Why not just buy a condo?

If your goal is to save money, a small, 800-900 ft^2 condo will still be cheaper than a 300 ft^2 custom built tiny house in most areas (and you won't be able to put your tiny house in a high real estate price area anyway). Yes, you will have association fees, but you don't have to worry about building maintenance and you get a pool, which is pretty neat.

If your goal is to be more environmental, tiny houses waste a lot of energy on heating/cooling because of the volume/surface area ratio. Condos share most walls with other units and are not exposed to the outside. Also, condos are more efficient way to use space as you can stack them.

And you'll have more space in the end.