Floors for beds and for iron golems are on the same height. I'm thinking about adding second floor for villagers. This floor would be 4 blocks above bottom floor. Text on https://minecraft.gamepedia.com/Iron_Golem#Spawning is that iron golem can spawn in 16x6x16 above or below right villager. So if a villager with met rules is 5 blocks (so it's up to 6) above bottom floor, iron golem should spawn on bottom floor.