John Pawson is not just a minimalist; he's the biggest minimalist around, and he's got one of the biggest baths. And what does he think about in his bath? He thinks about future bathrooms. If you go to Birmingham and put on virtual reality glasses, you can see what he has in mind. Nonie Niesewand mops up.

People think that minimalism is the equivalent of sensory deprivation. All that emptiness. No beer mats, beaded curtains, soft sofas or toys, squidgy cushions, lava lamps or ashtrays. Everything behind closed doors. Minimalists at home always get asked if they've just moved in, even when they have inhabited their cool white cell for years.

Minimalists love fine details: shadow gaps etched into walls as a fine line instead of skirting boards; plaster as smooth as silk; a pale palette; doors floor-to-ceiling height that click shut as silently as a car door, probably running on the same technology; designer pieces that say "Art". Understatement, but loads of light and space and warmth. They hide their sensuality behind hard exteriors. But don't imagine they give themselves a hard time. They do not, and nowhere does their quest for luxury in plain wrapping show as in the bathroom.

Here we see John Pawson, minimalist-in-chief, in his Notting Hill Gate house (now sold), luxuriating in the bath with wife and children as few of us could (ouch! those taps; oops! that overflow). His marble tub has no discomforts; it conceals a glass-fibre tilted tray so that when four people climb into it, and when the water goes over the side, it falls through chinks in the floor to run into the gutters. It is a fabulous bathroom, but really only a prototype for Pawson bathrooms to come.

From Sunday, you can visit a bathroom Pawson has designed for the year 2020, a place so minimal that it doesn't exist in real time. Yet you can see it at Interbuild in Birmingham and experience bathing in it, fully dressed on the bare stand, wearing a virtual reality helmet. First you enter this seamless stone box, three metres by two metres, through a gap carved in a block of stone, maybe from a meteorite since it's the year 2020. The bath is a plunge pool in the stone floor. A bench lines the windowless wall and the basin carved out of the bench at 40cm height means that you can stand at it, or sit down next to it to splash about. Just water and stone - at first glance it could have been designed for Fred Flintstone.

But fear not. Behind every minimalist there is a master technician. It may look austerely monastic but hidden details make bathing a luxurious experience. Showers tumble from an open sky. The retractable glass roof opens on a fine day, and solar-warmed water falls apparently from the sky rather than from the cunningly hidden plumbing and rainwater tanks on the roof. Pawson's inspiration was a 16th-century Mogal garden in Kashmir built by Shah Jehan, architect of the Taj Mahal, as well as the waterfall of "Eternal Tears", a piece of hydraulic engineering that resolves itself as a perpetual waterfall.

Basins fill without taps, activated by voice control. Copper pipes under the floor beam up fragrant steam through slits in the slabbed stone floor, based on the hammams of north Africa, totally hermetic spaces with a tiny door into which ovens pump steam "and inhabitants escape the cares of the street". It is a lot more energy efficient to use water to heat space rather than air - air conditioning and air heating are hugely wasteful, but simple, responsive plumbing can heat the stone floors as well as pump steam through them.

"My design treats water as a precious commodity, which it will be by then," says John Pawson. It also treats bathing as a ritual, the way that he discovered hot-tub bathing in Japan, where he worked in his twenties. So there are no intrusive taps: valves are voice operated, temperature requirements for different bathers are controlled by hidden computer software and plugs are little swivelling buttons that you press and the air comes whooshing out to seal it tight, the way the Renault 4 pioneered a rudimentary air ventilation system with a rubber button on the dashboard as big as a 10p piece.

"As usual with John's designs, when you start thinking about how to do it, it's difficult to achieve," says Jonnie Bell, Pawson's partner. Pawson is the architect who agonises over light switches lest they interrupt the flow of space in a wall, the author of a plump book called Minimum, which he packaged in a plain white cover with its title delicately embossed white on white, and whose favourite holiday destination is the desert.

Years ago when John Pawson and I travelled to his hometown, Halifax in Yorkshire, he visibly relaxed past the Watford Gap: "Trees clutter the landscape," he said, though now that he is landscaping gardens in Provence and Long Island with Jonnie Bell, he doesn't like to be reminded of it. But the trees he likes are often espaliered or pleached.

As you lie in the bath, still with your virtual reality helmet on, there is a view of the trees outside through a long, glazed, horizontal slit. The clear glass turns opaque when your voice activates its molecular coating, causing them to jiggle out of alignment and cloud the glass. The technology already exists for this glass but at present it has to be activated by an electrical impulse which is why it's called electro-chromic glass.

In the bathroom of 2020 the water drains through the steam slots in the stone floor to be recycled in a reed-bed in the courtyard, cleansed and returned to the water-table. This piece of eco-chic borrowed from the Marsh Arabs is pioneered by the Centre for Alternative Technology (CAT) in Wales (01654 702400).

You can take this time-warp journey to the past and into the future on the virtual reality programme designed by Gareth Lane and David Barker of Division Ltd. On stand number 1,830 at Interbuild you will be able to walk into the non-existent bathroom, sit on the bench, or step down the stairs into the bath. By pressing a button on the keyboard, you can fill the basin, empty it, switch on lights and adjust spots, make the glass change from clear to opaque, open and close the skylight, and simulate the shower coming on and off. The reed-bed in the garden correspondingly fills at the changes in water levels within. The designers of the virtual reality programme thought the bathroom was "a bit bland really", so they put in a towel and a couple of cans of deodorant, "things to pick up and play with if you want", and at a key press, let you change the surfaces from the rock specified to sandstone, pink marble or granite.

Interbuild, National Exhibition Centre, Birmingham (01203 426 508) from Sunday to 28 Nov.