How Much of a Slope Do You Need for a Walkout Basement?

A walkout basement is a desirable real estate amenity that adds habitable space to a house, quality of life to the family, and pleasing visual harmony between the house and the lay of the land. But a walkout basement requires expertise in site preparation and construction to ensure quality work. Soil properties are as important as the degree of slope of the building site when making the decision to build a walkout basement.

High angle view of Nidelva River, Trondheim, Norway
credit: Medioimages/Photodisc/Photodisc/Getty Images
Sloped terrain makes walkout basements possible.

Walkout Basements

Sloped sites and hillsides create the potential for construction of walkout basements. A typical basement is formed with a footing and foundation design, where the structure that supports the house is below grade. The foundation walls are usually the standard height for interior walls, at least in modern homes, and the basement can be finished for living space. A walkout basement utilizes the slope so that part of the basement can be at grade, allowing a doorway that provides access to the outside.

Walkout Basement Construction

The footings for a walkout basement are required to be below the frost line, and at a greater depth on the walkout side than for below-grade construction. Walkout basements present challenges with differential settlement: Variations in the soil can cause the upper part of a foundation to settle at different rates than the lower part, putting stress on the foundation. For steep slopes, retaining walls may be required to prevent erosion of the slope adjacent to the house and to prevent associated problems with moisture intrusion in the house.

Soil and Water

Soil and moisture management are significant concerns with walkout basements. Foundations have moisture issues from water in the soil seeping through the concrete, and extensive measures are needed to protect the structure. The type of soil is one of the deciding factors to determine whether to build a walkout basement, as importing material to replace poor soil adds to the cost of construction. Local officials may discourage walkout basements in regions with expansive -- clay -- soils.

Slope and Walkout Basements

Steep and moderate slopes can be incorporated into a walkout basement most economically when conditions permit. Generally, the greater the slope the less expensive the walkout basement will be; but the soil has to be viable as well. Walkout basements are more expensive with gradual slopes, as more excavation and grade work is required. It's best to determine the suitability of the degree of slope for a walkout basement with instruments, such as a survey transit. Have a professional assess the soil composition.