Shawn, I agree with everything that's been said but if you're like me, you're going to go ahead and build. I did. Unlike you, I only added two 14x22 foot rooms with sloped 10 foot ceilings, a 10x14 master closet and a sun room. If you're going to build anyway, one of the problems will be getting the area where the mobile and addition meet leak proof. Two by six walls will cost a little more but you will recoop the cost because you'll need a smaller HVAC unit. I built my rooms as stand alone rooms that were only attached to the mobile in the door ways. Any settling cracks were limited to the door openings. If I were putting a poured foundation under it, I would suggest that it be one continuous foundation, not one under the mobile and one under the addition. If you've never done this before you'll probably be wanting to move before it's finished unless you have a lot of help.