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.