In a reversal, city planners are assisting a group of neighbors trying to halt ongoing construction of an 800-foot tower across from the luxury high rise where many in the group live.

For over two years, the neighbors and a group they founded, the East River Fifties Alliance, has spent more than $1 million drafting an unusual do-it-yourself zoning rule, that could block the tower on East 58th Street near Sutton Place.

City...