If you place your building on different layers in photoshop - you can assign different values to the effects on the building. So one layer may have a bigger value for drop shadow, translating to a higher building. If you wanna get a really nice result you can take a copy of the "high-building-layer" and make a shorter shadow for those building whose shadow hits lower buildings indstead of the street. Here using masks can be good to erase unwanted shadows on both layers.