On the banks of the Thames at Henley, David Chipperfield is creating a minor masterpiece, a match of modern manners and time-honoured materials. But, in its final stages, his design is at risk. By Jonathan Glancey
Perhaps it's in the nature of rivers to bring out the best in everyone. Most of us like messing about on them, eating in riverside restaurants or just feel that bit cheerier when walking along their banks. Something in the Thames water gave us Terry Farrell's most delightful building - an Eighties' boat house in a post-modern classical style that would have had me snarling were it in a city centre. It looks a picture with boats rowing by on the Thames at Henley.

The Thames and Henley have now done the same thing for David Chipperfield, whose River & Rowing Museum on Mill Meadows is this traditionally uncompromising architect's most likeable work to date. The museum has been a long while in the making: commissioned in 1989, it does not open until summer 1998. In the meantime, those boating by can watch its progress and admire a building that, perhaps more than any other in Britain, reconciles the limitations of local planners with the adventure of modern architecture.

From the river, the great oak-clad roof of the first stage of Chipperfield's museum looks like an upturned boat. Or ark. This timber cladding gives way below the tree line to a ground-floor wall of glass which, in turn, rests on a concrete plinth raised up above the ground which, here, is a flood plain. If the river were to burst its banks and inundate Mill Meadows, the museum would presumably appear to float, a capsized ark on the flood waters.

In fact, its appearance takes its cue from traditional Japanese buildings as well as from the Oxfordshire barns that planners cited in discussion with Chipperfield.

Superficially, it has something of the look of the Japanese Pavilion at the '92 Seville Expo designed by Tadao Ando, an architect who works a comparable idiom to Chipperfield.

Most of Chipperfield's designs have been characterised by a rigorous use of concrete and an equally demanding structural logic. This is not to say they are unwelcoming. And certainly not unpopular. His most recent work in London is the new Wagamama noodle restaurant in Soho, which is never less than packed, and for many diners is their first meeting with an approach to modern architecture that links Britain with Japan and the architecture of the late Nineties with the revolution in design instigated by Le Corbusier.

The River & Rowing Museum is an unparalleled demonstration of how to match modern manners with time-honoured materials. As Chipperfield puts it, "the form makes reference to the traditional wooden barns of Oxfordshire and the riverside boat houses at Henley. It was clear from the beginning that any building proposal that departed from a `comprehensible' form would be resisted by planners. Our strategy was to work within this `cultural resistance' to propose a solution that found a resolution between convention and invention, figure and abstraction."

He has done precisely this. But what he hasn't done is to give shape to a building that is neither one thing nor the other. It does not look at all like the result of compromise. Perhaps the long delay between inception and construction has given everyone involved a chance to think through a building form new to Britain - one that the ripest heritage fruit and the most unrepentant modernist will find as delightful as it is satisfying.

The oak-clad roofs protect the first-floor galleries in which examples of rowing boats will be displayed together with a history of sport on the Thames. These spaces will be lit by skylights and will be intimate and enclosed.

The contrast with the public spaces below - entrance hall, cafe, bookshop, information and so on, couldn't be greater. These are sheathed in wall- to-ceiling glass, bringing a wash of light across oak floors and opening up lush views of surrounding greenery.

The combination of timber floors and timber cladding and a refined deployment of daylight makes for a building with something of the character of a contemporary art gallery and yet something too of a grand ocean-going yacht. Not the sort of yacht in which the interior mismatches the exterior (there are plenty of these nautical bordellos), but one in which the whole craft is is perfect trim, physically and aesthetically. Unlike so many modern buildings, the interior represents the exterior and vice-versa. There are few more ship-shape buildings to be found anywhere in Britain.

It could, and deserves to, set a precedent. It is exactly the sort of design that might just encourage us to stop building in faux "vernacular" styles. Oxfordshire barns were not built to be quaint, but to store grain. The fact that so many of them are handsome is because they are built with a robust confidence using local materials. Chipperfield's River & Rowing Museum is far more in keeping with its Oxfordshire setting than 90 per cent of the new "vernacular" housing that has been vomited over the county in new cul-de-sacs over the past decade.

This is the first Chipperfield building of consequence (pace Wagamama) in Britain. Plans for a radical new arts centre in Wales were squashed recently and Chipperfield has entered far too many competitions and not quite made the grade. In Germany, Japan and Spain he has met with success and critical acclaim. The difficulty for dedicated architects like Chipperfield in Nineties' Britain is clear: they have to compromise to make it.

There is, however, compromise and there is compromise. Sadly, Chipperfield's clients - keen rowers all - are not so keen on letting him design the museum's interiors and displays.

If they did stop him, they would be making a big mistake. For here is a chance to get a contemporary British building right from floor to roof. Allowed to complete what he started so convincingly, Chipperfield will create a minor masterpiece of end-of-the-century architecture.

The client body certainly owe their architect some respect, as he designed the building with precious little practical guidance and saw it to planning permission without being paid for his office's labours. Chaps (and chapesses) who mess about on the river should know about fair play and so encourage Chipperfield to complete a design that really ought not to be compromised too far at this late stage.

We have remarkably little respect for architects in Britain (compared with, say, Germany, Japan or the United States); when they get a building so right, we owe it to them to allow them to put the finishing touches on their designs. As it is in the nature of rivers to bring out the best in people, perhaps we can hope for tranquil waters at Mill Meadows by the opening of the River & Rowing Museum next summer.