The need for affordable housing in the United States has never been greater. …Research from federal mortgage backer Freddie Mac estimates that the U.S. is short 3.7 million housing units relative to current market needs, while the National Association of Realtors pegs the figure even higher at 5.5 million units. …Experts cite a variety of factors that contribute to difficulties adding housing stock. …While the national supply of housing has been lagging overall, some parts of the country have managed to add homes faster than others. States that have experienced the most housing growth in recent years are largely found in the Mountain West and Sun Belt regions, which tend to be less expensive than heavily developed coastal markets and have fewer regulations limiting construction. …Below is a complete breakdown of housing growth across nearly 800 U.S. counties (grouped by size) and all 50 states.