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 OHWM of any navigable waterway.
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 OHWM by the total surface area of that lot or parcel, and multiplied by 100. Impervious surfaces described in § 242-35 shall be excluded from the calculation of impervious surface on the lot or parcel. If an outlot lies between the OHWM 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.
Each lot is allowed up to 15% of impervious surface.
For properties where the general impervious surface standard applies under § 242-33, a property owner may exceed 15% of impervious surface but shall not exceed 30% of impervious surface, provided that the following is met:
A. 
Properties exceeding 15% of impervious surface but not 30% of impervious surface can be issued a permit for development with an approved mitigation plan that meets the standards found in Article XII.
A. 
Impervious surfaces that can be documented to demonstrate they meet either of the following standards shall be excluded from the impervious surface calculations under § 242-32.
(1) 
The impervious surface is treated by devices such as stormwater ponds, constructed wetlands, infiltration basins, rain gardens, bioswales, or other engineered systems.
(2) 
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.
B. 
To qualify for the statutory exemption, property owners shall submit a complete permit application that is reviewed and approved by the County. The application shall include:
(1) 
Calculations showing how much runoff is coming from the impervious surface area;
(2) 
Documentation that the runoff from the impervious surface is being treated by a proposed treatment system, treatment device, or internally drained area; and
(3) 
An implementation schedule and enforceable obligation on the property owner to establish and maintain the treatment system, treatment devices, or internally drained area.
For existing impervious surfaces that were lawfully placed when constructed but that do not comply with the impervious surface standard in § 242-33 or the maximum impervious surface standard in § 242-34, 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 impervious surface meets the applicable setback requirements of § 242-23 or 242-24.