Sammy bin Snoozin wrote:
I meant... what kind of underlayment, how thick, etc? Makes sense, but adding any thickness will bring the new section higher than the rest of the floor.

There are several things to consider for a floor tile installation as
Robert indicated. Not the least of which is the potential for
deflection or flex. I learned this the hard way once. I've done a few
tile installations over wood sub-floors since that snafu and I've
referred to this when I have.
http://www.ttmac.com/deflection_limitations.htm

Sometimes it's just no feasible for tile to be put over a sub-floor
given the existing construction, etc.