Ramsey County will be checking in with its designated developer every couple of months to make sure a proposal to build four towers along St. Paul’s riverfront goes through, according to a preliminary agreement approved by both parties.

The County Board selected AECOM in November to redevelop 5 acres on Kellogg Boulevard that once housed the county’s jail and several offices. They have agreed to a deal that will give AECOM six months — until July 22 — to complete its due diligence on the property, seek needed height and zoning variances with the city, apply for rights to cross Shepard Road and complete market studies for its plans to build a mix of hotel, office and residential space.

The entire project would be built in three phases and cost between $620 million and $900 million, according to AECOM estimates.

Ramsey County and AECOM plan to negotiate a final sales contract over the next six months, and the county hopes to have construction start as soon as possible if the deal goes through.

The county, which spent $17 million preparing the site, has selected a handful of preferred developers in the past only to see them walk away after determining that the project they planned wouldn’t make financial sense.

“Because we’ll be checking in every two months, we’ll have a good handle on it to make sure it’s progressing,” said Commissioner Rafael Ortega.

AECOM intends to build four towers that would rise from 12 to 36 stories high in a phased project that would reshape the St. Paul skyline.

Two of the towers would be built in the first phase — one with at least 250 apartments and a rooftop restaurant, and another with at least 80 condominiums and 150 hotel rooms, according to the preliminary agreement.

The exact size of the towers and number of apartments, condos and parking spaces will be hashed out over the next six months as the developer completes market and feasibility studies, according to the agreement. AECOM also would build a parking garage with at least 600 spaces.

Once the first two buildings are up, AECOM would build a land bridge — with tree-lined paths and open staircases — that would start at the top of the bluff and cross Shepard Road and the rail tracks below, to connect pedestrians on Kellogg Boulevard with the Mississippi River shoreline 80 feet below.

The preliminary agreement says both sides intend to close on the property by January 2020, with construction on the first phase starting three months later. The first two towers would open by 2022.