Hi, I was wondering if anyone knew the implications of a landlord not having a certificate of occupancy for a basement apartment. I currently live in one in Washington, DC, and my landlord is being very difficult about letting me out of my lease. I bought a house and have tried to work with her to find a replacement for me, but she's being unreasonable. I'm pretty sure she doesn't have a certificate of occupancy, and I was just wondering if anyone knew if that meant my lease is not legally binding. Could she successfully take me to court if I refused to pay my rent? Also, do you know if there is a general duty to mitigate damages in Washington, DC when someone has to break their lease for a legitimate reason (I bought a house). Thanks in advance!

You can take her to court. Without that COO, she cannot legally rent to you or anyone else. It will not wash if she says she is applying, she has to have that certificate.
If she threatens you, you can simply say, "let's go to court" and she will have to prove that she has this certificate.