A four-story mixed-use building with retail spaces on the ground floor and 28 dwelling units on the floors above could rise in Old Town Fort Collins. According to the Loveland Reporter Herald, the plan would demolish a pair of existing buildings on a 19,000-square-foot lot that is the result of acquisition of 335 and 345 E. Mountain Ave. The new structure’s usable space would total 46,500 square feet, according to Fort Collins planning documents. Architects submitted the idea to Fort Collins planners for a basic conceptual review earlier this month, but both they and city staffers stressed that the proposal is in the early concept stages…