I have an enclosed porch that was once a patio (I bought the house with the porch). The slab is approximately 13' x 21'. Because it was once a patio the 2x4's on the bottom of the walls are beginning to rot because the slab is 3" to 4" lower than the house slab. The roof of the room is part of the house roof, not an add-on. What I want to do is support the existing roof with 4x4's and remove the walls. I then want to raise the slab of the porch and add approximately 4' x 13' on one end of the slab for a half bath. I will then rebuild the walls to include the add-on. The porch has ceramic tile on the floor. After this explanation, my question is, can I pour concrete over the tile or do I need to remove it? Also, will the weight of the add-on and the extended roof weight be a problem as far a potential for cracking. I am trying to not have the added expense of removing the existing slab if possible, but I don't want problems later.

New concrete won't adhere to the tile or even the concrete under it sufficiently for the size and weight on it. It will crack. Concrete is actually porous. It's about 18% air spaces.
You don't say how deep the slab is, but I'll bet it's not enough to build a real room on. And then you'd get in trouble with the building inspector and assessor.
My house had a 12 x 24 roof extension over a carport and the middle post drove the footing down 10", bending the whole house roof down with it. The roof took it OK but it was pretty scary and noisy cracking sounds when it was being jacked back up.