According to recent statistics from the Census Bureau, about 3-5% of the housing inventory falls into the seasonal, recreational or occasional use category. These numbers are based on the 2010 Census and are the most current stats available.
The states that lead the list with the highest number of properties in these categories are Maine, Vermont, New Hampshire, Alaska, Delaware, Montana, Wisconsin, Florida, Arizona and Idaho. According to these statistics, approximately 4% of the housing inventory in Utah falls into this category. However, this percentage is much higher in the Deer Valley real estate market. According to a 2010 Park City Housing Market Assessment prepared by the Bureau of Economic and Business Research at the University of Utahs David Eccles School of Business, over 80% of the new residential units built in Deer Valley and Park City during the last decade were second homes.
For more information on housing data and statistics concerning the second home market, please click this Census Bureau link.