Hello Carolyn, I do not know what you want to plant I'm assuming it's shrubs.
18 inches is not a large overhang. Depending on the shrub I would plant the shrubs at least 2-3 feet from the house. One reason you want your shrub to have a back to it, second if you have a cement foundation the chemicals leach into the soil.
While the shrub may be small when it's planted, remember it going to grow yearly. It doesn't take long for some shrubs to fill out, then they are up against the house anyways. You also need to be able to prune the shrub to keep it looking it best. When planted up against the house, your going to have a plant thats full in the front, with probably very little foliage on the back side.
Read the planting intructions, see how wide and tall the plant you are going to plant gets. Then make your decisions accordingly.