In order to be considered a qualified (non-taxable) distribution, the amount taken from the 529 for room and board expenses must not be more than the greater of-
1) The allowance for room and board, as determined by the school, that was included in the cost of attendance (for federal financial aid purposes) for a particular academic period and living arrangement of the student;
or
2) The actual amount charged if the student is residing in housing owned or operated by the school.
As long as you don't exceed the school room and board COA for off-campus living, you will be ok.