I recently added a fifth bedroom to my septic and my town's regulations have changed in the 10 years since we put it in, so we are able to do it with no issue. It was something like 250 gallons per bedroom, so we had 1000, but now it's 200 per bedroom and 1000 is enough for 5 (this is just an example). Your town's health department can tell you, and they may have it online if you don't want to alert them. Hopefully you have information about the size of your system.

Building code wise, you will need an egress window in a basement bedroom.