While in the process of working on some nifty mapping stuff, I wanted to produce some KML data that I could overlay onto a Google Map. I’m so glad that Google My Maps will allow you to easily export your polygons as a KML file.
So easy, just go to Google Maps, go to My Maps and then create a new map. Then use their super easy marker and polygon tools. The polygon tool works like a very primitive pen tool, but is good enough to draw most buildings on a map.
When you’re ready to export what you’ve drawn on your map as KML just click on the View in Google Earth link and it should produce a nice KML file for you.