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Editor's Note: See § NR 115.05(1)(e), Wis. Adm. Code.
Establish impervious surface standards to protect water quality and fish and wildlife habitat and to protect against pollution of navigable waters. County impervious surface standards shall apply to the construction, reconstruction, expansion, replacement or relocation of any impervious surface on a riparian lot or parcel and any nonriparian lot or parcel that is located entirely within 300 feet of the ordinary high-water mark of any navigable waterway.
[Section NR 115.05(1)(e)1., Wis. Adm. Code] Percentage of impervious surface shall be calculated by dividing the surface area of the existing and proposed impervious surfaces on the portion of a lot or parcel that is within 300 feet of the ordinary high-water mark by the total surface area of that lot or parcel, and multiplied by 100. Impervious surfaces described in § 520-38 shall be excluded from the calculation of impervious surface on the lot or parcel. If an outlot lies between the ordinary high-water mark and the developable lot or parcel and both are in common ownership, the lot or parcel and the outlot shall be considered one lot or parcel for the purposes of calculating the percentage of impervious surface.
[Section NR 115.05(1)(e)2., Wis. Adm. Code] Except as allowed in §§ 520-37 through 520-39, allow up to 15% impervious surface on the portion of a lot or parcel that is within 300 feet of the ordinary high-water mark.
[Section NR 115.05(1)(e)3., Wis. Adm. Code] A property may exceed the impervious surface standard under § 520-36, provided that the following standards are met:
A. 
For properties where the general impervious surface standard applies under § 520-36, a property owner may have more than 15% impervious surface but not more than 30% impervious surface on the portion of a lot or parcel that is within 300 feet of the ordinary high-water mark.
B. 
For properties that exceed the standard under § 520-36 but do not exceed the maximum standard under Subsection A, a permit can be issued for development with a mitigation plan that meets the standards found in Article XII.
[Section NR 115.05(1)(e)3m., Wis. Adm. Code, and § 59.692(1k)(a)(5), Wis. Stats.] Impervious surfaces that can be documented to show they meet either of the following standards shall be excluded from the impervious surface calculations under § 520-35.
A. 
The impervious surface is treated by devices such as stormwater ponds, constructed wetlands, infiltration basins, rain gardens, bioswales or other engineered systems. The treatment devise must be able to handle stormwater discharge from storms up to and including a ten-year, twenty-four-hour storm event.
B. 
The runoff from the impervious surface discharges to an internally drained pervious area that retains the runoff on or off the parcel and allows infiltration into the soil. The pervious area must be able to handle stormwater discharge from storms up to and including a ten-year, twenty-four-hour storm event.
[Section NR 115.05(1)(e)4., Wis. Adm. Code] For existing impervious surfaces that were lawfully placed when constructed but that do not comply with the impervious surface standard in § 520-36 or the maximum impervious surface standard in § 520-37, the property owner may do any of the following:
A. 
Maintain and repair the existing impervious surfaces;
B. 
Replace existing impervious surfaces with similar surfaces within the existing building envelope;
C. 
Relocate or modify an existing impervious surface with similar or different impervious surface, provided that the relocation or modification does not result in an increase in the percentage of impervious surface that existed on the effective date of the County shoreland ordinance, and the impervious surface meets the applicable setback requirements in § NR 115.05(1)(b), Wis. Adm. Code.