Ramsey County’s latest proposed redevelopment of the St. Paul riverfront also could re-sculpt the downtown skyline.

The county’s chosen developer wants to build an office tower of up to 40 stories on the edge of the Mississippi River bluffs, making it potentially the tallest building in downtown.

Representatives of Los Angeles-based AECOM shared their lofty vision for the county-owned site Tuesday with the Ramsey County Board. They said demand is strong, with businesses looking for this kind of sleek, urban office space close to condos, apartments and restaurants.

The entire project, to be built in phases, could cost between $620 million and $900 million, according to AECOM’s early estimates.

“We feel we are on track, moving in the right direction,” said Brian Dusek, AECOM’s managing principal.

Commissioners seemed keen on the idea of a “catalytic” project that could create a whole new section of downtown.

But they also asked lots of questions: Could the project include affordable housing? Would the outdoor areas be accessible to people with disabilities? How would the project affect the rest of downtown?

“What we want is growth. We don’t just want to move people around,” Commissioner Victoria Reinhardt said.

The riverfront plan is still in development. AECOM officials will report back to the county in June; they are already in talks with St. Paul officials about getting the needed height variance for the development.

AECOM first shared its dramatic vision for the St. Paul riverfront in January, and much of it remains the same. It would take the nearly 6-acre plot along Kellogg Boulevard that not long ago housed the jail and county offices and build a lid that would expand the site to 12 acres stretching down the bluffs.

Its plan is to build four towers overlooking the Mississippi River. Tower one, closest to the Wabasha Bridge, would include a 150-room hotel and 80 condos. Tower two would have 300 apartments.

Towers three and four, if built to capacity, could include 1.3 million square feet of high-end office space. Tower three could rise 24 stories, and tower four would be the tallest, at 40 stories.

Currently Wells Fargo Place is downtown St. Paul’s tallest skyscraper, stretching 37 stories high.

The site would include a smattering of retail stores and restaurants and 8 acres of terraced public space, much of it green and parklike. The walking and biking trails along the river would remain and be connected to downtown through the site.

The lid, perhaps the most novel part of the plan, would be supported with concrete towers and extend above Shepard Road.