OP, we live in Silver Spring by the Metro (like right around the corner from it). Our house is smallish (1,600 sq ft?) and our AP room is upstairs with the rest of us and we all share the bathroom. I think our house is pretty updated and we keep it very neat and clean. The AP room is really cute - I decorated it the way I would decorate a room if I were single

and it has a pretty big closet given the size of the room. We also share our one car, but DH and I rarely use it since we take bus/metro most places anyway. We're not in a "hot" area really, although our AP spends most every weeknight at bars in SS and then it's an easy walk home. There are tons of other au pairs too. And it's also really easy to get to the hot DC areas, and we always have other suburban APs staying with us because they can't get to their giant private suites after clubbing until 3am.
So that's our set up. We're very clear in our profile (so before the AP even talks to us she knows) about our situation. We've had a number of candidates say they don't want to share a bathroom, which is completely fine. But all three times we've matched, we've also had our pick of who we want. Like we'll have 5 or 6 candidates that we like that would come to us if we said the word. They all know exactly what they're getting and we make sure to tell them that other people will have bigger houses with private AP areas. We play up the accessibility, the shopping and bars right out our front door, and the fact that we're actually a nice family that plays by the rules.
We've had three successful au pairs with this set up. Now I will say that one of them made comments probably once a week or so about the set ups her friends had. I think she was unhappy with her choice, but failed to see the whole picture. She seemed to think she could have found a family that never had her work and was really nice to her where she had a private suite and car. I've yet to truly hear about this family (honestly most of the ones that live in a huge house in Great Falls seem to want to treat their APs like maids in our experiences - not that it always happens, but it's a story I hear over and over!). This AP was our worst AP all around and we definitely chose poorly with her. The other two seemed to appreciate what they were getting and didn't seem to mind the small accommodations, at least not enough to say anything or be unhappy.
I think you'll be fine if you make sure you pick someone who doesn't have it in her head that she's coming to America where everyone has huge houses, and six luxury vehicles, and money will be handed to her for everything. There are definitely those au pairs out there. Look for the ones that are coming to be part of a supportive family and who are looking to have a great time in the city.