CoordinatorLayout ignores margins for views with anchor

I suggest an elegant solution for you:

<android.support.design.widget.FloatingActionButton
    ...
    android:translationY="-20dp"
    ...
/>

Try putting it in a linear layout that have padding:

<LinearLayout
  width=".."
  height=".."
  paddingBottom="20dp"
  app:layout_anchor="@id/appbar"
  app:layout_anchorGravity="bottom|center_horizontal">

  <android.support.design.widget.FloatingActionButton
        android:layout_width="70dp"
        android:layout_height="70dp"
        app:fabSize="normal" />

</LinearLayout>