How They Are Billions Building Access Works
When building colonist dwellings in Numantian's game They Are Billions, each building must have what's known as "free access". Free access means roughly that buildings can't be packed too tightly together.
The game will prevent a new building if either of two conditions are met:
- An existing building would have more than 3 direct neighbors. A direct neighbor is a building (or natural feature like rocks or forrest) that occupies any of squares 2,3 or 5,6 or 8,9 or 11,12. In other words, a direct neighbor is one that is built not diagonally but rather next to a given building.
- An existing building would have more than 5 surrounding neighbors. A surrounding neighbor is one in any direction, including diagonally, from a given building. In other words, a surrounding neighbor is a building or natural feature that occupies any of the 12 squares.
For example, building is prevented here because of rule (1). If something were built in the red squares, then building C would have more than 3 direct neighbors.
In the example below, building is prevented because of rule (2). If something were built in the red squares, then building D would have more than 5 surrounding neighbors. The question from this Steam thread matches this building layout.
These rules answer the situation from this Steam forum post, pictured below. Building B would have more than 5 surrounding neighbors if something were built in the red squares. The rocks here actually do not have anything to do with the free access violation.
Natural features (like rock, forest, water...) do affect building access (although not in the example above). The counting, however, gets more complicated. Natural features increase buildings' neighbor counts, but the features themselves can have infinite neighbors.
Orientation: first a non-bug. The graphical orientation of a building (which way the door faces) is purely cosmetic. This has no affect on the free access of the building.
Ordering: since the game runs these access rules against existing buildings before the new building is placed, it creates a surprising result: the order in which you build matters. You can build in situations the game would otherwise prevent, by building in a specific order.
Compare these two layouts. In the first example, the building is prevented due to rule (2) being run on building E. But in the second example, since building E has not yet been built, neither rule would be violated. This allows you to sometimes achieve "impossible" layouts.
Impossible layouts might lead to a more profitable Market or Bank housing layout, but I have not experimented with this.
Offset neighbors: there are also situations that while strictly speaking follow the rules above, probably aren't what the developer intended. In the example below, building is prevented by rule (1) because building C would have more than 3 direct neighbors. But since buildings A and B are offset direct neighbors, it ends up seeming like a strange free access prevention.
I hope this has helped explain the free access rules in They Are Billions.