Building a shop. exterior wall height vs interior wall height

[cvse346]

The shop is 36x50 with an exterior wall height of 17'. Inside the shop will have a mezzanine that will extend 16' ft out from the back wall allowing me to have 8ft ceilings below and on top. I am trying to create the walls for the bathroom to an 8ft height. The program keeps making the walls 17' the same height as the exterior walls. How can I change the inside wall height only.
Thanks in advance