I have a small garden ( aproximately 20 ft. by 20 ft. sq. ). I am interested in growing nuts (preferably self-pollinated). I have also heavy clay under my soil.
What Is the Smallest Edible UK Nut Tree?
Certified GKH Gardening Expert
If I were you, I would look at growing a dwarf nut tree. Most fruit and nut trees can also be bought as dwarf trees, which stay considerably smaller and still produce. I myself have a dwarf orange tree that I have had for near 15 years and it has never gotten any larger than 4' and still produces.
This article will give you a good introduction and some suggestions: