I have a 100 amp and a 70 amp breaker boxes in my house.
Can I replace the 70 amp breaker box for a 200 amp breaker box with 30 spaces for the new addition?

Legally you can, but does this make ANY sense to you??

You have an existing 100A service.
You also have an existing 70A sub-panel.
You need several more spaces for an addition.

It is obvious that you need a service upgrade to 200 amps.
This should have been included in your budget for the addition. Didn't your electrician suggest this?

Thanks for that response. The reason why I'm asking is because I have yet to ask an electrician. I've been doing the job myself. But part of my question was if I can replace the 70 amp breaker box and keep the 100 amp breaker box.

Hi,

First thing you really need to do is map your branch circuits so you can tell what panel to is serving what connections.

Next you can start to plan where the new wiring needs to go.

But part of my question was if I can replace the 70 amp breaker box and keep the 100 amp breaker box.Like I said, yes, you legally can.