Use the Copy Parallel tool 
in the Tools gallery on the Parcel Record Workflows tab to
create parallel, offset copies of selected parcel lines. You can use the Copy Parallel tool can when constructing parcel boundaries and building parcels from seeds. A common scenario for using the Copy Parallel tool is a boundary line adjustment where a parcel boundary is offset for narrowing or widening of the parcel.
When there is an active record in the map, the Copy Parallel tool is enabled to work with parcel features. An additional Extend or Trim option is available in the Copy Parallel pane. Use the Extend or Trim option to prevent dangles and gaps when creating parallel lines. Choose Extend or Trim and specify a tolerance within which to extend or trim the copied lines. The copied parallel lines are extended or trimmed to snap to the first adjacent line encountered within the specified tolerance.
For example, in the image below, the offset parallel line copied from the selected line needs to be extended to snap to the northernmost parcel boundary.
Learn more about using the Copy Parallel tool in a parcel boundary line adjustment workflow