Garage Conversion

My house is 46 years old and on a concrete slab foundation. The garage portion (next to the kitchen) is 4 1/4 inches lower than the rest. We have tiled the kitchen with ceramic tile and now wish to continue the tile into what was the garage but will now be the laundry room. For the subfloor I am thinking:
1. Build a structure of 2X3's with 8 inches of space between the boards. The 2X3's would be standing up on their side so that the height is actually 2 1/2 inches. Should they be a certain type of wood?
2. Lay down 3/4 inch exterior grade plywood and screw in to 2X3's every 6-8 inches, (1/8 inch gap between boards).
3. Lay down 1/2 inch exterior grade plywood (the opposite direction & leaving 1/8 inch gaps) and screw to the 3/4 inch plywood but not the 2X3's.
4. Apply un-modified thin set mortor on top of the plywood.
5. Lay down 1/2 inch HardiBacker Board leaving 1/8 inch gaps between the boards. Screw it down with those big backer board screws and apply that mesh tape & thin set to the gaps.
That all adds up to 4 1/4 inches which will give me the proper height. Originally I was going to use 2X4's for the bottom structure (which would have been 3 1/2 inches high) then 1/2 inch plywood and then 1/4 inch backer board for a total of 4 1/4 inches. But, after reading your instructions I realized that I would not have the 1 1/8 inch minimum under the tile. With the scenario above I will have 1 3/4 inches under the tile. Is that too much? Also, if you have a better suggestion please let me know. I eagerly await your response.

Just install 8 layers of 1/2" backer board and top that off with 1/4" backer board to acheive 4 1/4 inches

Cement board is not structural like plywood so dont count it in the equation when thinking how much support is under the tile. With a 3/4" subfloor and a 1/2" underlayment, both plywood, you have adequate thickness. The cement board is there to give you a superior bonding surface

I could be wrong but I beleive this post is a joke.

Come on Robin Wisdom.