An apartment development in the works for the River Market is the final chapter in transforming what was once a gritty railroad yard into a new neighborhood. The 300-unit development promises the latest in energy efficiency while allowing residents to get their hands dirty working their rooftop gardens. “We want to make it affordable for downtown workers and students up to empty-nesters from Johnson County who want a great view of the river and downtown,” developer Jonathan Arnold said. The development, which is now before the City Plan Commission, would restore the historic street grid of the River Market, the area where Kansas City was born. It’s also being built to last. The structural frame of the four- and seven-story buildings will use concrete, not wood. The 16-inch-thick walls will not only make the apartments quieter, but require 70 to 80 percent less energy to heat and cool the units.