Retaining walls are typically built to create a flat area on a sloped parcel or to control erosion by holding back soil, but they can also be an attractive addition, adding colors and textures to the landscape. The most common materials for building retaining walls are wood, interlocking blocks, stone or poured concrete. Costs will vary depending on materials used and the circumstances -- the ease of access to the site, the slope, the height and width of the wall, soil conditions and similar factors.

Typical costs:

• The most recently developed material for building a retaining wall is pre-cast interlocking concrete stone blocks (also called modular or segmental units) which lock together without using mortar. Materials and labor run anywhere from $15-$30 or more a square face foot, or around $750-$1,500for a wall 30 inches high by 20 feet long (50 square face feet). The more expensive blocks are stronger and will stack higher. Do-it-yourself materials cost about $10-$15 a square face foot. ThisOldHouse.com provides a video[1] showing how to build a block wall, calling the project "challenging" and estimating it will take two people 8-10 hours for a low wall 10-16 feet long.

• Material and labor for a wood retaining wall runs approximately $20-$25 a square face foot (and there's usually a minimum charge for small projects), which would be about $1,000-$1,300 for a 50-square-face-foot wall. Plain lumber is vulnerable to termites and will rot quickly, so a wooden retaining wall is usually made of redwood, pressure-treated fir or used rail ties. Better Homes & Gardens provides do-it-yourself instructions for building a retaining wall of railroad ties or large timbers[2] or pressure-treated wood[3] .

• Materials and installation for a stone retaining wall start around $25-$40 a square face foot and go up depending on materials, site conditions, and other factors. It costs $1,250-$2,000 or more for a wall 30 inches high and 20 feet long. Building a stone wall is as much art as it is science, and because it's hard to estimate exactly how much actual labor will be needed most contractors work on an hourly basis rather than for a flat fee -- so the total cost may fluctuate considerably, depending on circumstances. The Building Stone Institute explains how to build a stone retaining wall without mortar and ThisOldHouse.com describes the process for building a stone retaining wall with mortar[4] .

• Poured concrete costs about $30-$40 or more a square foot (usually with a minimum charge for small jobs), or about $1,500-$2,000 for a wall 30 inches high and 20 feet long. ConcreteNetwork.com provides an overview[5] of the various types of poured concrete walls, while Better Homes & Gardens gives basic instructions[6] for pouring your own concrete retaining wall.

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What should be included:

• There are a wide range of materials to choose from, Better Homes & Gardens offers an overview[7]of the various types.

• Regardless of the materials used, a retaining wall requires three key things -- a good base; good compact backfill (the material filled in behind the wall once it's completed); and good drainage throughout the entire wall.

• Since any water trapped behind the wall is extremely heavy, all retaining walls should be backfilled with coarse gravel that drains quickly. Depending on the type and size of the wall, professional installation can take a few weeks to a month or more -- and a do-it-yourself retaining wall project usually requires a great deal of digging and heavy lifting.

• Check with your local planning department -- many building codes require a permit for a retaining wall, and may have a height limit on a do-it-yourself wall project.

Additional costs:

• Hiring an engineer to help design a retaining wall can run about $200-$500 or more, depending on the complexity and extent of the project. Hiring an engineer is strongly recommended for walls over 3 feet high.

Shopping for a retaining wall:

• Manufacturers of interlocking block systems include Allan Block[8] , Geostone[9] and Versa-Lok[10] ; they provide links to local dealers.

• Search for a state or regional association[11] of the American Nursery and Landscape Association for referrals to garden centers and landscape firms in your area.

• Get several estimates, and ask for (and verify) references. Check if there are any complaints with the Better Business Bureau[12] . The written contract should include a detailed outline of the work required and materials used, dates within which the project will be done, and a payment schedule.