Just a simple 499 sq ft city approved plan, single story, 1 bed/1bath/full kitchen adu with living space.

Or better said, they are vastly inferior to single family properties.- With $100K, you have a lot of options. 

There is a single family home that was divided and one half has been rented and an ad on small "house" that has also been rented mostly long term.

Furnished Finder attracts travelers who are single or traveling as a couple.

Not for a single airbnb like you are seeking (you and everyone else) but because if you start a BNB or a guest house or a hotel, you can get 40% of your investment back in the form of a tax CREDIT that you can sell for about 90 cents on the dollar. 

Purchase price: $75,000 Cash invested: $10,000 Sale price: $120,000 This is a 2 bedroom 1 bathroom single family detached house is South Georgia.

I was thinking that by giving a single builder rights to build on the land, I would get 1% of each contract price, something along those lines.

The pro development and affordable housing advocates use the term NIMBY, but I suspect it is the rare person that buys in a single family zoned area that would be thrilled to have 17 units (qp16 additional units added) in the lot next to them.My initial protege was the land source on this effort.  

There is no single website to easily find them (that I'm aware of) and if there was, I'd imagine you'd be competing with 1000s of other investors.  

I do expect greater development of condos and townhomes and higher density single family communities. - Acreage will continue to be a very hot commodity.