Bottom Space Fill

The BottomSpaceFill functionality prevents there from being any air blocks below a structure. If present, all air blocks under a structure will be set to the desired fill block. You can configure fill blocks based upon what biome the structure is spawning in. All ignore blocks, such as plants and trees, will also be replaced by the fill block. Water will not be replaced unless you have the IgnoreWater property set to true under StructureProperties.

BottomSpaceFill:
  'SNOWY_TAIGA,SNOWY_PLAINS': SNOW_BLOCK
  'DESERT': SANDSTONE
  'default': DIRT

You define fill blocks in a Key-Value pair. The keys are a comma separated list of biomes. The value is what material of the fill block. So in the example above, the fill block is snow when in the SNOWY_TAIGA and SNOWY_PLAINS biome. You can find a list of valid biomes here and a list of valid block materials here.

'default': DIRT

If you want to set a default block fill type for any biome not explicitly stated, you can use the 'default' option. If 'default' is not present, then any biomes not listed will not have any BottomSpaceFill.

Custom Implementations

Developers can create custom implementations of the BottomSpaceFill property to suite their needs. Check out the Developer API page for more information