Hi, I'm a residential builder in East Austin as well as currently going through ADU permit process for my tiny house that'll be on the backyard of my primary so I can answer some of these.
1. Other than lot sizes > 5750sqft and at least SF-3, watch out for set back (distance between your ADU and main house needs to be at least 10 feet as well as from the sides), tree requirements (if there are trees that are 19" diameter 4.5 ft from ground as well as if it's a heritage tree, you cannot take them down and even trimming can be pain as city of austin arborists might flag you and they are expensive)
2. My properties are all located within 1/4 miles from a transit stop, in which case you don't need to provide additional parking space. To see if your property would require in your neighborhood: https://www.arcgis.com/apps/we...
3. There are ADU companies that have existing relationships with mortgage companies. Good news is that compared to few years ago, ADU/tiny houses are now way easier to get financing, making it a great time to invest in ADU especially if you're going to turn it into a STR (with only type II permit as an owner occupied). If they don't know, I've used 21st century mortgage which services tiny home/ADU.
4. ADU can be either 1100 sqft or 15% of the lot size, whichever is smaller. Also both house and ADU combined cannot be over 40% FAR ratio (so if you're looking from aerial view, both structures combined cannot be over 40% of the entire lot size). Also, if your property is in a flood zone or historical site, permit can experience delay, as well as make sure to check deed restrictions and HOA regulations. Sometimes it's not so easy to find out restrictions on ADUs.
Also getting insurance quote can be trickier with ADU especially if you're going to run as a STR. For ADU permit applications, you'll need architectural plans, elevations, as well as Texas certified structural engineer's seal of approval. Hope these help.
