Yo, use the hoe to raise the ground and flatten to just the hight of your floor.

Now place the camp fire on that ground. Level it back down to below the floor. Then place your floor block.

The fire doesnt get destroyed or damage anything.

now make a chimney, and be creative!

My most recent house is a circle, so the chimney is the center roof, and I have logs that were used to support, angled at the top with an opening to let smoke out.