[...] many residents such as Callahan have voiced concerns about traffic, parking and the juxtaposition of a modern, four-story building in a neighborhood filled with craftsman bungalows. [...] with developers looking for new places to break ground, more apartment projects are taking root in the city's established residential neighborhoods — in some cases causing tension with residents, who complain about noise, congestion and the erosion of neighborhood character. "There are inherent problems when you start to warehouse a lot of people in a small space," said Marty Mauk, president of Northeast Neighbors, a group that has raised concerns about apartments being built on Hubbell Avenue. Apartment construction around the city has surged since the recession, as more people move to the region and as young people delay homeownership — either by choice or because of financial barriers. Since 2012, metro-area cities have issued building permits for more than 7,000 apartment units, according to the Home Builders Association of Greater Des Moines. Rental projects are in the works along Ingersoll Avenue, Southwest Ninth Street and Hubbell Avenue, in the River Bend neighborhood, around Drake University and in the neighborhoods surrounding the confluence of the Des Moines and Raccoon rivers. In the Drake neighborhood, residents originally voiced opposition to an apartment complex proposed for Forest Avenue at 32nd Street, calling the project over-development, ''a monstrosity and an eyesore. Officials at the nonprofit Neighborhood Development Corp., which is building the complex, said they went through nearly 15 designs to make the project more palatable to neighbors. On the city's northeast side, several apartment projects near Hubbell Avenue have faced backlash from neighbors who have raised concerns about increased traffic, crowded schools and a concentration of income-restricted rentals. Real-estate investors are