Who are you calling a carbuncle?

Should you spot an open-top Routemaster trundling around London on Monday, don't dismiss it as just another load of snap-happy tourists. On board will be Ken Livingstone, Sir Peter Hall and architect Terry Farrell, along with many other movers and shakers responsible for shaping our cities.

Should you spot an open-top Routemaster trundling around London on Monday, don't dismiss it as just another load of snap-happy tourists. On board will be Ken Livingstone, Sir Peter Hall and architect Terry Farrell, along with many other movers and shakers responsible for shaping our cities.

The bus tour heralds the start of Urban Design Week and, turning its back on the standard tourist trail, will instead visit areas of the city which demonstrate vibrant and successful urban design with commentary from its eminent speakers on board.

This third year of events is the largest so far with organisers Urban Design Alliance promising a wide geographical spread. A spokesman says: "We will provide enlightening guides and examples of how places might be better shaped in the 21st century." Brian Raggett, Chairman of UDAL 2000 opens the conference at which Nick Raynsford will speak on 21 September at the Bridgewater hall in Manchester.

Debate on urban design is timely as yet more plans unfold for transforming the more nightmarish aspects of London's urban landscape into the stuff of architects' and planners' dreams. Nowhere more dream-like than Elephant & Castle, where the bubblegum-pink shopping centre and the vast Heygate estate are about to undergo major regeneration.

Recently unveiled drawings for the borough show a distinct lack of concrete and no hint of puce. In their place gleaming towers filled with flats, shops and offices abound. Even the monstrous roundabouts are replaced with pedestrian plazas and transport links, designed by Norman Foster in Southwark's estimated 10-year plan.

There's a similar picture in many northern cities, too. Yet Bruce Walker, managing director of Nicholson Estates, says the phrase "new development" is a misnomer: "It suggests the possibility of daring architectural styles and use of contemporary materials, but in reality brown-field developments can be constraining."

Typically developers find that, because of planning restrictions, buildings can't be demolished and they must find ways to meet the challenges inherent in converting office into residential space. At Century Buildings, in the St Mary's Parsonage Conservation area in Manchester, Nicholson Estates is creating 121 apartments within a Grade II listed Edwardian building, and the adjacent glass-fronted property constructed in the 1960s.

Mr Walker says: "Here the challenge has been to convert the listed building and modify the contemporary building whilst ensuring that elements from different eras complement each other. We were determined that the additions would not emulate the original period of the building, but would be strikingly modern."

Nicholson Estates cleaned and restored the façade of the original Edwardian building and, after consultation with English Heritage, Manchester City Council granted consent for the addition of two penthouse floors designed in an "uncompromising modern style". English Heritage's report specified that these should "have a minimum impact on both the exterior and interior of the building, and retain features of interest whilst allowing the building to be brought back into positive use". Nicholson Estates have fulfilled the brief but planning permission and constraints about developing listed buildings can give developers sleepless nights.

"Not always so," says Tom Nicholson managing director of Try Homes who are currently developing Warnham Court Mansion in Horsham, Sussex. Their challenge is to refurbish the Grade II listed house built in 1828 while retaining its key Georgian characteristics. "The conversion aims to ensure that the original room sizes and panelling will not be interrupted," says Mr Nicholson.

Warnham Court, at one time a school, posed particular problems. Along with retaining its immense period detail, there was also a Sixties-built classroom block to contend with. Mr Nicholson says that removing this ugly addition was not, in this case, problematic and he hopes that it will act as a blueprint for future developments: "Some councils see Fifties and Sixties additions as reflections of that building's history, and while we may think that this strengthens the argument for removing them, in some cases they too are listed." Fourteen townhouses will replace the Sixties block in a development which will total 29 units.

Warnham Court does not have an urban setting but Mr Nicholson sees similarities: "There are an awful lot of exciting developments going on and the same principles apply to all. The challenge is to get people to accept the concept of urban design in rural developments which are on brown-field sites and pose similar challenges to city sites.

"Good design makes best use of them and eases pressure on green-field sites."

* Urban Design Week 18-24 Sept, 020-7636 9107