You may choose to build a deck on the side of a hill for many reasons. Besides the benefits that come with any type of deck, including more usable outdoor space, a deck on a hill will ensure that water collecting around and below the deck drains away from your home. It also allows you to build a deck where it makes the most sense for your home without moving large amounts of earth to grade the landscape.
Framing a deck on a hillside is more challenging than it is for a deck on flat or slightly graded ground. This is because the deck, which still needs a level surface, will have a vertical support structure that consists of support posts of many different heights. Each support post must be cut to the precise length before installation. Longer posts also require special bracing to ensure that they can support the weight of the deck above.
For a deck on the side of a hill, use temporary lumber supports to place the deck rim in its correct position before you install any support posts. This will ensure that the rim is level and the deck surface is square. Depending on the distance between the ground and the deck rim at the low end of the hill, you may need to use reinforced concrete posts instead of concrete footers with pressure-treated lumber posts. Cross braces can also strengthen wooden posts without the need for additional concrete pouring.
A deck that you build on the side of a hill may require additional structures to make it useful. For example, if you want to be able to access the deck from your yard, you'll need to build a stairway. This can be a stairway set into the side of the hill, or a wooden stairway that attaches to the deck's rim and connects it to the ground below. For this type of stairway, your deck rim must be able to support more weight. It also means you should incorporate a stair railing into your design.