Penny, First time I've heard of crown on a joist moving that quick in a new house.( crown of a joist is where the heart of the joist is on the bottom )"that dont sound right" all trees have a heart in the center, now when they cut the trunk into planks you can see the rings that radiate from the center (heart) this is on all planks, look at the end of a plank you will see what I mean. When you look down a lenght of timber (thick plank) it will have a bend in it. like a bow, the top of the bow is called the crown, this is always on the top, then over the years it will straighten it self out. ---- tiles on wooden floor, there are many ways to do this. cover the floor with 3/4"- 18mm WBP plywood. ( 3/4" thick plywood a bit much, no not realy, remember your floor will move (wood) but your tiles will not, then the cracks will start. screw the plywood to the floor dont nail,nails will move after a time You will now have two levels, one in the bathroom the other where you come in. the carpenter will if the know his job will fit the floor starting at the doorway. When the tiles are fitted you will have a step of 3/4" + the thickness of the tile. fit a hardwood strip inside the doorway a lighter/darker colour than the floor, this is so you dont trip over it each time you go in. " How do I know all of this" I used to build wooden chalets in the French Alps and I learned these tricks over the years. I Hope all goes well for you.