Depends upon jurisdiction and who's asking. Fire code is different from tax assessor's concerns. Local real estate brokers usually know the criteria as far as advertising (when is a den or office also a possible bedroom). Some places require living space to have at least 6-foot ceilings and a bedroom to have a source of heat and a solid closing door to separate it from other rooms, same with a bathroom. Under that rule, a cold sleeping porch or attic/basement with beds do NOT qualify as bedrooms and any space beneath a low ceiling cannot be counted as living space. Some very old houses only have cupboards for storage in bedrooms, not necessarily built in.

I have lived in houses that had no closets at all, the local custom being armoire/wardrobe stand-alone storage in any room that needed it.

DEPENDs on your state some require 70sq ft others 90. But it must have a built-in closet to be defiined as such. A den could have an EFFECTIVE use as a bedroom though!

Generally, a bedroom must have a window of sufficient size to qualify for safety egress in the event of a fire, must have a permanent closet (not a free standing unit), must have a door, and must not be a room which is passed through to gain access to another room.

Some areas also have minimum sizes for rooms to be considered as bedrooms.