What makes a room a bedroom?

Before we do anything we will obviously find out the correct answer for our area, but I was just curious what other people have been told about this. We are looking to buy a house, and need a 4 bedroom. Everything for sale where we are looking is only 3 bedroom, but some of them have an office or some space that we could easily turn into a four bedroom. The fourth bedroom will be for my step son, who is only allowed to spend 2 nights a month overnight with us (though we see him more - and would gladly keep him all the time). We don't want to have to upgrade the septic and spend all that time and money, and to see ahead of time if the land even perks for a 4 bedroom. So what defines a bedroom? Some people tell me that it is simply having a closet. So it we didn't put a closet in his room and just did a wardrobe, would we be safe? Or would we have to pull out the bed and wardrobe before an appraiser came to look at the house? I am just curious what other people have been told or know. We will get the correct information when we find a specific house we want to buy.