5% is quite common. However in some areas that would be very low. Mesa, Arizona for example has 35% vacant housing units. It might also depend on which side of the fence you are asking from? Seller's naturally estimate a very low or non-existant vacancy factor to keep the NOI up. A buyer would tend to estimate vacancy higher to reduce the NOI and thus reduce the price. If you are a buyer you will no doubt ask the seller to show you the income and expenses. From that you can calculate the actual vacancy.
Paul Sylvester, CCIM
May 20, 2009