i don't see anything wrong with charging an adult child room and board. my parents were clear that so long as we were attending school, if we chose to live at home it would be free of charge but beyond that we would be expected to pay room and board (granted, i don't think they ever considered any of us would pursue post grad degrees). i lived with my mother for several years after i graduated and reccognized i was responsible to meet this financial obligation.
op-has your dd EVER lived on her own and payed living expenses? it seems like many college grads have no clue on the realities of the costs b/c they were in dorms with bundled in meal plans/utilities/all inclusive and/or the parents were paying all the costs if they were in an apartment or shared housing. i know it was a real wake up call for my oldest when faced with seeing/paying the monthly utility bills, seeing how much groceries cost (esp. the odds and ends ingredients, spices, cleaning and hygene products that i always had on hand at home), learning the apartment complex required renter's insurance and how much it costs, getting the yearly lease renewal and seeing how much rent was going up

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i don't think you owe your dd any kind of explanation as to why you want to begin charging her-just that you and you dh have come to the decision that is IS TIME. you don't have to share your income and expense information with her (a landlord would not do so). you and your dh need to come to an agreement as to what is a realistic amount. i would not suggest it being crazy low b/c then it's not taken seriously and it becomes more of a gesture on your dd's part than an adult obligation. nothing crazy high but before you speak with her run the numbers on what you are spending on the household, keep in mind that you are providing food, utilities, laundry facilities, cleaning supplies, insurance (your homeowners is covering her as well), upkeep on the home...the figure may sound higher to her than she perceives renting on her own would cost but remind her of what is included. if she opts to stay, collect the room and board and do what you will with it (i'm not a fan of the saving it to give back to the adult child-then it's just mommy and daddy playing piggy bank). if she opts to go out on her own i suspect she will quickly learn that she has to curtail her expenses and her financial support of her bf/friends, in fact as she has to pay mandatory adult expenses and has less spending money she may tire of financialy carrying others (i've known many a young woman to ditch the boyfriend who did'nt carry his own weight when she reached the point of carrying her own fully and independantly).