Developers are planning to build what could be Miami Beach’s tallest building on the city’s front doorstep — beside the Alton Road flyover at the end of the MacArthur Causeway.

Designed by international architecture firm Perkins + Will, the residential tower would rise 50 stories at the foot of the flyover. A six-story waterfall and three-acre public park would take the place of a dilapidated hospital and parking garage that now greet the millions of visitors streaming into South Beach from the MacArthur.

With only four units per floor, the condos would go for $2 million, with penthouses running in the $10 million range, according to Matthew J. Allen, executive vice president of the Related Cos., which would develop the project in cooperation with Crescent Heights.

For the project to get city approval, commissioners would have to recommend to change a city ordinance limiting height in the area. Now, 75 feet is the maximum height allowed. If the City Commission is interested, city staff would have to come up with a new proposed ordinance, which the commission would have to vote on twice. Then, the city’s Planning and Design Review Boards would have to approve the plans.

According to Emporis, an international provider of building data, the Blue and Green Diamond condos in Mid Beach are the tallest in the city. They reach 44 stories, or about 560 feet. The Akoya, the second-tallest building, stands at 47 stories and 492 feet.