Simple 5x5 Iron Farm

A minimal iron farm for three villagers and one zombie, dug into flat ground. The zombie sits in a boat below the villagers, golems spawn on the flattened area and wash into lava over a single hopper. It costs almost nothing past the beds and the buckets, but the ground around it has to be flat and spawnproofed. Java only.

• The ground has to be genuinely flat and spawnproofed. A cave below takes all the golems, and one builder recommends putting the villagers 8 blocks above ground, or slabbing every cave within 8 blocks of anywhere a villager can stand.
• Path blocks around the outside stop golems spawning there, and slabs do the same.
• Do the shovel dirt step on the two sides away from the hopper. The creator says that is what stops golems spawning inside blocks.
• The zombie burns because hoppers do not block sunlight. Shade him properly or give him a helmet, and name tag him or leave him in the boat so he cannot despawn.
• Villagers must have slept in the last 20 minutes or golems stop. This is why several builds died within the first hour.
• Put the water in three corners of the hole rather than the way shown, or golems spawn where the water cannot push them out.