Minecolonies version

minecolonies-1.12.2-0.10.134-ALPHA-universal

Actual behaviour

Citizens wander into construction sites when idle or unemployed and fall into holes when standing on a block the BuilderNPC removes, and/or get solid blocks or lava placed on them and die.

Suggested behaviour

Option 1: Block "Construction Tape" is solid and fence-height but BuilderNPCs can jump just high enough to path over them.
Option 2: Block "Construction Tape" only allows players and assigned BuilderNPC to walk through it.
Option 3: Block "Construction Tape" only allows players to walk through until the "Build" button is clicked on a placed hut, then the assigned BuilderNPC is allowed to walk/teleport through it.
Option 4: Block "Construction Tape" only allows players to walk through ever. When the "Build" button is clicked on a placed hut, a box of tools is placed interrupting the ConstructionTape and only the assigned BuilderNPC can walk over the box of tools.

2 sounds doable and easy to understand

Just add this block as solid when doing pathfinding for non-builders?
AbstractPathJob#isPassable(@NotNull final IBlockState block) looks like the place for doing that

Sorry for pinging :D wasn't intentional

The problem is allowing citizens to pass in one direction to leave the build area but not in the other to enter it.

sounds like natural selection to me

I thought a lot about this and it is not feasible unfortunately, it would have to allow only one way crossing and they would still find a way.