Depending on the type of retaining wall, it might not be possible.
You may need to contact a landscape "structural engineer" as it were. Years ago, our neighbors built a retaining wall to extend their sloping backyard three times. The first two times, the retaining wall buckled. The third time was a "charm" and a different contractor. They moved away a while ago and I have no idea who they used.
A deck may give you more usable space and may be less expensive, so I would research those options too.