1. It's always better to analyze the area where you map - not to rely just on satellite/aerial maps, moreover if it's already tagged area and you can see what are a rules.
2. You've specified the type - a house by inserting it as a value for their tags. And for now OpenStreetMap thinks these building are meant to live there. Community gardens are not a residential area. Please read about tag values - if you are not sure about THE TYPE of the building, just type building=yes.