Mayor Nutter's inspiring but so far unfulfilled promise to improve the way we plan and build this city is about to be tested.

The test involves the proposed "extreme makeover" of Dilworth Plaza, the prominent public space on the west side of City Hall. Today, it is a jumble of ramps, steps, railings, sunken courts, and raised platforms, all constructed out of massive blocks of gray granite. Once the latest thing in urban design, these elements have not worn well, making this one of Philadelphia's most disappointing public places.

Now the Center City District has made a promising proposal for overhauling Dilworth Plaza. It would create a continuous, open piazza, with no breaks in the paving level from the curb to the facade of City Hall - a versatile public space suitable for rallies, ceremonies, outdoor markets, and ice skating in the winter.

The design also aims to bring sunlight and clarity to the dark, labyrinthine underground interconnections among SEPTA's subway, trolley, and regional-rail lines. It also would add ease, dignity, and a bit of democratic spirit to movement between the subway and pavement levels. It could be a fitting gateway to the network below.

The plan would tame the intersection of Market and 15th Streets, and it would "green" Dilworth Plaza with more planted surfaces and contemporary water-management technology. Two graceful glass shelters over the SEPTA entrances and a big, multifunction fountain could be great works of civic art.

With so much to gain, will Philadelphia - the city that loves to hate itself and often ignores or mismanages its opportunities - get this one right?

This is likely to be the largest public work of urban design undertaken in the city during these difficult times, and it won't succeed without oversight and coordination by the City Planning Commission. Success will depend on addressing several tough but not impossible issues:

Most important, this proposal must be part of a bigger plan for the City Hall environs. It has to be integrated with at least conceptual planning for the adjacent public spaces: City Hall's other three sides, courtyard, and ground-floor public rooms, as well as other plazas to the west and north. The same thinking has to be applied to all these areas, which will be seen and experienced together.

A proposal to put a deck over the present sunken courtyard must be carefully scrutinized. It threatens to create a space as gloomy and confusing as the worst parts of the city's current underworld.

The planning must include major public art. Locations have to be determined by the architects, and there should be a process to identify artists of great talent.

A proposed lawn will be subject to a good deal of shade and pedestrian use. We have to consider whether it should be crisscrossed by pathways or perhaps paved.

This project will be undertaken amid challenging economic conditions, and the plaza was built with very expensive materials only 30 years ago. At the outset, therefore, we have to know the full cost of the project, and we must explore opportunities to reuse existing materials and features.

The Center City District and its CEO, Paul Levy, deserve credit for sponsoring this initiative and for hiring two leaders of Philadelphia's design community, architect Stephen Kieran and landscape architect Susan Weiler. The Historical Commission has conceptually approved the design's impact on City Hall. The next hurdle will be a review by the Art Commission, perhaps as soon as July 1.

That's when we will see if Philadelphia has the wisdom to ask - and the skill to answer - these hard questions. If we can do that, we can create a great work of urban design, just as we managed to do 70 years ago during the Great Depression - an era in which we thought boldly and built well.

David B. Brownlee and George L. Claflen Jr. are vice chairmen of the Design Advocacy Group of Philadelphia, a volunteer organization that seeks to encourage public discussion about design and advocate for design excellence. They can be contacted at dbrownle@sas.upenn.edu and gclaflen@claflenassociates.com.