How to Build a Retaining Wall: Materials, Cost & Planning
A 20 ft × 3 ft retaining wall needs roughly 135 blocks, 1.5 cubic yards of gravel backfill, and costs $300–$900 DIY. Always use gravel behind the wall for drainage and step blocks back for stability.
A retaining wall does two jobs: it holds back soil on a slope and it adds structure to your yard. Done right, a DIY retaining wall under 4 feet tall is a manageable weekend project. Done wrong, it leans, cracks, and eventually fails — usually because of poor drainage or a missing base.
This guide covers planning, materials, cost estimates, and the step-by-step process for building a block retaining wall. For exact material quantities, use our retaining wall calculator.
Do You Need a Permit?
In most jurisdictions, retaining walls under 4 feet (measured from the bottom of the footing to the top of the wall) do not require a building permit. Walls 4 feet and taller almost always need a permit and should be engineered by a licensed professional.
Choosing a Wall Material
| Material | Cost/sq ft of wall face | DIY Friendly? | Notes |
|---|---|---|---|
| Interlocking concrete blocks | $3–$6 | Yes | Most popular DIY option, no mortar needed |
| Natural stone | $8–$15 | Moderate | Heavier, irregular — takes more time to fit |
| Timber / landscape ties | $5–$10 | Yes | Easier to cut, but rots in 10–15 years |
| Poured concrete | $12–$25 | No | Strongest, best for walls over 4 ft — needs forms |
| Boulder wall | $10–$20 | No | Requires heavy equipment for placement |
For most DIY projects, interlocking concrete blocks are the way to go. They are engineered to stack and step back for stability, require no mortar, and come in multiple textures.
Materials You Will Need
For a typical 20 ft long × 3 ft tall block retaining wall:
| Material | Quantity | Estimated Cost |
|---|---|---|
| Standard wall blocks (12" × 4") | ~135 blocks | $200–$400 |
| Cap blocks | ~17 | $50–$85 |
| Gravel backfill (¾" clean) | 1.5 cubic yards | $45–$80 |
| Base gravel (crusher run) | 0.5 cubic yards | $15–$25 |
| Landscape fabric | 50 linear ft | $20–$35 |
| 4" perforated drain pipe | 24 ft | $15–$25 |
| Construction adhesive | 2 tubes | $10–$15 |
| Total materials | $355–$665 |
Use our retaining wall calculator to get exact block counts and gravel amounts for your dimensions. Calculate gravel backfill separately with the gravel calculator.
Step-by-Step: Building a Block Retaining Wall
1. Plan and Mark the Layout
- Use stakes and string to mark the wall line.
- Call 811 to mark underground utilities (free in most states).
- Determine the highest and lowest points — the wall height varies along a slope.
2. Dig the Trench
- Width: block depth + 12 inches behind for gravel backfill.
- Depth: 6 inches below grade for the base (bury the first course below ground level).
- Level the bottom of the trench with compacted crusher run.
3. Lay the Base Course
- Spread 4–6 inches of compacted crusher run in the trench.
- Use a level on every block — the base course dictates the entire wall. If this row is off, every row above it will be too.
- Place blocks lip-side down for the first course.
4. Stack Remaining Courses
- Stagger joints like bricks — offset each row by half a block.
- The built-in lip on each block creates a slight setback, which leans the wall into the hill for stability.
- Check for level every 4–5 blocks.
5. Backfill with Gravel
- Line the back of the wall with landscape fabric to keep soil from clogging the gravel.
- Fill behind the wall with ¾-inch clean gravel, 12 inches deep.
- At the base, lay a 4-inch perforated drain pipe (holes down) on top of the base gravel, sloped to daylight at the end of the wall.
6. Place Cap Blocks
- Glue cap blocks with construction adhesive (one bead along the back edge).
- Caps create a clean, finished top edge and lock the final course.
7. Finish
- Fold the landscape fabric over the top of the gravel backfill.
- Backfill the remaining space above the gravel with native soil.
- Grade soil away from the wall so surface water drains to the sides.
Retaining Wall Cost: DIY vs. Professional
| Wall Size | DIY Materials | Professional Installed |
|---|---|---|
| 20 ft × 2 ft | $250–$450 | $1,200–$2,500 |
| 20 ft × 3 ft | $355–$665 | $1,800–$3,500 |
| 40 ft × 3 ft | $700–$1,300 | $3,500–$7,000 |
| 50 ft × 4 ft | $1,200–$2,200 | $5,000–$12,000 |
Professional installation makes sense for walls over 3 feet, curved layouts, or hillsides with drainage issues.
Common Retaining Wall Mistakes
- No gravel backfill: Soil against the back of the wall traps water, causing hydrostatic pressure that pushes the wall outward.
- Skipping the base: Stacking blocks on bare dirt leads to settling and leaning within the first season.
- Not leveling the first course: Every error in the base row compounds as you stack higher.
- Building too tall without engineering: Walls over 4 feet need geogrid reinforcement and professional design.
- Forgetting the drain pipe: Even with gravel, water needs a path to exit. A perforated pipe at the base is essential.
Related Guides & Calculators
- Retaining Wall Calculator — exact block and gravel quantities
- Gravel Calculator — for backfill and base gravel (see our driveway gravel guide for more on gravel types)
- Concrete Calculator — if you are pouring a concrete wall instead of stacking blocks (see our concrete cost guide)
- Fence Calculator — for fencing above retaining walls
- Paver Patio Guide — for the patio area above or below your wall
Ready to run the numbers?
Use our free calculators to get exact material quantities and cost estimates for your project.
Try the Calculators