Block vs. Poured Concrete vs. Boulder Retaining Walls: Cost, Pros & Cons and Spokane's 4-Foot Permit Rule
Hillside lots in the South Hill, Five Mile and Spokane Valley foothills, and much of north Spokane County, often need a retaining wall to hold a flat yard, a driveway or a shop pad. The three common materials cost about the same per square foot, but they differ a lot in what they can hold back and in when the city or county will want engineered plans. Here's how they compare, using published 2026 prices and the actual local code language.
Side-by-side comparison
- Installed cost (HomeGuide, 2026): Modular / segmental block: $15–$35 per sq ft of wall face; Poured (reinforced) concrete: $20–$45 per sq ft; Boulder / natural stone: $25–$50 per sq ft (1, 2)
- CMU (cinder block) wall: Modular / segmental block: $20–$35 per sq ft (1)
- Look: Modular / segmental block: Uniform, many colors and textures; Poured (reinforced) concrete: Smooth, board-formed or stamped face; Boulder / natural stone: Natural and rugged
- Space needed behind the wall: Modular / segmental block: Geogrid layers for taller walls; Poured (reinforced) concrete: Least. A footing and stem wall handle the load; Boulder / natural stone: Wide, because the base boulders are deep
- Best for: Modular / segmental block: Terraced yards and garden walls up to about 4 ft; Poured (reinforced) concrete: Driveway cuts, shop pads, walls next to structures or parking; Boulder / natural stone: Natural slopes and large-scale landscaping
- City of Spokane permit exception: Modular / segmental block: Up to 4 ft (footing to top) if the manufacturer's guidelines are followed, there's no surcharge, and backfill is gravel; Poured (reinforced) concrete: Up to 3 ft above grade with a footing as deep as the wall is tall, level backfill and no surcharge; Boulder / natural stone: Up to 4 ft only if 15 ft or more from property lines (3)
Bottom line: on a straight per-square-foot basis the three overlap. The deciding factors are what the wall supports and how close it is to a property line.
What "per square foot" means on a retaining wall
Retaining walls are priced by wall face area: length × exposed height. A 40-foot wall that's 3 feet tall is 120 sq ft:
- Modular block: 120 × $15–$35 = $1,800–$4,200
- Poured concrete: 120 × $20–$45 = $2,400–$5,400
- Boulder: 120 × $25–$50 = $3,000–$6,000
A taller wall usually costs more per square foot, too. Excavation, drain rock, geogrid, engineering and equipment access all grow with height, so a 6-foot wall usually costs more than twice as much as a 3-foot wall.
The permit rules, in the code's own words
City of Spokane (BUILD-08)
The City's earth-retaining structures guidesheet requires a building permit and plan review, with plans sealed by a Washington-licensed professional, for retaining walls. A geotechnical analysis may also be required. There are three exceptions (3):
- Reinforced concrete walls up to 3 ft above grade, with the foundation buried at least as deep as the wall's height above grade, level backfill, and no surcharge (no parking, walkway or structure loads on top)
- Any retaining wall up to 4 ft from the bottom of the footing to the top of the wall, located at least 15 ft from property lines
- Modular block walls up to 4 ft from the bottom of the footing to the top, built to the manufacturer's guidelines, backfilled with clean gravel or crushed rock, with no surcharge or sloping backfill. Up to two terraces are allowed, spaced at least 2× the wall height apart.
Spokane County (unincorporated) and Spokane Valley
Spokane County's BP-25 exemption list includes retaining walls up to 4 ft measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge (4). Spokane Valley keeps the IRC R105.2 exemption language as written (5).
The two traps
- The 4 ft includes the buried footing. A wall that shows 3.5 ft above ground and sits on a 1-ft footing is 4.5 ft by code.
- Surcharge. A driveway, parking pad, shop or sloped yard above the wall puts extra load on it. That's when an "under 4 ft" wall needs engineering anyway.
Which wall makes sense when
Driveway cut or shop pad: The wall will carry vehicle loads, so treat it as surcharged and engineered. Reinforced poured concrete usually works best because it handles concentrated loads and needs the least room behind it.
Terraced backyard: Modular block in two short terraces can stay within the City's exception if the spacing and backfill rules are met, and it's usually the lowest-cost option.
Large rural slope: Boulder walls look natural and suit acreage, but the 15-ft property-line rule matters on smaller lots.
Next to a foundation or basement: Talk with an engineer. The wall and the house foundation interact.
Drainage is what makes a wall last
Whatever the material, retaining walls usually fail from water pressure behind the wall, not from the wall itself. Ask every bidder:
- Is there free-draining backfill (clean gravel or crushed rock) behind the wall? The City's modular-block exception requires it (3).
- Is there a perforated drain pipe at the base, and where does it outlet?
- Is there a filter fabric between the native soil and the drain rock?
- For poured walls, are there weep holes or a drain board?
Retaining wall planning checklist
- Measure total height from the bottom of the footing, not from grade
- Mark the distance to every property line
- List anything above the wall (driveway, parking, structure, slope)
- Confirm your jurisdiction: City of Spokane, Spokane Valley, County, Liberty Lake or another city
- Get sealed plans if any exception doesn't apply
- Have a drainage plan in writing
- Call 811 before excavation
FAQ
What are Spokane County's retaining wall requirements? Walls up to 4 ft from the bottom of the footing to the top are exempt from a building permit unless they support a surcharge. Setbacks and land-use rules still apply (4).
Do I need an engineer for a retaining wall in the City of Spokane? Yes, unless the wall meets one of the BUILD-08 exceptions. Plans must carry a Washington-licensed professional's seal (3).
What is the cheapest retaining wall? In HomeGuide's 2026 data, modular interlocking block has the lowest range at $15–$35 per sq ft installed (1).
How much does a concrete retaining wall cost? $20–$45 per sq ft of wall face installed, before engineering or unusual excavation (1).
Get a wall that's built to last
Raven Buildings forms and pours reinforced concrete walls and handles excavation, drainage and the flatwork on top. Call 509-350-2016 or request a quote. See our concrete flatwork and custom home builds pages.