Your doing it all wrong see, what you gotta do is… 
… took me a while to figure out the easiest, free process, (#usual disclaimer, may not be best way, if someone has better idea pls post) Looking outside traditional gaming tools there is a world of under utilized resources.
Basic workflow:
- using open street map select area required, this contains building placement and height data and road layout(sometimes trees to) if you’re lucky, export “whatever”.osm
2)if you only require layout then import directly into your modelling app using a osm importer, but for height and additional data use osm2world to convert to obj, then import to your app of choice to refine.
3)Then depending on game-play/level of detail required etc you can use the simple building shapes as templates.
Quick note: the first rule of open software, confuse the user by doing something backwards, i.e select export before selecting area to export, really?
Click on export , select area, export .osm , load into osm2world http://osm2world.org/ convert to obj import into 3d software,
from there depending on what your goal is you can apply module building assets or textures,