One issue is – how to finance it. You typically need a construction loan for all the materials, then when the house gets final inspection, you convert the construction loan to a regular mortgage.
Building it yourself can mean a 2 or 3 year project instead of a builder popping it up in a few months. This means – you probably can’t get a construction loan because they usually only stretch to 12 months, I think. So, you need the cash.
I suspect the comment where the poster suggested you may end up spending the same amount of money as the market value is correct. The appeal to me is that I get to design it myself. I am a closet architect. Also, the idea of a small house on a big lot is appealing to me and it seems the only way to get that is to do it myself. Finally, you can build a small house today, then add on as families and income grow.
Also, you don’t need illegals to do it cost-effectively, just finding some good subcontractors is fine, and helps speed up the project.