Many yards have areas that collect water, a problem that a new patio or path can actually make worse. Water doesn't soak through the hard surface of a patio or path, so it runs into the yard. This runoff can create problems if you don't correct it before you start construction. Easy-to-install solutions can carry the water away.
A swale or French drain channels the water away from the site to a place where it won't cause problems. You can divert water only to another place on your own property, however, not to the neighbor's yard. If you can't divert the water to a good location, you'll have to run a French drain to a dry well.
A dry well collects water and lets it seep slowly into the surrounding soil. Look to local codes for the required dry-well size for your area. The size could be subject to conditions of your property or neighborhood. Even if your yard doesn't have runoff problems, you can avoid creating them by installing a gravel-lined trench around the patio.
Be sure to check the location of underground utility lines before you install any kind of drainage system.