This flat in the basement was very nicely decorated, the bathroom was very modern and everything was spotless clean. However, there's 3 points that we didn't enjoy:
1) The house's main heating is next door, and it was so loud that we actually had trouble sleeping. Even watching TV in the living room was tricky, as you had to turn the volume high up. However, note that this might be a problem in winter only, and not an issue for your summer vacation in D.C.
2) The surrounding area is still a bit edgy, and we didn't really feel safe at night. It's also roughly 30mins away by public transport from the major attractions (e.g. the national mall), so please keep this in mind.
3) The notion of "private" here is a bit tricky. The flat was indeed 100% separate from the rest of the house, but you could hear people above walk around in the morning hours, which is not nice if you want to have a lie-in. Also, we had no personal contact with the host, which I believe counteracts a bit the airbnb spirit. I appreciate being welcomed and shown around at least once, to give the whole thing a more personal touch.