You will need 2 spaces to subfeed your other panel. Unless you plan on having everything on at once you should be fine subfeeding off the exhisting panel. When I get my code book back I can tell you exactly what you will need. Formulas are too many to keep in my head. Plus I been living commercial and residential is a little different when rating panels. If I lived closer I would do it for free, you buy material of course. I like installation for some odd reason.