According to
http://realestate.wharton.upenn.edu/research/papers/full/493.pdf
The quarterly standard deviation of unleveraged housing returns is 3.4 percent, which is 6.8 percent annualized. With a 20% downpayment, that’s a 34% stdev.
The SP500 has a standard deviation of around 20%, so to get the same amount of risk I want to be 1.7x leveraged.