NASA knows that there's not much room for packing stuff onto a spacecraft. That's why it's testing expandable living modules on the ISS prior to sending them to Mars for living spaces. The Bigelow Expandable Activity Module will ride along in an upcoming SpaceX resupply mission to the ISS.
From there it will be unpacked and attached to the side of the station. After that it'll be filled with air to expand from just over five feet in depth and almost eight in diameter to 12 feet deep and over 10 feet in diameter and have its pressure equalized with the rest of the station. In the video below, NASA says that it's all going to be done pretty slowly given that it's the first experiment of its kind.