Use A Custom Unity Terrain

Documentation for Unity Asset Store version v1.20

Static/Background Custom Unity Terrains

You can add custom Unity terrains to your editor and/or game scenes. By default existing terrains will be used as not editable static background objects. If you want to allow the user to modify only a part of the terrain, then you could create your own background terrain, which is bigger than the user's terrain (or make a nine patch terrain where the centered terrain is the terrain that is modified by the user).

Default/Template Custom Unity Terrain

To make your custom terrain editable, select the 'LE_ConfigTerrain' instance (attached to 'LE_LevelEditorMain') of your editor scene and assign your custom terrain to the 'Custom Default Terrain' property in the inspector view. Doing so will skip the terrain creation phase in the terrain editor.
The level editor is completely independent of the UI implementation. The behaviour described above is true for the built-in UI example. Setting the 'Custom Default Terrain' will force the editor to call 'LE_GUIInterface.Instance.delegates.SetTerrainUIMode(ETerrainUIMode.EDIT)' which will hide the terrain creation UI in the example. If you want to use a custom UI, then you can find more information in this article.

This feature was introduced in v1.10 and is not available in older versions.