Depends on purpose of considering it. If it is up to you, call it what you want. It only needs to be determined one way or another if it is required for some purpose, like a government housing assistance program (rental or financing). They (e.g. HUD) will have guidelines to detemine it. The local realtor association may also have guidelines as to what can be marketing via their MLS as a bedroom, so you could check there it if helps your case. In the end, a homeowner can call any room anything they want, but do you agree and value it the same as they do? If the space is what you need and the value is there and there are no lender requirements regarding bedrooms, call it whatever you want. But more to your point, guidelines that do address this often require some type of emergency egress, which it sounds like this has. Best of luck to you!