey climes, of solar panelling - part of the original plan for the block, but later rejected.

Third Linacre decided to make its own contribution to the reduction of global warming. The new hall will produce, through gas and electricity use, more than 42 tonnes of carbon dioxide a year, exacerbating the greenhouse effect. Trees, planted or saved from the chainsaw, can absorb that carbon dioxide. How much rainforest is needed to mop up Linacre's carbon pollution? A square metre of rainforest absorbs roughly one kilogram of carbon dioxide a year. Linacre decided to 'save' twice as much carbon as it released, at a cost, in forest adoption, of pounds 10,000. The college's CO2 emissions are thus being offset by 40 acres of threatened Tasmanian eucalyptus forest.

The Linacre project follows many of the newly discovered precepts of green architecture: reliance on passive heat and light gain, for example, through south-facing windows, thermal mass, natural ventilation. Many materials were recycled - roofing tiles, bricks newsprint - in the form of cellulose- fibre roof insulation. Where designers have chosen hi-tech, it is energy- efficient hi-tech: gas-condensing boilers, low-energy light bulbs, gas- fired tumble-driers and fridges, sensors that switch off lights when nobody is about. But there is lower technology too: some bricks in the facade have built-in nesting-boxes. Hence the fourth unique point claimed for the project - that of being the first multi-occupancy residential building to gain the top rating of 'excellent' in the green assessment method devised by the Building Research Establishment and ECD - is hardly surprising. What is surprising, on the face of it, is why this should have happened at Linacre.

The explanation lies in an upsurge of environmental enthusiasm among its graduate students in the late Eighties, coupled with the arrival of Sir Bryan Cartledge, Britain's former ambassador to Moscow, fresh from witnessing the ecological disasters inflicted on the Soviet Union. The college set up an environmental working party and held an environmental audit; it launched the increasingly prestigious Linacre Lectures on environmental subjects; it has recently committed pounds 100,000 - about 5 per cent of its investment funds - to a green unit trust (with 'better-than- average' results). And when Oxford admits the first students on a one- year environmental studies MSc course in October, Linacre hopes to be a principal destination.

Perhaps the college's most unusual achievement, however, has been to insert so much greenery into a building scarcely distinguishable in style from its neo-Georgian neighbours. Despite the traditional facades, dictated by the planners, the project has attracted much scepticism from the rest of Oxford. The extra cost of the greenery is put at 1 per cent of the pounds 2m total cost. But the hall will use 35 per cent less gas and 25 per cent less electricity than a conventional building. So the extra investment will pay for itself in less than two years. Or as Sir Bryan puts it, other bursars will be much less sceptical 'when they start comparing their gas bills with ours'.

GREEN BUILDING OF THE YEAR: Entries for this award, sponsored by the 'Independent on Sunday' and the Heating and Ventilating Contractors' Association, should be submitted by 18 February. Details from HCVA, Esca House, 34 Palace Court, London W2 4JG (071-229 2488).

(Photograph omitted)