The following is NOT really "scratch building," but I just noticed on Thingiverse a set of free designs by one designer which have [free] printable terrain of what looks like good quality, some for Necromunda, some for GorkaMorka, some for Mordheim.
https://www.thingiverse.com/Terrain4Print/designs
Necromunda:
Grimdark Block Structures.
Shanty Houses
Shanty Walkways
Shanty Barricades
Shanty Junk
Gothic Spaceship Wreck B Engines
Modular Control Panels
Two-Armed Construction Robot
Mordheim:
Ulvheim B3 Stairs
Ulvheim B2 Modeular Ruins
Ulvheim B1 Modular RUins
Whattle Fence 2
Ulvheim Building and Ruins
Whattle Fence
Stone Wall
GorkaMorka:
Gothic Spaceship Wreck C Nose
Shanty Walkways
Shanty Obstacles
Shanty Junk
Shanty Barricades
Gothic Spaceship Wreck B Engines
Gothic Spaceship Wreck A Body
Barbarian Tent
Modular Control Panels
Two-armed Construction Robot
Desert Sci-Fi Buildings
Shipwreck Terrain
Modular Arabic Style Village
Edit: The creator says he printed them at about a 5% infill for his general gaming use, and did only the briefest of sanding jobs. I especially like the Ulvheim Building And Ruins set, in that it is pretty effectively modular, to make two- or three-story ruined buildings, or even to use as the "base" stonework story for some foamcore "lath-and-plaster" or "half-timber" upper structures. Separately, I also like the GorkaMorka "crashed Imperial spaceship" sections, to represent bits of the Hulk scattered about the Skid on Angelis.