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20.20.460 Impervious surface.
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A. Purpose. Limits on the total amount of impervious surfaces associated with site development are desirable to protect critical areas and limit stormwater runoff, which are impacted by the increased levels and rates of surface flow generated by impervious surfaces.

B. Applicability. The impervious surface limits contained in LUC 20.20.010 and Chapter 20.25 LUC, and the standards of this section, shall be imposed any time a permit, approval, or review including land alteration or land development including subdivisions, short subdivisions or planned unit developments, a change in lot coverage, or a change in the area devoted to parking and circulation is required by this Code, or by the International Building Code.

C. Modifications to Impervious Surface Limits. The impervious surface limits contained in LUC 20.20.010 and Chapter 20.25 LUC may be modified pursuant to a critical areas report, LUC 20.25H.230, so long as the critical areas report demonstrates that the effective impervious surface on the site does not exceed the limit established in LUC 20.20.010 and Chapter 20.25 LUC; provided, that impervious limits within the Shoreline Overlay District may be modified pursuant to a shoreline special report or shoreline variance, as provided for by LUC 20.25E.050.C.2.

1. Garages on sites sloping uphill should be placed below the main floor elevation where feasible to reduce grading and to fit structures into existing topography. Garages on sites sloping downhill from the street may be required to be placed as close to the right-of-way as feasible and at or near street grade. Intrusion into the front setback, as provided in LUC 20.20.025.B, may be required. On slopes in excess of 25 percent, driveways shall be designed to minimize disturbance and should provide the most direct connection between the building and the public or private street; and

2. Changes in existing grade outside the building footprint shall be minimized. Excavation shall not exceed 10 feet. Fill shall not exceed five feet subject to the following provisions: all fill in excess of four feet shall be engineered; and engineered fill may be approved in exceptional circumstances to exceed five feet to a maximum of eight feet. Exceptional circumstances are: (1) instances where driveway access would exceed 15 percent slope if additional fill retained by the building foundation is not permitted; or (2) where the five-foot fill maximum generally is observed but limited additional fill is necessary to accommodate localized variations in topography.

D. Exemptions. The following are exempted from determining maximum impervious surface. These exemptions do not apply to any other Land Use Code requirement, including setbacks and limits on maximum lot coverage by structure, building code, utilities code or other applicable City of Bellevue codes or regulations.

1. Decks/Platforms. Decks and platforms constructed with gaps measuring one-eighth inch or greater between boards, so long as the surface below the deck or platform is pervious;

2. Rockeries/Retaining Walls. Rockeries and retaining walls shall be exempt from the maximum impervious surface limits;

3. Stabilization Measures. Shoreline stabilization measures shall be exempt from the maximum impervious surface limits; and

4. Landscape Features. Fences, arbors with lattice or open roof materials and similar structures, individual stepping stones placed in the ground but not cemented or held together with an impervious material, and gravel mulch shall be exempt from the maximum impervious surface limits.

E. Performance Standards.

1. Design shall minimize topographic modification. Structures shall conform to the natural contour of the slope. The foundation shall be tiered to conform to the existing topography and step down the slope with earth retention incorporated into the structure where feasible. Standard prepared building pads, i.e., slab on grade, shall be avoided; and

2. Garages on sites sloping uphill should be placed below the main floor elevation where feasible to reduce grading and to fit structures into existing topography. Garages on sites sloping downhill from the street may be required to be placed as close to the right-of-way as feasible and at or near street grade. Intrusion into the front setback, as provided in LUC 20.20.025.B, may be required. On slopes in excess of 25 percent, driveways shall be designed to minimize disturbance and should provide the most direct connection between the building and the public or private street; and

3. Changes in existing grade outside the building footprint shall be minimized. Excavation shall not exceed 10 feet. Fill shall not exceed five feet subject to the following provisions: all fill in excess of four feet shall be engineered; and engineered fill may be approved in exceptional circumstances to exceed five feet to a maximum of eight feet. Exceptional circumstances are: (1) instances where driveway access would exceed 15 percent slope if additional fill retained by the building foundation is not permitted; or (2) where the five-foot fill maximum generally is observed but limited additional fill is necessary to accommodate localized variations in topography.

F. Existing Impervious Surfaces. Impervious surfaces legally established on a site prior to December 31, 2016, and which exceed the limits set forth in LUC 20.20.010 and Chapter 20.25 LUC shall not be considered nonconforming. Proposals to increase impervious surface on a site shall conform to the limits of LUC 20.20.010 and Chapter 20.25 LUC; where a site already exceeds the allowed amount of impervious surface, the additional impervious surface shall not be approved unless an equal amount of existing impervious surface is removed such that the net amount of impervious surface is unchanged.

G. Innovative Techniques. Surfaces paved with permeable pavement or other innovative techniques designed to mimic the function of a pervious surface shall not be included in the calculation of impervious surface areas, so long as the technique is designed by a professional engineer licensed by the State of Washington and the plans are approved by the Director. These surfaces, however, shall be included in the calculation of maximum hard surface areas. The Director may require a maintenance plan and long-term performance assurance device to ensure the continued function of the permeable pavement or other technique. (Ord. 6417, 5-21-18, § 17; Ord. 6323, 11-21-16, § 6; Ord. 5876, 5-18-09, § 13; Ord. 5683, 6-26-06, § 9)

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