A hillside is not the ideal place to build a garage. Excavation and foundation work may be more expensive than on a more easily accessible site, and stability can be an issue, particularly if the site is in an area prone to earthquakes or landslides. However, it may be your only option on a small and steeply graded property.
Building Into the Hillside
If access to the garage is at the bottom of the hill, the garage can be excavated into the hillside. This option encompasses extensive excavation work, but results in the most stable finished garage. A backhoe may be used to excavate a garage-size hole into the hillside. A garage is built in this space from cinder blocks or similar strong and moisture-resistant material. Pay special attention to the strength of the back wall, where the pressure from the hill may be formidable. When the structure is completed, the fill from the excavation is backfilled around and on top of the structure. Fast-growing grass and other deep-rooted plants are planted to prevent erosion.
Building on Top of the Hillside
If access to the proposed garage is at the top of the hill, a garage needs to be built on stilts so that the car enters it from the highest point on the hill. The framework must be extremely strong in order to support both the building and the car. One option is to excavate a flat area into the hillside where the garage will go, build up a story with cinder block or cement, then build the garage on top of that. You then have the added use of a storage space beneath the garage.
Building Sidways on the Hillside
If access to the garage is from the side of the hill, a small driveway and area for the garage needs to be cut into the side of the hill. This technique presents a large possibility of causing erosion or damage to the landscape, so care must be taken to minimize the amount of excavation that is done. By flattening an area that is wide enough to accommodate a driveway and the width of a garage, you give yourself an area to build a garage that is situated crossways in relation to the hill; you can backfill on the uphill side of the garage. A building such as this should have a strong foundation that is solidly rooted deep in the soil or directly to bedrock.