This is a hidden Side Tale in the Iki Island Expansion for Ghost of Tsushima: Director’s Cut. These are completely optional but if you’re aiming for all Trophies, must be completed towards that goal.
How to find the Peasant
This Side Tale is a unique one, it will never show on the map or your journal, even once you’ve begun and it will only show up once you’ve completed all the steps and are ready to turn it in. To make matters even more complicated, the person you need to find to begin this Tale can be in a number of different locations and you’ll need to check all of them to see which one it is, these are the locations:
In the woods - There are two locations for this meeting, either the woods at Tatsu’s Ladder or the woods at Weeping Mother’s Meadow. He will request 3x Yew from you in this location.
You must now find him in the field - Head along the path leading south from Katayama Market. He will request x6 Predator Hides, obtained from Wild Boars or Bears.
The final location is at the beach - Head to the beach west of the Yahata Lighthouse. This time he will request x10 Iron, obtained from completing the Mongol Territories on Iki Island.
Talk to the builder at his homestead
At this point, the Tale will appear in your Journal and the builder will inform you of where he plans to build his house. This is close to where your current position is, simply head south into the forest and you’ll find the now built house. Speak to the builder and he will revel in joy at the house the two of you built, bringing this Tale to an end.