Hi @tjxme !
Interesting question! Firstly, to understand the source data, are your polygon geometries located at 0-m elevation? (just geometry, disregarding the attribute table) If no, you can use Geoprocessing toolbox → set Z value (might be called slightly different), and set your polygons Z coordinate to 0 (or even 77).
If the geometries are flat but you actually want them to be elevated, you can set Z value in the same way, choosing “set value from the field” and point to “ELEVATION” field.
Finally, you can add Speckle “Transformation” to extrude your polygons from whenever they currently are, and choose the field “DERIVED HEIGHT” for extrusion.
Additionally, if you want your building elevation to be correct relative to each other, but adjusted to 77m as “zero level”, just add a New Field in the attribute table, using the Field Calculator formula “elevation - 77”. Then you can use this new field instead of ELEVATION.
Let me know if I understood you correctly, or if the issue is somewhere else🙃