Depending on the age of the home, a typical hydronic heat system is embedded within 1.5" to 4" of gypcrete or concrete.
Also, terrazzo is not usually installed as a tile. Terrazzo is similar to concrete in that it is a poured self-leveling installation procedure. It has a higher aggregate count of decorative stone and or shells. After it is cured it is polished.
Removal of the terrazzo may be difficult. Removal must be done by hand and with great care. Cracking gypcrete subfloor during the removal of the terrazzo tile could lead to broken seals and joints in the hydronic heat piping.
Hardwood flooring and engineered floors can be placed of hydronic heated floors. Since your floor is terrazzo, I would not expect to find wood nailers installed to allow for a proper solid hardwood floor to be installed.
I would recommend using an engineered hardwood. These tend to be more stable, minimizing the effects of expansion and contraction during times of heating or cooling.
Many engineered flooring systems are currently installed as a glue down application over hydronic heated floors here in the States as well as in Europe. When selecting your flooring ask and review the manufacturers installation specifications. These specification will outline the limitation of their flooring as it relates to certain types of installation.
Keep in mind, wood is an insulator and will not radiate the heat into the habitable spaces as efficiently as tile.
Harold J Remlinger, AIA, LEED AP BD+C, NCARB DesignTeam Plus, LLC