If you have a sloping or uneven yard, consider building a retaining wall. Retaining walls provide an aesthetically pleasant means of implementing landscaping design, while simultaneously preventing the erosion and downslope movement of rock and/or soil. Most retaining walls that are under 3 feet tall are considered ornamental, and thus do not require a building permit. Laws and rules for retaining walls vary in 20099, District Of Columbia so check your local zoning laws or homeowner’s association rules before you build.