It appears that this addition will be attached to your house, if it is, the sub-panel does not need a main, if it is a detached building, it may need a main, see 225-31, 225-33.

As for the closet issue, depends how you feel about your mother-in-law and if the addition is attached to or well separated from your house. [Linked Image]

Tom

Few things are harder to put up with than the annoyance of a good example.