Hi bushsnuffler
The best thing to do if you want to put a hog house straight on the ground is to put some wooden runners on the bottom, but you can also raise it up on bricks/paving slabs. Hogs have no problem climbing up bricks so if you were worried about the entrance you could put another brick there as a step.
Yes, it’s good to use untreated wood, but, coating the hog house in waterproof material could actually make it rot more quickly.
Yes, boiling water is essential to clean out hog houses to get rid of any parasite eggs – chemicals not appropriate.
https://www.hedgehogstreet.org/cleaning-out-boxes/
