The ideal placement for a rain garden is under downspouts, which wash water off the roof as rainwater is channeled down through the gutter system. The EPA notes the importance of contacting local officials before constructing a rain garden. You’ll know if there are areas you can safely dig and areas that should be avoided due to buried power lines, plumbing tunnels, septic tanks, etc. Tree roots should also be considered when planning the design (via UF/IFAS). Remember also that a rain garden is defined as a hollow or low place. So if you already have a natural recessed area available, it’s a good idea to place your garden there instead of digging another spot.
Like any other type of garden, a rain garden needs plenty of direct sunlight. Most homes have multiple downspouts as part of the overall gutter system, so choose to locate your yard on the side of the house that gets the most light. If that side is along your neighbor’s property line, consider incorporating them into your plans so they can both benefit from a single rain garden.