Ordinance Text
8-2-30: HILLSIDE (HS) OVERLAY DISTRICT:
A. Purpose: The purpose of the Hillside Overlay (HS) District is to:
1. To provide for public safety and welfare in the development and design of building sites, roadways, and other service amenities;
2. To preserve and enhance the hillside landscape by encouraging retention of natural features, such as drainage swales, streams, slopes, ridgelines, crests of hills, rock outcroppings, vistas, and natural formations;
3. To provide safe ingress and egress for vehicular, bicycle, and pedestrian traffic to and within the hillside areas, while at the same time minimizing the scarring effects of hillside development; and
4. To use the planning, design, and engineering professions and the natural sciences including botany, biology, ecology, soils, and geology to mitigate potential hazards, and to enhance the existing and future appearance and resources of the hillsides.
B. Applicability:
1. Trail development that does not exceed eight feet (8') in width and three feet (3') cut, or three feet (3') fill shall be exempt from the provisions of this Section.
2. Except for as allowed in subsection B1 of this Section, the regulations of this Section apply to any grading, filling, clearing, or excavation of any kind where slopes exceed twenty-five percent (25%).
3. The Hillside areas shall be designated in two (2) levels:
a. Level 1: Areas where the slope is at least twenty-five percent (25%).
b. Level 2: Areas that meet Level 1 criteria and that have special hazard areas as described in subsection E2.
C. Application Requirements:
1. Application: An application, as set forth in Chapter 6 of this Title, shall be submitted to the Director on forms provided by the Development Services Department. Level 2 requires all information described in subsection E2 to the County Engineer and the Director.
2. Preliminary Grading Plan: Levels 1 and 2 require a preliminary grading plan designed to ensure that the properties within the development are able to conform to the requirements of a final grading plan as set forth in subsection E. The following items shall be included in the preliminary grading plan:
a. Contour lines at five-foot (5') intervals; the maximum allowable side slope on any cuts and fills is 2:1;
b. The location of all proposed or existing structures and roads;
c. Any areas of cut or fill;
d. Any areas with characteristics listed in subsection E2 of this Section; and
e. A narrative indicating how the proposed design complies with the purpose statement of the Hillside Overlay District.
3. Slope Stabilization And Revegetation Plan And Report: A licensed landscape design professional shall prepare the slope stabilization and revegetation plan. The report shall include a complete description of the existing soils, existing vegetation, the vegetation to be removed and the method of disposal, the vegetation to be planted, soils amendments and pH adjustments, and slope stabilization measures to be implemented. The plan shall include an analysis of the environmental effects of such operations including the effects on slope stability, soil erosion, water quality, and fish and wildlife.
4. Engineering Hydrology Report: A professional engineer registered in the state of Idaho shall complete an engineering hydrology investigation and report. This individual should be experienced and knowledgeable in the science of hydrology and in the techniques of hydrologic investigation. This report shall include the following information:
a. An adequate description of the hydrology of the site, conclusions on the proposed development, and opinions and recommendations covering the adequacy of sites to be developed. The report shall include results of field investigations of the site, unless existing information is determined by the County Engineer to be sufficient to satisfy the purpose of this Section.
b. The Hydrology Report shall use the best available data and standard engineering practice.
5. Soils Engineering Report: Any area proposed for development shall be investigated to determine the soil characteristics.
a. This report shall include the following information:
(1) Data regarding the nature, distribution, and strength of soils.
(2) Conclusions and recommendations for grading procedures, design criteria for corrective measures, and opinion and recommendations covering the adequacy of sites to be developed.
(3) Results of field investigations of the site unless existing information is determined by the County Engineer to be sufficient to satisfy the purpose of this Section.
b. The investigation and report shall be completed by a professional engineer registered in the state of Idaho. This individual should be experienced and knowledgeable in the practice of soils mechanics.
c. Recommendations included in the report shall be incorporated into the design plan and specifications.
6. Engineering Geology Report: Any area proposed for development shall be investigated to determine its geological characteristics. This report shall include the following information:
a. A description of the geology of the site, conclusions, and recommendations regarding the effect of geologic conditions on the proposed development, and the opinions and recommendations covering the adequacy of sites to be developed. The report shall include results of field investigations of the site, unless existing information is determined by the County Engineer to be sufficient to satisfy the purpose of this Section.
b. The investigation and report shall be completed by either a professional geologist registered in the state of Idaho or by a professional engineer registered in the state of Idaho. This individual should be experienced and knowledgeable in the principles and practices of engineering geology.
c. Any area in which the investigation indicates geologic hazards shall not be developed unless the project engineer can demonstrate to the County Engineer, based on the required engineering reports, that these hazards can be overcome in such a manner as to prevent hazard to people, hazard to property, adverse effects on the safety, use or stability of a public way or waterway, and adverse impacts on the natural environment.
d. Recommendations included in the report shall be incorporated into the design plan and specifications.
7. Visual Impact Report: This report shall be prepared by a licensed design professional in the state of Idaho and shall be submitted with the development application. The report shall include the following information:
a. The view from key vantage points along public roadways or public viewing areas that depict the existing view (prior to development) and the proposed view (after development).
b. Proposed screening methods such as architectural design, designated building envelopes, height restrictions, landscaping, fencing, construction materials, and colors.
c. The existing vegetation and the proposed method of preserving and/or replacing such vegetation.
d. A statement detailing how the proposed development or subdivision minimizes grading through careful site and roadway design.
8. Other Pertinent Data: Any other pertinent data deemed necessary by the engineer of record or by the Director, after consulting with the engineer of record, to satisfy the stated purpose of this Section and that is reasonably related to the health, safety, and welfare of the general public and persons who might purchase the property being developed.
D. Process:
1. The preliminary grading plan, prepared by the engineer of record, shall be submitted with the development application. The Director shall forward the preliminary grading plan and other documents required by this Section to the County Engineer for review and recommendation.
2. Upon review of the study, the County Engineer shall forward a recommendation on the grading plan to the Director. The Director shall apply the standards listed in subsection E of this Section and the required findings listed in subsection F of this Section to determine whether or not to approve, approve with conditions, or deny the preliminary grading plan.
3. No grading, filling, clearing, or excavation of any kind shall be initiated until the preliminary grading plan is approved by the Director, and the development has received final approval by the decision-making body. For subdivisions, Board approval of the preliminary plat is deemed the final approval.
4. When required by the Director, special inspections and special testing shall be performed to verify conformance with this Section. The cost of special inspections and special testing shall be paid by the applicant or owner.
E. Standards:
1. General Standards:
a. Construction shall be scheduled to minimize soil disturbance between December 1 and April 15.
b. The County Engineer may require the grading operation and/or project schedule be modified if delays occur which result in weather-generated problems not anticipated at the time approval was granted.
c. All development shall take into account land use planning, soil mechanics, hydrology, civil engineering, the environment, architectural and landscape design, and related disciplines as applied to hillside areas.
2. Development Of Special Hazard Areas: Any area that presents one or more of the following limiting factors shall not be permitted to develop unless the engineer of record can demonstrate to the County Engineer, based on the required engineering reports, that these limitations can be overcome in such a manner as to prevent hazard to life, hazard to property, adverse effects on the safety, use or stability of a public way or drainage channel, and adverse impact on the natural environment:
a. Landslide areas or scarps, or areas of active landslides.
b. Lines of active faults.
c. Soils with a high shrinkage-expansion potential and hydro- compactible soils.
d. Soils with unified classification of ML, CL, OL, MH, CH, OH, and Pt as listed in ASTM.
e. Natural slopes greater than twenty five percent (25%).
f. Water table within six feet (6') of the surface at any time of the year.
g. Any area of hydrologic hazard as determined by the engineering hydrology report.
3. Vegetation:
a. Vegetation shall be removed only when absolutely necessary, such as for the construction of structures, filled areas, roadways, firebreaks, or as required by this Title.
b. An effort shall be made to conserve topsoil that is removed during construction for later use on areas requiring revegetation or landscaping, e.g., cut and fill slopes.
c. Vegetation or a chemical or mechanical stabilization method sufficient to stabilize the soil shall be established on all disturbed areas, except for proposed rights of way, as each stage of grading is completed.
4. Grading And Stabilization:
a. All retaining walls with a total vertical height of four feet (4') or more, including footing, shall be designed in accord with the regulations of the Ada County Building Code.
b. All slopes that are stabilized by mechanical or chemical means shall conform to the surrounding terrain and shall be given aesthetic treatment that is consistent with the purpose of this Section.
c. Large tracts shall be divided into smaller workable units on which construction can be completed within one construction season so that large areas are not left bare and exposed during the winter-spring runoff period.
d. All disturbed soil surfaces shall be stabilized or covered prior to December 1. If the planned impervious surfaces (e.g., roadways, driveways, etc.) cannot be established prior to December 1, a temporary treatment adequate to prevent erosion shall be installed on those surfaces.
5. Hydrologic Controls:
a. Curb and gutter, pavement, and appurtenant roadway drainage facilities shall be designed to control roadway runoff to accommodate the 100-year design storm.
b. Watercourses shall be riprapped or otherwise stabilized below drainage and culvert discharge points for a distance sufficient to convey the discharge without channel erosion.
c. Any material from construction, including soil and other material, shall not be deposited within any floodway or watercourse.
d. All new water courses shall be designed to accommodate the 100-year design storm.
e. With the exception of roadway crossings, approved drainage structures, and recreation and open space uses that do not involve the destruction of vegetative cover, development shall be prohibited within the 100-year floodplain for major watercourses.
6. Sediment:
a. All construction must meet current EPA and IDEQ erosion and control rules.
b. The overall drainage system shall be completed and made operational at the earliest possible time during construction. No certificate of occupancy shall be issued on the property until the drainage system is complete.
c. Alterations of major watercourses shall be prohibited, except for approved roadway crossings and drainage structures.
d. Natural or improved open channels shall be preserved, or provided for, in watercourses, except that at roadway crossings, conduits may be permitted.
7. Roadways And Circulation:
a. Roadway alignments shall be designed to create the minimum feasible amounts of land coverage and the minimum feasible disturbance of the soil.
b. Roadway alignments shall be designed to minimize removal of existing deep rooted perennial vegetation.
c. Roadway alignments shall be designed to follow natural terrain.
d. The width of the graded section shall extend five feet (5') beyond the travel right of way line on both the cut and fill sides of the roadway for slope rounding purposes. If a sidewalk is to be installed parallel to the roadway, the graded section shall be increased by the width of the sidewalk plus five feet (5') beyond the sidewalk.
e. Combinations of collective private driveways, cluster parking areas, and on street parallel parking bays shall be used where possible to attempt to optimize the objectives of minimum soil disturbance and minimum impervious cover.
f. Approval of the applicable transportation authority is required for publicly dedicated streets.
g. Paving shall commence within one year of County Engineer approval of a drainage study or drainage plan.
8. Alternatives: The Director may approve, or recommend approval of, an alternative development proposal if:
a. The overall design, as proposed, meets, or exceeds the intent and the requirements of this Section and shall not be detrimental to the public health, safety, or welfare; or
b. The project involves grading, filling, clearing, or excavation of less than four hundred (400) square feet.
9. Maintenance: The owner of any private property on which grading or other work has been performed, pursuant to a grading plan approved subject to the regulations of this Section, shall maintain in perpetuity and repair all graded surfaces and erosion prevention devices, retaining walls, drainage structures, means, and devices not subject to the jurisdiction of the applicable transportation authority, and plantings and ground cover installed or completed. Such requirements shall be incorporated into the protective covenants for a subdivision and the conditions of approval for development applications.
F. Required Findings:
1. The site is physically suitable for the design and siting of the proposed development. The proposed development shall result in minimum disturbance of hillside areas;
2. The grading and excavation proposed in connection with the development shall not result in soil erosion, silting of lower slopes, slide damage, flooding, severe scarring, or any other geological instability or fire hazard that would adversely affect the public health, safety, and welfare;
3. Areas not suited for development because of soil, geology, vegetation, or hydrology limitations are designated as open space use;
4. Disruption of existing native vegetation and wildlife habitat is minimized; and
5. The proposal sets forth sufficient and adequate mitigation for the identified visual impacts beyond the normally expected impact of hillside development.
G. Waiver Of Standards:
1. The Director, upon the recommendation of the County Engineer, has the authority to grant a waiver if the engineer of record can demonstrate conclusively that any of the standards required by this Section are not necessary in the proposed development, and that the omission of such requirements would not result in any of the following:
a. Hazard to life.
b. Hazard to property.
c. Adverse effects on the safety, use, or stability of a public way or drainage channel.
d. Adverse impact on the natural environment.
2. The request for a waiver of standards shall be in writing and shall state the reason for the request. The request shall be signed and have the professional seal of the engineer of record. The justification for the waiver shall be based on the engineering reports required in subsection C.
H. Engineer Of Record Responsibilities:
1. To ensure that the intent of this Section is attained through the principles and practices of civil engineering, the applicant or owner shall retain a professional engineer currently licensed in the state of Idaho to serve as the engineer of record. It shall be the responsibility of the engineer of record:
a. To prepare the preliminary grading plan.
b. To incorporate into the grading plans all recommendations contained in the soils, geology and hydrology reports, and the slope stabilization and revegetation plan and other reports as required in this Section.
c. To inspect and certify all work within the project.
d. To act as coordinating agent for liaison between other professionals, the owner, and the Director.
e. To communicate any substantial plan changes due to field conditions with the final substantial completion letter.
2. Prior to and during grading operations, the engineer of record shall submit all necessary reports, compaction data, soils, geology, and hydrology recommendations to the Director.
3. If, in the course of fulfilling the specified responsibilities, the engineer of record discovers that the work is being accomplished to a substantially lesser standard than required by this Section or by the approved final grading plan, the noncompliance shall be reported in writing to the Director within three (3) working days with recommendations for corrective measures, if applicable. The Director may require corrective action within a specified period of time. If any deficiency is deemed by the Director to be an imminent threat to the public health, safety, or welfare, that work shall be stopped immediately until such deficiencies are corrected.
4. If the engineer of record, the soils engineer, the geologist, the professional landscape architect, or the hydrologist of record is replaced during the course of work, the work shall be stopped. Work may resume when the replacement individual has agreed to accept the responsibility for certifications of the work within the areas of their individual technical competence.
5. If work is stopped during inclement weather, all open, and/or unfinished work on the project shall be protected to the satisfaction of the Director.
6. At the conclusion of the approved work, the engineer of record shall submit a report to the Director stating that the work has been executed in compliance with the approved plans. The report shall have the signature and professional seal of the engineer of record.
(Ord. 1002, 10-28-2025)
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