Attached is a sample Release 18.2 model archive of an area created with splines and its offset. The basic steps are: new sketch plane create sketch projection concept >lines from sketches … to create a line body of the boundary create > body transformation > scale… to make the smaller set of lines create sketch3 to be able to do the extrude in along that line extrude with imprint faces set -be sure merge topology is off to get separate faces The scale setting determines the amount of offset and trial and error is the only way I see to get a specific offset distance.

Attachments:
1. 2055835.zip