I have a lot of 6mm Sci Fi stuff that I'm currently trying to sort through and I figured it was a good opportunity to take some photos as I do so. Here's a collection of the Sci-Fi Buildings that was originally made by Ground Zero Games, but are now sold here by Daemonscape Games. I see the new manufacturer also make a great looking trenchline and fortifications too. I've included a Rhino APC in the photos for scale comparison.
Here's a set of City Block buildings.
They are modular so you can assemble them in different ways.
They also make quite a few different warehouses, storage tanks, generators and so on, including the following.
And a picture of them altogether.