Three developers have submitted proposals for occupying the thin air above the Seventh and Grand parking ramp in downtown Des Moines, and they all seem to agree that the city is lacking upper-end apartment units. Denver-based Linden Street Partners proposes a 5-story, 160-unit apartment building that would cater to office workers, including those with families. In addition, the firm said it has identified a need for a 20-room boutique hotel. Minneapolis-based Sherman Associates Inc. proposes a “leading market-rate housing development and architectural icon” for downtown. Sherman’s $51 million plan calls for a 13-story structure with 195 apartment units. Des Moines-based Nelson Construction & Development suggests a 10-story structure with 150 apartments, three rooftop courtyards and a restaurant and lounge. A second phase could include 112 apartments and an additional courtyard. Nelson estimates the cost of the project at $36.4 million.