For single-family, cottage dwellings, duplexes or triplex style dwellings, the minimum setback shall be 30 feet from all property lines in the residential districts, unless the Planning Board determines that a reduced setback is necessary to achieve the purposes of this section and will not have a detrimental impact on the neighborhood. The minimum setback from all property lines for assisted-living residence, independent living units, or any buildings in a continuing-care retirement community shall be 50 feet in all districts, except that the minimum setback shall be 100 feet from the side or rear lot line, as applicable, abutting an existing single-family dwelling. Nothing in this section shall preclude the Planning Board from reducing or waiving minimum setback requirements between buildings to internal lots created within the SLC. The first 15 feet of all setbacks shall be preserved as natural vegetation or professionally landscaped trees/plantings.