An attic can be considered a bedroom once it has a solid floor put down and meets the requirements of space. To be considered a bedroom, the attic must have at least 70 square feet of space. This is generally enough space to fit a single bed and a bedside table.
If you are turning an attic into a bedroom, make sure it has enough ventilation and light. Skylights are a great addition that adds character to the room and ceiling fans should only be used if the ceiling is high enough.