North Dakota is probably the most vulnerable to a downturn in housing because of low oil prices.
New home sales were down by 6.3 percent in North Dakota between January and October of 2015 compared to a year earlier. Housing prices have not crashed yet, but there tends to be a bit of a lag with housing prices. JP Ackerman of HouseCanary says that it typically takes 15 to 24 months before house prices start to show the negative effects of an oil downturn.
According to Arch Mortgage, homes in North Dakota are probably 20 percent overvalued at this point. They also estimate that the state has a 46 percent chance that house prices will decline over the next two years. But that is probably understating the risk since oil prices are not expected to rebound through most of 2016.