What if my roommate let them in?

Someone who shares authority over a premises can generally consent to a search of the areas they share. A roommate can typically consent to common areas.

Where it gets narrower. Authority doesn't extend to spaces that are exclusively yours — your bedroom, if it's yours alone, or a locked container that belongs to you. A roommate can't consent to what they have no authority over.

And presence matters. Where a physically present occupant objects to a search, that objection carries substantial weight even if another occupant consents. The practical lesson is that being there and saying no is meaningfully different from being absent.

Also relevant: whether the person who consented actually had the authority they appeared to have. Guests, ex-partners who've moved out, and visitors sometimes give consent they weren't entitled to give, and the officer's reasonable belief about that authority becomes the question.