The grade to which any work shall be done shall be that which is shown on the plans or profiles therefor or it may be done on the grade formally established by the legislative body. If any official grade has already been adopted or established for any of the streets, avenues, or other places or property proposed to be improved, it shall be lawful for the resolution of intention to provide that said work shall be done to new grades or grades different from those so established or adopted, and shall refer to plans, profiles or specifications for the description of the grade at which the work is to be done.