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.
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?.