The intent of the hillside development overlay standards is to provide for supplementary regulations to underlying zoning districts to ensure that development occurs in such a manner as to protect the natural and topographic character and identity of these areas, environmental resources, the aesthetic qualities and restorative value of lands, and the public health, safety, and general welfare of the citizens of Silverton by ensuring that development does not create soil erosion, sedimentation of lower slopes, slide damage, flooding problems, and severe cutting or scarring. It is the intent of these development standards to encourage a sensitive form of development and to allow for a reasonable use that complements the natural and visual character of the city. (Ord. 08-06 § 3, 2008)

A. General. The hillside development standards apply to all areas that have an average slope of 12 percent or greater and areas classified as having moderate or high susceptibility to shallow and deep seated landslides by the Department of Geology and Mineral Industries (DOGAMI). Notwithstanding any provisions of the development code to the contrary, hillside lots in subdivisions and partitions receiving tentative approval on or after the effective date of the ordinance codified in this title shall comply with the minimum lot size standards set forth in this article. Hillside lots in other areas may, at the option of the developer or owner, meet such standards in lieu of those specified elsewhere in the development code. A topographical survey of the lot shall be furnished by any person wishing to meet the standards of this article. For development of any property after the effective date of the ordinance codified in this title, the community development director or building official may require a topographic survey showing two-foot intervals wherever there is a question as to whether the lot is a hillside lot.

B. Exemptions.

1. An excavation or fill that is less than three feet in depth, or which involves less than 50 cubic yards of volume; or

2. Emergency actions required to prevent an imminent danger to public or private property, as determined by the community development director or designee; or

3. Any land use or activity that does not require a building or grading permit or land use approval. (Ord. 10-02 Exh. A § B, 2010; Ord. 08-06 § 3, 2008)

All development proposed within the hillside development overlay district is required to provide for the preservation and, if possible, enhancement of the site’s natural features during all phases of the design and development process. This includes consideration of soils, vegetation, hydrology, wildlife habitat, views and visual orientation, both from the site and to the site, and unusual or unique natural features. (Ord. 08-06 § 3, 2008)

Prior to issuance of a land use decision or a building permit on land that exceeds 12 percent average slope or contains areas classified as having moderate or high susceptibility to shallow-seated and deep-seated landslides by DOGAMI, a geotechnical study prepared and stamped by a geotechnical engineer and certified engineering geologist, following applicable guidelines for preparing engineering geologic reports in Oregon as adopted by the Oregon State Board of Geologist Examiners, indicating that the site is stable for the proposed use and development shall be submitted to the city. Peer review of a geotechnical study required by this chapter shall be performed by a professional or professional firm of the city’s choice that meets the qualifications listed above. The peer review will occur at the applicant’s expense.

The study shall be approved for a specific development plan and shall be valid for five years. Should the time limit expire for the approved study, if environmental conditions associated with the site change, or if the proposed development activities change, an amendment to the study shall be submitted for review. The study shall include the following information:

A. Index map.

B. Project description to include location, topography, drainage, vegetation, discussion of previous work and discussion of field exploration methods.

C. A scale-drawing site plan of the property, showing all natural physical features, topography at two-foot contour intervals, steepness of slopes, location of all test excavations or borings, watercourses both perennial and intermittent, ravines and all existing and manmade structure or features all fully dimensioned, trees six-inch caliper or greater measured four feet from ground level, rock outcroppings and drainage facilities.

D. A scale-drawing map for the property showing areas with zero to 11.9 percent slopes, 12 to 14.9 percent slopes, 15 to 24.9 percent slopes, 25 to 29.9 percent slopes, 30 to 34 percent slopes, and 34.1 to 100 percent slopes. The map shall include a square foot calculation for each slope category area on the site.

E. Site geology, based on a surficial survey, to include site geologic maps, description of bedrock and surficial materials, including artificial fill, locations of any faults, folds, etc., and structural data including bedding, jointing and shear zones, soil depth and soil structure.

F. Discussion of any off-site geologic conditions that may pose a potential hazard to the site, or that may be affected by on-site development.

G. Suitability of site for proposed development from a geologic standpoint.

H. Specific recommendations for cut and fill slope stability, seepage and drainage control or other design criteria to mitigate geologic hazards.

I. If deemed necessary by the engineer or geologist to establish whether an area to be affected by the proposed development is stable, additional studies and supportive data shall include cross-sections showing subsurface structure, graphic logs with subsurface exploration, results of laboratory test and references.

J. A scale-drawing grading plan, including all of the features and detail required for the site plan listed in subsection (C) of this section, but reflecting preliminary finished grades and indicating in cubic yards whether and to what extent there will be a net increase or loss of soil.

K. Signature and registration number of the engineer and/or geologist.

L. Additional information or analyses as necessary to evaluate the site.

M. Inspection schedule for the project. (Ord. 10-02 Exh. A § B, 2010; Ord. 08-06 § 3, 2008)

The developer has two options for the development of steeply sloped land. The first option, Option “A,” is designed to correlate minimum lot sizes to the average slope of the development area. The second option, Option “B,” is designed to allow, through a planned development, a density transfer bonus to stimulate development on those portions of the development area where the slope of the land is less than 12 percent. A combination of Options “A” and “B” may be used through a planned development.

A. Option “A”: Average Slope – Minimum Lot Size. In slope areas of 12 to 14.9 percent slope, the minimum lot areas shall be 10,000 square feet. In slope area of 15 to 24.9 percent, the minimum lot size shall be 12,000 sq. ft. In slope areas of 25 to 29.9 percent slope, the minimum lot size shall be 15,000 square feet and for areas of slope between 30 and 34 percent the minimum lot size shall be 20,000 square feet. All hillside lots created by following Option “A” shall meet the lot area and dimension standards of Table 2.6.140.

Areas with average slopes of greater than 34 percent prior to grading shall not be subdivided or partitioned further unless the area being divided is included in addition to the square footage required for building envelopes. However, open space, greenways and recreational trails may be developed in these areas. For those lots of record which have an average slope of greater than 34 percent and have no area less than 34 percent slope, the maximum residential development shall be one dwelling unit per lot of record. When density is transferred from areas in excess of 34 percent slope, density transfers shall be allowed at a rate of one dwelling unit per acre.

B. Option “B”: Planned Development Density Transfer Bonus. In order to promote the preservation of natural slopes greater than 12 percent, development density transfer is encouraged when dividing land. The density transfer is only feasible where there are sizable portions of the development area which have slopes less than 12 percent through a planned development process per Chapter 4.5 SDC. Determination of the density transfer bonus is a three-step process:

1. Determine the area of the parcel where the average slope of the land is:

a. Less than 12 percent.

b. From 12 to 14.9 percent.

c. From 15 to 24.9 percent.

d. From 25 to 29.9 percent.

e. From 30 to 34 percent.

f. In excess of 34 percent.

2. Determine the number of potential lots for the total development area which could have been permitted for the portion of the parcel where the average slope is greater than 12 percent, if the average slope option had been considered by using Table 2.6.140. Fractions shall be rounded to the nearest whole number.

3. Multiply the number of potential lots by 1.2 to determine the density that may be transferred to those sections of the development area where the slopes are less than 12 percent. Development shall occur in conformance with the provisions of the planned development process found in Chapter 4.5 SDC. Land of greater than 12 percent average slope subject to density transfer provisions shall be maintained as permanent open space or dedicated for park use. The findings and recommendations of the report required in SDC 2.6.130 shall be considered and utilized during the development process. (Ord. 21-04 § 14, 2021; Ord. 08-06 § 3, 2008)

All buildings and buildable areas proposed for hillside lands shall be designed and constructed in compliance with the following standards:

A. Building Envelopes. All newly created lots, either by subdivision or partition or property line adjustment, shall contain building envelopes conforming to the following standards:

1. The building envelopes shall contain a buildable area with a slope of 34 percent or less unless the division or lot line adjustment is for open space or conservation purposes.

2. Building envelopes and lot design shall address the retention of the following percentage of the lot in landscaping or in a natural state:

3. Building envelopes shall be designed and located to maximize tree conservation while recognizing and following the standards for fuel reduction if the development is located in wildfire lands.

4. Building envelope locations shall be located to avoid ridgeline exposures, and designed such that the roofline of a building within the envelope does not project above the ridgeline.

B. Building Setbacks. Setbacks shall be the horizontal distance from the property line to the nearest vertical wall. Site plans and building elevations submitted for building permits shall include the topography of the site at two-foot intervals and building elevations in relation to the existing and proposed finished grade.

C. Building Design. To reduce hillside disturbance through the use of slope-responsive design techniques, buildings on hillside lands shall incorporate the following into the building design and indicate features on required building permits:

1. Hillside Building Height. The building height shall be determined by the vertical distance from the lowest-grade plane to the average height of the highest roof surface. In no case will the height of the structure exceed 35 feet.

2. Fire Suppression. All buildings served by streets or roadways in excess of 10 percent grade shall have a fire suppression system installed.

3. Building Plans. All structures on hillside lands in excess of 14 percent shall be designed by an engineer or architect. Foundations shall be designed in accordance with a site-specific geotechnical report. (Ord. 21-04 § 15, 2021; Ord. 08-06 § 3, 2008)

Exposed soil which is not under continuous construction must be revegetated with temporary or permanent vegetation so that the soil is not left exposed for more than 60 days. Within six months of issuance of a certificate of occupancy, vegetation must be reestablished. If irrigation is not provided, then the exposed soil must be planted with species which can survive without irrigation. Vegetative cover or any alternative cover (rock, masonry, etc.) must be maintained in perpetuity. (Ord. 08-06 § 3, 2008)

All cut and fill slopes must not exceed a two (horizontal) to one (vertical) ratio. Slopes which are steeper (i.e., 1:1.5 or 1:1) may be conditionally approved by the public works director upon certification, by a qualified soils engineer or geologist, that the slope will remain stable under foreseeable conditions. The certification must delineate any specific stabilization measures deemed necessary by the soils engineer or geologist. As a condition of approval, a designated state certified person on the grading crew shall be employed by the developer to determine soil stability on all cut and fill slopes and take appropriate measure where unstable soils are encountered. (Ord. 08-06 § 3, 2008)

All development on lands classified as hillside lands shall be required to conform to the following items:

A. Plans. All grading, retaining wall design, drainage, and erosion control plans for development on hillside lands shall be designed by a geotechnical expert. All cuts, grading or fills shall conform to the most current State Structural Code. Erosion control measures on the development site shall be required to minimize the solids in runoff from disturbed areas. Temporary erosion control measures shall be taken throughout the course of development and construction, and permanent measures shall be taken thereafter to prevent erosion from foreseeable sources.

B. Time Period. For development other than single-family homes or duplexes on individual lots, all grading, drainage improvements, or other land disturbances shall only occur from May 1st to October 31st. Excavation shall not occur during the remaining wet months of the year. Erosion control measures shall be installed and functional by October 31st. Up to a 30-day modification to the October 31st date and a 45-day modification to the May 1st date may be made by the community development director, based upon weather conditions and in consultation with the project geotechnical expert. The modification of dates shall be the minimum necessary, based upon evidence provided by the applicant, to accomplish the necessary project goals.

C. Grading – Cuts. All cut slopes on areas classified as hillside lands shall be designed by a registered structural engineer, and the following standards shall apply:

1. Cut slope angles shall be determined in relationship to the type of materials of which they are composed. Where the soil permits, limit the total area exposed to precipitation and erosion. Steep-cut slopes shall be retained with stacked rock, retaining walls, or a functional equivalent to control erosion and provide slope stability when necessary. Where cut slopes are to be laid back (1:1 or less steep), the slope shall be protected with erosion control netting or a structural equivalent installed per manufacturer’s specifications, and re-vegetated.

2. Exposed cut slopes, such as those for streets, driveways, accesses, or yard areas, greater than seven feet shall be terraced. Cut faces on a terraced section shall not exceed a maximum height of four feet or five feet if building permits are obtained. Terrace widths shall be a minimum of three feet to allow for the introduction of vegetation for erosion control. Total cut slopes shall not exceed a maximum vertical height of 15 feet.

a. The top of cut slopes not utilizing structural retaining walls shall be located a minimum setback of one-half the height of the cut slope from the nearest property line.

b. Cut slopes for structure foundations encouraging the reduction of effective visual bulk, such as split pad or stepped footings, shall be exempted from the height limitations of subsection (D)(1) of this section.

3. Re-vegetation of cut slope terraces shall include the provision of a planting plan, introduction of topsoil where necessary, and the use of irrigation if necessary. The vegetation used for these areas shall be native or species similar in resource value, which will survive, help reduce the visual impact of the cut slope, and assist in providing long-term slope stabilization. Trees, bush-type plantings and cascading vine-type plantings may be appropriate. On slopes likely to be extensively disturbed by later construction, an interim ground cover shall be planted or other suitable temporary measures taken, to be supplemented by the permanent ground cover or shrubs and trees when the site is finally developed and landscaped.

D. Grading – Fills. On all fill slopes on lands classified as hillside lands, the following standards shall apply:

1. Fill slopes shall not exceed a total vertical height of 15 feet. The toe of the fill slope area shall be a minimum of six feet from the nearest property line.

2. Fill slopes shall be protected with an erosion control netting, blanket or functional equivalent. Netting or blankets shall only be used in conjunction with an organic mulch such as straw or wood fiber. The blanket shall be applied so that it is in complete contact with the soil, so that erosion does not occur beneath it. Erosion netting or blankets shall be securely anchored to the slope in accordance with manufacturer’s recommendation.

3. Utilities. Whenever possible, utilities shall not be located or installed on or in fill slopes. When determined that it is necessary to install utilities on fill slopes, all plans shall be designed by a geotechnical expert.

4. Re-vegetation of fill slopes shall utilize native vegetation or vegetation similar in resource value and which will survive and stabilize the surface. Irrigation may be provided to ensure growth if necessary. Evidence shall be required indicating long-term viability of the proposed vegetation for the purposes of erosion control on disturbed areas.

E. Re-vegetation Requirements. Where required, all re-vegetation of cut and fill slopes shall be installed prior to the issuance of a certificate of occupancy, signature of a required survey plat, or other time as determined by the hearing authority. Vegetation shall be installed in such a manner as to be substantially established within one year of installation. If such measures are considered ineffective, the director of public works or his designee shall prescribe alternative measures to control erosion. Upon the failure of the person responsible to institute such measures within 30 days of notice to do so, the director of public works shall obtain competitive bids and let a contract in the name of the city of Silverton for the performance of such work. All costs incurred thereby shall become a lien against the property to be assessed, collected, and enforced as provided for dangerous buildings in Chapter 15.08 SMC.

F. Maintenance, Security, and Penalties for Erosion Control Measures.

1. Maintenance. All measures installed for the purposes of long-term erosion control, including but not limited to vegetative cover, rock walls, and landscaping, shall be maintained in perpetuity on all areas which have been disturbed, including public rights-of-way. The developer or subdivider shall record covenants for the property to ensure the ongoing maintenance of erosion control measures.

2. Security and Penalties. Except for individual lots existing prior to December 1, 2005, after an erosion control plan is approved by the hearing authority and prior to construction, the applicant shall provide a performance bond or other financial guarantees in the amount of 120 percent of the value of the erosion control measures necessary to stabilize the site. Any financial guarantee instrument proposed other than a performance bond shall be approved by the city attorney. The financial guarantee instrument shall be in effect for a period of at least one year, and shall be released when the community development director and public works director determine, jointly, that the site has been stabilized. All or a portion of the security retained by the city may be withheld for a period up to five years beyond the one-year maintenance period if it has been determined by the city that the site has not been sufficiently stabilized against erosion.

G. Site Grading. The grading of a site on hillside lands shall be reviewed considering the following factors:

1. Hazardous or unstable portions of the site shall be avoided.

2. Existing natural drainage systems shall be utilized, as much as possible, in their natural state, recognizing the erosion potential from increased storm drainage.

3. Flow-retarding devices, such as detention ponds and recharge berms, shall be used where practical to minimize increases in runoff volume and peak flow rates due to development. The needs for an emergency overflow system to safely carry any overflow water to an acceptable disposal point shall be considered for each facility.

4. Stormwater facilities shall be designed, constructed and maintained in a manner that will avoid erosion on site and to adjacent and downstream properties.

5. Alternative stormwater systems, such as dry well systems, detention ponds, and leach fields, shall be designed by a registered engineer or geotechnical expert and approved by the city’s public works director or building official. (Ord. 21-04 § 16, 2021; Ord. 08-06 § 3, 2008)

In all slope areas, impervious surface drainage from roofs, driveways, and parking areas must be directed to a city storm drain or other city-approved drainage system. Development activities must not block the flow of stormwater in natural drainage ways without prior approval of an alternative solution by the public works director. With application for tentative plan approval for a subdivision or partition, the applicant shall submit a conceptual plan for stormwater control. The plans shall include but are not limited to the following items as appropriate: location of curbs, gutters, inlets, catch basins, detention facilities and stabilized outfalls. Prior to the signing of the final plat, the applicant shall submit a permanent and complete stormwater control plan. (Ord. 08-06 § 3, 2008)

A. Where practical, streets must be contoured to hillside areas in order to minimize environmental and scenic disruption. Street grades shall generally be 10 percent or less. Street grades of up to 12 percent may be permitted for a distance of no more than 300 feet only where topographical conditions make it impractical to meet the 10 percent standard. No intersections are permitted where street grades exceed 10 percent. Street grades over 12 percent will require variance approval by the approval authority. In no case shall a street grade exceed 15 percent for any distance. Street rights-of-way may be 40 feet in width with a minimum 28-foot-wide curb-to-curb improvement, with a five-foot-wide curb line sidewalk on one side and 10-foot-wide utility easements on both sides. The area within the utility easement shall be graded to meet the requirements of the private utilities.

B. A minimum three-foot-wide level shoulder shall be provided adjacent to all exposed public improvements such as sidewalks and streets in areas classified as hillside lands.

C. All constructed cut or fill slopes adjacent to street or sidewalk improvements must be entirely within slope easements, given to the city of Silverton, and shall be shown as such on both preliminary and final subdivision plats in areas classified as hillside lands. All retaining walls and other improvements, including landscaping, within the slope easement area shall in all cases be the property owner’s responsibility to maintain or replace.

D. Retaining walls necessary for public street improvements in areas classified as hillside lands and on public rights-of-way shall be designed and constructed as per the city’s standard specifications. The developer shall be required to reimburse the city for an independent geotechnical/structural design review by a qualified engineer, selected by the city, on all walls over five feet in height. (Ord. 08-06 § 3, 2008)

The maximum difference in elevation between the curb line and finished floor level of the garage or carport for driveways serving individual hillside lots shall be on a slope of 15 percent or less. The building official may permit a difference in elevation not exceeding a 20 percent slope; provided, that there is no slope exceeding 25 percent between any two points in the driveway, and that adequate vertical curves or ramps are used in the driveway to assure usability by a standard-size American automobile. A driveway serving a residence located more than 50 feet from the public road shall conform to the following standards, which are necessary for adequate access for emergency vehicles:

A. Be improved with an all weather surface with a minimum width of 12 feet and four feet of unobstructed clearance on both sides. The unobstructed clearance areas may consist of four-foot graveled shoulders on both sides;

B. Have a maximum grade of 12 percent for no more than 300 feet;

C. Driveways 150 feet in length or more shall be provided with a vehicle turnaround meeting the approval of the Silverton fire district; and

D. Provide adequate sight distance at the intersection with a public street. (Ord. 08-06 § 3, 2008)

Refer to Chapter 3.6 SDC for tree requirements. (Ord. 25-09 § 2, 2025; Ord. 08-06 § 3, 2008)

If the review body determines that the applicant’s plan adequately implements the policies of the comprehensive plan, the review body may modify the standards of this code as they apply to the entire proposed development, within the following limitations:

A. Front, side and rear yards may be reduced (when in conformance with the fire and building safety codes); provided, however, where attached dwellings are proposed there shall not be more than five dwelling units in any group.

B. The reduction of public right-of-way, pavement width, and/or requirements for the installation of sidewalks as specified in Chapter 3.4 SDC, Public Facilities, may be made if provisions are made to provide off-street parking in addition to that required in other portions of this code. Any reduction of these minimum street and sidewalk standards must be approved by the public works director. The additional parking requirements are as follows:

1. Detached Dwelling Units and Duplexes. Two additional off-street parking spaces.

2. Attached Dwelling Units. One-half additional off-street parking space for each bedroom more than one in each unit. In cases where a half space occurs in a total figure, the standard shall be increased to the next whole figure. (Ord. 21-04 § 17, 2021; Ord. 08-06 § 3, 2008)

In addition to the variance review criteria listed in Chapter 5.1 SDC, the following review criteria must also be met:

A. The variance is consistent with the stated purpose and intent of the physical and environmental constraints of the hillside development provisions.

B. The amount of deviation from standards is the least amount necessary in order to provide a practical solution to the environmental constraints of the hillside area.

C. The variance provides for an improvement in design or other characteristic better than would result from complying with the established hillside regulations. (Ord. 08-06 § 3, 2008)