Complete and proper street grading can be a challenge in Seattle because of steeply sloping and variable topography. Construction of street improvements requires grading the right-of-way to the standard design cross section at the permanent street grade. SDOT establishes grades for permanent improvements of each street and alley to minimize (as much as possible) adverse impacts on adjacent private property.

Private developments must be designed to accommodate this planned permanent street grade so when a street is fully improved with pavement, curbs, and sidewalks, the street grade will not

• Result in driveways that are too steep;
• Require retaining walls to protect foundations and landscaping; and
• Create the possibility of an inaccessible or unsafe condition.

Grading for Standard Design Cross Section : The standard design cross section consists of a crowned roadway centered in the right-of-way, sloping down at 2% from the crown to the gutter line, with a 6-inch high curb and a 2% slope up from the top of the curb to the right-of-way line. See the standard design street details in 4.10: Design Cross Section. New development shall be designed to accommodate the revised design cross section.

Permanent street grades : SDOT establishes permanent street grades at the right-of-way line for most of Seattle’s public rights-of-way. These standard profiles are available at the Seattle Public Utilities Records Vault. The approved street elevations form a design control for on-site development. In some cases, such as in steep slope areas, the graded section may not extend all the way to the right-of-way line or the roadway may be offset from the centerline. The standard profiles indicate when the grade line is not congruent with the right-of-way line.

Whether street improvements currently exist, are to be installed in conjunction with the project, or will be constructed at some time in the future, driveways and other permanent on-site structures shall be designed and constructed to accommodate the permanent street elevations at the right-of-way line. If the project does not require street improvements, refer to the Building Grade Sheet on-line tool to ensure your project meets the permanent established street elevations.

Centerline Profile : The centerline profile shall have a constant slope from cross-street to cross-street, with vertical curves as needed at street intersections. Additional slope changes within the block are permitted only when a constant slope cannot be obtained or when needed to accommodate street drainage.

Maximum Slope : The project shall be designed so that any new grading on site will not exceed the maximum slope permitted if it becomes necessary in the future to adjust site grades to accommodate grading for street improvements. The maximum slope permitted without a retaining wall is two horizontal to one vertical (2H:1V). Grades steeper than this require installation of a reinforced concrete retaining wall.

Survey : Because of the relationship between street grades and site grades, it is essential that survey information for both on-site and off-site improvements be based on NAVD-88 datum, using City of Seattle bench marks and monuments as reference points. For specific details go to -Washington Council of County Surveyors. When no profile has been established for the streets abutting and leading to the development site, the developer shall provide a survey of the street area by a licensed surveyor for the purpose of establishing the proposed centerline profile. SDOT must approve the centerline profile of the street and accompanying centerline elevations proposed by the developer's professional civil engineer.

The survey shall extend the full length of the block plus 50 feet on either side, show adequate cross section, and be based on NAVD-88 Datum. Include NAVD-88 marks with id numbers, descriptions, locations and elevation. Get more complete information on survey requirements.

Maximum/Minimum Roadway Grade : The following tables show crown elevations along the centerline of a traveled way.

If the proposed roadway cannot be kept below the maximum slope the project may meet the street improvement exception criteria defined in the Land Use Code, SMC 23.53.

Vertical Alignment : Vertical curves shall be based on appropriate design standards (City and County Design Standards, AASHTO or WSDOT Design Manual) and shall not be less then the following minimum vertical curves:

Horizontal Alignment : Design speeds are established by the City Traffic Engineer based on current engineering standards and practices. A minimum horizontal radius with a maximum 4% super elevation for urban conditions are as follows:

Regrading for Construction : When the area of the site adjacent to a public right-of-way is to be altered or is disturbed during construction, any regrading on the site and in the street shall be consistent with the established street elevations shown on the standard profiles. The design of on-site improvements such as foundations, footings, floor elevations, building entries, driveways, and utility service connections shall be compatible with all grading that will be required to install future street improvements. Curbs damaged during construction shall be replaced with a 6-inch high curb. This is especially critical to project design when the building and doorways are at or near the property line, where driveway slopes are at or near the maximum allowed, and where building height is at or near the maximum permitted by the Land Use Code.

• Consult the standard profiles in the early stages of project development because private property elevations must meet the approved street elevations at the right-of-way line.
• Profiles for many Seattle streets have already been determined by SDOT, including some streets not yet open to traffic. In some cases, permanent street elevations have not yet been established and must be created by the project applicant and approved by SDOT. This will require survey of the existing project site.
• It is highly desirable to consider ADA compliant crosswalk slopes in the design and placement of vertical curves. Therefore, placement of the In order to make a more level and therefore more accessible crosswalk, the point of vertical curvature (PVC) should be carefully considered.
• In general, the point of vertical curvature (PVC) shall not encroach in to a cross street any further than the cross streets’ center of pavement.
• Foundations and footings shall be designed and constructed so they will not be uncovered or undermined by future grading required for street improvements.