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One of the very few things architects agree about is urban regeneration. To them, it means high-density, mixed-use, sustainable development on brownfield sites within conurbations. But how to achieve regeneration that really is sustainable is a trickier question. I spent last weekend with 150 architects in a former ocean liner terminal in Rotterdam, where we were presented with descriptions of some of the pitfalls that lie in wait for the would-be urban regenerator.

The occasion was the annual conference of the RIBA, and it was an inspired decision to hold it in Rotterdam, as the city is Europe's unofficial capital of urban regeneration. First there was the large-scale rebuilding of the city centre after it was bombed flat at the start of the Second World War, then the city began a second wave of regeneration in the 1980s and 1990s by transforming its extensive riverside docks.

The ocean liner terminal was a pioneering structure of concrete shells with inspiring views over the Maas, the river where the city's trade is still transacted by means of large barges. It stands between two landmark skyscrapers of more recent vintage that are much less inspiring: an illogically lop-sided office tower by Renzo Piano and a dull grey tower by Foster & Partners.

A line-up of Rotterdam's urban movers and shakers, invited to show off the city's recent achievements, proceeded to heap criticism on the city's rash of high-rise buildings. "A collection of some of the worst skyscrapers; they haven't done much for the urban fabric," commented the Californian Aaron Betsky, who runs Rotterdam's world-famous architectural museum. "High-rise by bad architects, with B-versions by Renzo Piano and Norman Foster," said architect Kees Christiaanse. "These buildings have voids at street level," added landscape architect Adriaan Geuze, creator of a vibrant square in the city centre. "The tissue in between has been neglected, and there is no rich subculture to colonise it."

Liverpool, a city of similar size to Rotterdam and arguably more in need of regeneration, came in for even more withering criticism from another panel of pundits. Jim Gill, chief executive of the city's urban regeneration quango, Liverpool Vision, presented an upbeat case that the city now had its big chance, with its selection as European capital of culture 2008, the planned development of Will Alsop's scheme for the landmark Fourth Grace on the Pierhead, and what he called "a significant change in the corporate government of the city".

Manchester is taken to be the capital of successful regeneration. But it is protected only by a thin layer of branding. The city is actually losing people. Manchester is the Museum of Loft Living and the city of New Labour

But Gill's optimism could not allay the ingrained scepticism of other speakers. Tom Bloxham, the chairman of developer Urban Splash, which was responsible for the lively Concert Square in the city centre, reminded delegates of Liverpool's long history of post-war failures. "Hundreds of millions of pounds of public money have been spent in Liverpool, and the results are absolutely crass. I worry whether Liverpool city council really knows what architecture is about."

David Dunster, professor of architecture at Liverpool University, continued the theme that Liverpool might fail to grasp the opportunity. "Liverpool is the capital of nostalgia," he said. "It wallows in its sense of being the underdog." Their scepticism was underlined by the fact that the fourth invited speaker, Baron Isherwood, failed to attend: the week before, he had been sacked from his post as development director of the regional development agency for north-west England.

Thames Gateway was the conference's third area of discussion. With the government earmarking the 40 mile region on either side of the Thames estuary for 200,000 homes and the same number of jobs, it is one of the most ambitious development programmes ever undertaken in the UK. But local government minister, Nick Raynsford, passed the buck of transport infrastructure to transport minister Alistair Darling and, when asked what vision the government had for the area, could only point the Greenwich Millennium Village as an exemplar of "exciting new developments".