Fireplaces, Heating Stoves, Flues and Chimneys - Remodeling fireplace wall; few Qs...

In a month I will be remodeling a wall with a fireplace and I want to make double check a few points. The house is in Saint Louis County unicorporated and is about 50 years old. The fireplace wall is cinder block and actually shares the cinder block wall with a fireplace in the adjacent room and another fireplace in the basement below. the firebox opening is 3' wide by 2'5" high with a 4' wide by 2' deep (extends 6" beyond the firebox opening on each side) concrete hearth that is almost level with the subfloor. Currently, the wall is covered with 1/2" furring strips with paneling attached to that. The plan is to build out the fireplace about a foot and put a stone finish on it and the hearth. Also, we want to build out the rest of the wall with a drywall finish. After doing much research and investigating I still have a few questions...

1. Can we actually extend out the fireplace with cinder blocks and the add the stone finish?

2. The plan for the hearth has a larger footprint than the current concrete hearth that is ALMOST level with the plywood subflooring. I am concerned with laying slate or any other stone on top of two different media along with the small height difference (less than 1/8" difference). Is there something I should do with the subfloor?

3. This is not an exterior cinder block wall. Should the sole plate of the new framed wall be pressure-treated? I did not think so but answers have been inconsistent online.

4. Should the frame of the new wall still be offset from the cinder block wall by 1/2" or so?

Any answers or advice will be greatly appreciated. Thank you again.


Dan