Hillside Development Overlay

The purpose of the HS-O district is to ensure the development of hillsides and foothills is consistent with the Comprehensive Plan and to ensure protection from hazards due to slope, erosion-prone soils, unstable soils, earth movement, and other geologic and hydrologic hazards.

These provisions shall apply to development on properties where the slope exceeds 15 percent or where adverse conditions due to slope stability, expansion soils, high water table and springs, erosion, or sedimentation are present as determined by the Planning Director or City Engineer.

See More On Sensitive Land Overlay Districts

Categories of Hillside Development Permits

The Planning Director (with input from the City Engineer) shall determine whether an application may be processed as a Category 1, 2, or 3 permit.

Category 1

Category 1 Permits are issued by the Planning Director for minor, routine construction on prepared building pads and single lots that do not involve significant grading. For example:

i. Single-Family Detached Dwellings or accessory structures placed on lots needing little modification, in a development for which a Category 3 permit has previously been issued.
ii. Single-Family Detached Dwellings or accessory structures placed upon lots of record that comply with approved building envelopes and limits to grading and for which Category 2 Permit criteria are not exceeded.

Category 2

Examples of the development requiring a Category 2 Permit are:

i. Exterior additions to existing structures;
ii. Grading with significant modification of approved topography including:

A. A retaining wall that is greater than four feet of exposed height or more than one retaining wall when the horizontal distance between retaining walls is less than 10 feet and the total of all exposed retaining walls exceeds four feet in height; or
B. An excavation or fill that exceeds the limits as defined by the International Building Code Chapter 18 and Appendix J as amended by Chapter 9-1 of the Boise City Code;

iii. Access roads or driveways in excess of 100 feet in length or in excess of 15 percent grade. Such driveways shall be reviewed for impacts on drainage and soil stability, emergency access, access to the public street and potential physical impacts on neighboring properties; or
iv. Multiple retaining walls located within setbacks, per Section 11-04-09, Landscaping, Fencing, Walls, and Screening.

Category 3

Category 3 Permits are for PUDs, Preliminary Plats, or grading involving modification of approved topography beyond that allowed under Categories 1 and 2 including:

i. Projects where the Planning Director, with input from the City Engineer, determines that slope stability or drainage problems exist.
ii. Projects involving modification of pre-graded lots in excess of 30 percent of the volume of previous excavation or fill or 30 percent of the surface area by square footage.
iii. Projects involving modification of lots with natural topography in excess of 30 percent of the surface area of the lot. 
iv. Projects not defined as a Category 1 or 2 but that fall under the purview of this Section 11-02-07.3.F.

Hillside Development Restrictions

Any area that presents one or more of the following limiting factors shall not be subject to development unless the project engineer can demonstrate satisfactorily to the City Engineer, based on the required technical reports, that these site limitations can be overcome in such a manner as to minimize hazard to life, hazard to property, and adverse effects on the safety, use, or stability of a public way or drainage channel. Such site limitations to be overcome shall include, but not be limited to the following:

(a) Landslide areas or scarps, or areas of active landslides;
(b) Lines of active faults;
(c) Areas with expansive soils or collapsible soils;
(d) Slopes greater than 25 percent; or
(e) High water table and springs.

Hillside Development Standards

  • Standards Applicable to All Categories of Hillside Development Permits
  • Grading Standards
  • Re-Vegetation and Erosion Control Standards
  • Hydrologic Controls
  • Roadways and Circulation
  • Maintenance
  • Adjustments
  • Inspection and Enforcement

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