The length of your wall is what's known as linear footage. Square footage considers both length and height. To estimate your wall's square footage, multiply your desired length by the height, and then multiply that figure by 1.12. This estimate covers the wall you'll see above ground and the portion that provides support underground. This gives you an excellent starting point for estimating retaining wall cost.

Concrete specialists and hardscape contractors both are equipped to build retaining walls. Hardscapes are those hard surfaces in your landscape, such as retaining walls, patios and sidewalks. A landscape architect may also be involved. Each type of professional includes different services in their project cost, but most include free estimates to get you started.

Concrete installation professional Henry Fuapau of 4Sons Concrete in Portland, Oregon, says his company's typical pricing ranges from $23 to $28 per square foot to install a concrete retaining wall. That covers basic excavation and materials, including steel reinforcement and framing to hold the poured concrete. Tall retaining walls often involve multiple concrete pours, which adds to your final wall costs.

Here are some questions concrete specialists ask to determine additional costs for your retaining wall:

• Does an existing wall need removal? Demolition means higher labor costs, but an old wood or timber retaining wall usually requires less labor to remove than an old concrete wall.
• Are trees or shrubs growing on the site? Most basic wall pricing assumes the area is relatively clear and ready for excavation. If brush covers your retaining wall site, you may pay more to have it cleared.
• Where is the wall located? The easier it is to access your job site, the lower your costs will be. Contractors save time and money when they can pull trucks and machines right up to the site. If crews have to haul equipment and materials by hand, labor costs—and your costs—rise.
• Do you have a design or landscape plan for the wall? If you don't have a plan and aren't sure what you want, your contractor may bring in a designer or landscape architect. If you have a design and know exactly what you want, you can skip the added cost.
• How complex is your wall design? Curves, steps, built-in seating and other special elements in your design require extra framing, labor and materials. Interesting textures, added color, or wall openings to plant trailing perennials can increase costs accordingly. . 4Sons Concrete charges an extra $16 to $18 per square foot to handle additional design work, such as steps, to a wall.

A poured concrete wall for your landscaping is similar to a sidewalk, only vertical. A mixture of water, portland cement, gravel and sand, the wet concrete pours into a frame and then dries. Prefabricated concrete blocks stack together, much like mortarless stone, to form a retaining wall. Because block walls don't use mortar, they don't require a masonry expert.

Like concrete block, poured concrete is available in different colors, textures and finishes. You can even mimic the weathered, rust-red tones of corten steel, if you desire. Because poured concrete forms a single, solid mass, it has greater strength than walls built with individual blocks. Many landscape contractors reserve stack block for shorter projects, such as raised garden beds that don't hold back as much earth as a tall retaining wall. Aside from concrete, cinder blocks and versa-lok bricks are additional, cost-effective options.

Prior to installation, your concrete or hardscaping specialist will meet with you on-site to consult about the design and review specifics of the job, such as extra excavation or labor to remove obstacles, correct drainage, manage a slope, and prepare the fill that goes behind the wall. If you need to correct something like a slope, you'll need a wall contractor that can handle land grading, or a hire an additional pro to take care of that prep work. Ensuring proper drainage before work begins will help prevent landscape flooding down the road.

Once the prep work is complete, your contractor will create the base to support your wall. This sits several inches below the soil surface. For proper support, the depth of the base should equal at least 1/8 of the wall's finished height. For example, if your final above-ground wall height is 48 inches tall, the base goes at least 6 inches below the surface.

Your local building codes determine whether you or your contractor need to hire a structural engineer for your wall system. Most states require a structural engineer on any retaining wall project that's more than 4 feet tall. Rates for engineers commonly run $50 to $150 per hour, depending the engineer's experience and your geographic region. Some engineers will charge a flat fee instead.

Taller walls require deeper, broader footings with more reinforcement and different fill materials. If a wall fails and collapses, it presents a serious safety hazard. If it connects to your home, it can threaten its structure and add significant repair costs. A structural engineer will design and build the wall according to accepted engineering practice to prevent failures and other problems.