I currently have a full 200 AMP service. I 'm going to add a Mother-in-Law Quarters. May I add another 200 AMP sub panel? I can't have a separate meter because of zone restrictions. I'm thinking about a tankless water system for the addition. I'm just looking at my options before I go looking for contractors.

Most 200A panels will not accept a 200A branch breaker. Most are rated for 100-125A max branch breakers.

WHY do you need 200A to the M-I-L part of the house? Because of the water heater?

I would SERIOUSLY reconsider the on-demand unit.

That has been in the back of my mind, not having the tankless if it is going to be a lot of extra expense and work for the electrical. However my current 200 is maxed out. I added a RV garage and that took up my extra slots.