Originally posted by Werric:

They are in game but you need to have the stone cutters table to be able to build them. As your re-building a stone building I assume you already know about the stone cutters table.

Unless I'm mistaken from the screen shot and those are not the stone floor tiles we can build.

The ones on the screenshot are 4 times bigger than the 2x2 we can build, and have a pattern.
Edit for clarity; the 16 squares highlighted in light blue are a single slab