I managed to build an Iron golem farm in survival vanilla (1.9). later I realised that golems were spawning outside of the spawning platform.
see images below to get the scenario.
For temporary I use suffocation method to kill them and obtain iron.
however soon I plan to prevent them from spawning outside the platform because it is written on wiki
But what can I do to prevent them from spawning?
First I thought of placing half slabs everywhere.(Here I don't know what should be spacing between slabs)
The second thought was to dig the terrain aside the farm (But I don't know how many blocks deep)
EDIT :
I decided to place half slabs. then added dark oak half slabs around the spawning platform. Still again golems were spawning outside the spawner. after few minecraft days, I saw a golem spawned inside a half slab.
Now this leaves only one option digging around the spawner platforms. However I don't know how deep should I dig?.
Best Answer
For a Golem to spawn, it needs 4 blocks of height before the next solid block, and 2 blocks of horizontal space (Golems can spawn between blocks horizontally, so they may fit into 2-wide passage). Golem spawning is not impeded by half-slabs, other transparent blocks, or liquids, so you can't cover the floor with spaced half-slabs like it works with Spiders.
This leaves you with only two options:
Did I already tell Golems can spawn in transparent blocks? :-)