Spokane Hillside Landscaping: What to Plan First

A sloped lot on the South Hill, in the Latah Valley or up on Five Mile Prairie can become a level, usable yard with terraces, boulder walls and stone steps. The ground decides how. Rock close to the surface, clay layers that can move when wet, and City rules for steep slopes all shape the plan, so they're worth settling before anything is built.

What's under a Spokane hillside

Much of Spokane sits on basalt, lava flows of the Columbia River Basalt Group; Spokane Falls is carved from one of them. Between and beneath the flows lies the Latah Formation, siltstone and claystone laid down in lakes the lava dammed. The USGS geologic map of Spokane County notes that the area's landslides sit mostly on the sides of prairies and bluffs, with the Latah Formation generally underneath them.

On parts of the South Hill, basalt comes close to the surface, so excavation is planned around rock instead of discovered halfway through. The same rock has been Spokane's wall stone for about a century: the City-County Historic Preservation Office's survey of native basalt points to Manito Park's stone bridge from the 1920s and the rock walls along Aubrey White Parkway, dry-stacked without mortar in the 1930s.

Check whether the slope is regulated

Inside city limits, the geologically hazardous areas code, SMC 17E.040.030, classes several kinds of ground as landslide hazard areas, including:

• any slope of 30 percent or more;
• slopes over 15 percent where permeable soil sits on clay or rock and springs or seepage show up;
• Latah Formation ground subject to landslides, and areas of uncompacted fill;
• slopes showing creep: trees with curved trunks, fence posts leaning downslope, retaining walls that lean or have broken.

Clearing and grading in those areas, even 50 cubic yards or less, falls under the City's critical areas rules and starts with a critical areas checklist (SMC 17E.040.050). The same code notes that irrigation adds water and weight to a slope, which is why the watering plan on a hillside matters as much as the walls.

Outside the City, Spokane Valley requires an engineered grading permit for grading in a geologically hazardous area (land disturbance checklist), and Spokane County's permit handout, BP-25, notes extra requirements for work in critical areas such as steep slopes.

Terraces and boulder walls

Terracing breaks one steep grade into a few level steps, each held by a wall, and it is the usual way to turn a slope into space you can use. Shorter walls are easier on the permit side too. Inside the City, the guide sheet for earth-retaining structures, BUILD-08, exempts modular block walls up to four feet from the bottom of the footing, with no more than two terraces spaced at least twice the wall height apart, clean gravel or crushed rock behind them and nothing loading the top.

Boulder walls suit Spokane's rock, but the City treats them differently: a wall that isn't modular block or reinforced concrete is exempt up to four feet only if it stands at least 15 feet from property lines. So a boulder wall near a property line generally needs a permit and sealed plans; confirm with the City's Development Services Center. In Spokane Valley and the county, the limit is four feet from the bottom of the footing, and a surcharge, such as a driveway at the top, takes the exemption away. Our retaining walls page covers how each jurisdiction handles walls.

Drainage and erosion

Water is the main thing to manage on a slope: it scours bare soil, softens clay and builds pressure behind walls. A hillside plan follows where water arrives, from the street, the roof and the lot uphill, and gives it a path down that won't cut a gully. Gravel backfill and drains behind walls, mulched or planted slopes instead of bare ground, and downspouts routed away from wall tops each do part of the work, and planting with deep roots helps hold the soil between the walls.

City drainage rules can apply as well. Moran Prairie and Five Mile Prairie are City-designated Special Drainage Districts (SMC 17D.060.130). The City Engineer can designate a lot as an Area of Drainage Concern because of poorly draining soil or topography (SMC 17D.060.135). And in the R1 and R2 zones, a lot needs an engineer's drainage plan once hard surfaces pass 60 percent of it, or 40 percent in an Area of Drainage Concern (SMC 17C.111.205).

Steps and paths

Steps are what make a terraced yard usable. Stone steps set into the slope, with even risers and a landing where the path turns, are easier to climb and safer after dark. Light matters here: the U.S. Naval Observatory puts sunset in downtown Spokane at 4:00 p.m. on the winter solstice, so step and path lighting earns its keep. Where steps meet a wall, they're designed with it rather than cut in afterward.

What to settle before building

1. The jurisdiction. City of Spokane, Spokane Valley or unincorporated county. A Spokane mailing address doesn't settle it; Spokane County's SCOUT property dashboard does, as the City's permit process page explains.
2. Property lines. They decide where setbacks fall and, inside the City, whether a wall is within 15 feet of a line.
3. The slope and the soil. How steep it is, where the rock and clay are, and whether there's seepage or creep. That decides whether the slope is a landslide hazard area and whether an engineer or a geotechnical report belongs in the plan.
4. Where the water goes. From the street, the roof and the neighbors uphill, down to somewhere it can soak in or leave without doing damage.
5. Utilities. Washington law requires marking the dig area in white and notifying 811 at least two full business days before digging (RCW 19.122.030). Avista typically owns lines up to the meter; private lines past it need a private locator.
6. Structures near the slope. Under the City's slopes and setbacks guide, BUILD-10, a structure near a slope of 3:1 or steeper sits back from the top by at least a third of the slope's height, up to 40 feet, and from the toe by at least half its height, up to 15 feet.
7. Access and phasing. How equipment and materials reach the slope and, if the work is phased, which parts go first. Grading, walls and drainage usually lead, because everything above them depends on them.

If you have a slope you'd like to use, start with a free on-site consultation. Call our Spokane office at (509) 352-3433 and we'll walk it with you.