What you are calling the 'build elevation' in the 3D window corresponds to the user origin. Just move your mouse to a hotspot that you want to be the new working origin - the top of that pony wall for example - and press alt-shift (Windows) or option-shift (Mac) and the 3D cursor will be placed there. Any elements you now model will have their heights relative to that point.
This works in all versions of AC. The new thing in 15 is that the 3D grid can display itself at that height with guidelines.
Cheers,
Karl
One of the forum moderators
AC 28 USA and earlier • macOS Sonoma 14.7.1, MacBook Pro M2 Max 12CPU/30GPU cores, 32GB