merge 2 building parts without connecting line?

Hello,

we have several 3D buildings that need to be extended downwards. That's why we applied the extruder in conjunction with the terrain model. After that we aggregated the two parts (aggregator).

However, then when we turn on the contours, there is the undesirable effect that there is a line between the 3D building and the extruder building part. (see example, highlighted in yellow)

How can we remove this line?

Or is there another way to extend the buildings downwards?