This is basically the same thing as a crawlspace, just deeper. Modular homes do not have a steel frame or I-beams and sit on either an 8 or 9 foot wall. The walls are generally concrete except in daylight or walkout areas. In order to have bedrooms in the basement, egress/daylight windows are required in each bedroom. This allows for a safe and accessible escape in case of fire.
For a HUD (doublewide home) there will need to be steel I-beams going across the top of the basement to support the home. Because the home will have the frame and I-beams, the walls should be at least 9 feet tall to have 8-foot ceilings. Doublewides are not normally placed on basements anymore. Most factories no longer offer the option to build them basement ready.
All of our modular homes are placed on crawlspaces and basements using a crane.