please post a file that shows your problem.
it s hard to help with 2d screenshot, that does not distinguish between curves and surface.
kind regards -tom
dear @user2134
looking at your input curve - they are composed with 1 to 3 segments (polycurves). they have very vivid curvature graph and are only G1 (tangent) continuity between the segments (line to curved part).
not sure what your design intention is - if you build single nurbs - curves with all same structure, loft and other surface commands will give very clean result, with very nice options to modify the surface after it s initial creation. (_setPt, _matchSrf, _moveUVN, gumball…)
to answer your initial question:
No - as fare as i know, there is no option to restrict a surface to be constraint within a certain coordinate-domain in z while it is created.
you can _setPt the controlpoints that a beyond your limits with the desired z-value.
you can create separate surfaces for the planar areas, define a helping surface for tangent or curvature condition at boundary or other surface approaches than a single loft.
hope this helps - to get more specific - you may need to post the file from the initial screenshots.
kind regards - tom