House building questions

Hi @noblehousesrt.  Here is a link to a portion of a building guide that will explain how to create an open story (there are many more aspects to that particular guide that you may find helpful as well).  As for the grid increments, you are indeed constrained to go no smaller than the solid square grid line.  The grid square in game is usually said to be equivalent to about 2-1/2 sq. feet.  For more building inquiries/discussions, I invite you to take a look at our home on the forums: http://forums.thesims.com/en_US/categories/the-sims-4-gallery