have you thought of doing a vertical garden bed? You can hang it from the wall like wall art and you should be able to grow heaps of herbs in that. You can keep them simple and use a sprayer to water the herbs, or go a little fancier and rig up a frame with a small pond pump to syphon water from a reservoir in the base back to the top (which will then filter through the growing medium, watering all the plants and gather again at the base ready for pumping back to the top!!) - people generally use something like shade cloth to keep the soil or other growing medium trapped in place and cut holes in it to fit the herbs in. You can make them yourself or spend LOTS of money buying ready made.

Just a thought for if you run out of room on your windowsill Growing your own herbs can get very addictive, and uses a lot of space when you start growing everything that you want!