Below is a table of plants giving the common name and latin name. All of these plants are beneficial to butterflies at some stage in their life. Even if you have a windowbox you can help by planting it with one or more of these.
Your butterfly garden can be any size at all - these days every little bit helps. So think of the space down the side of you garage, or at the back of your compost bins, or behind the shed. In these little used spaces you can allow the less attractive (to humans) but vital (to wildlife) plants to grow. And in your garden itself you can plant up some of the more attractive species of wildflowers, herbs and edible plants.
Do remember that the beautiful butterfly which delicately sucks nectar though its coiled tongue was previously a chrysalis, cocoon or pupa needing an undisturbed habitat to metamorphose in, and before that a caterpillar (a bag joined at one end by a mouth, and the other by an anus), whose sole aim and desire was to eat as much of its food plant as possible, and before that an egg that, like the chrysalis, required an undisturbed location.
Most of these plants will benefit other beneficial insects such as bumblebees, hoverflies, bees, as well as butterflies.
If you are concerned about the invasiveness of any of these plants then you can do what I do with my mint (a thug if ever a plant could be called one). I plant it up in a large plastic pot and put that in the ground with just a centimetre of pot rising above the surface. This is enough to stop the mint spreading and taking over the entire bed. I do have to dig up any seedlings though, but I am rewarded by the wonderful smell of crushed mint as I do so!