However, I was curious to know if anyone has looked into how treehouses appraise, if at all? 

Fire Marshalls are by far the worst people to deal with, and they know this.

I know there is an ATI exemption that reduces the taxable value by ~25%.Anyone else went through this and can advise on how to move forward with this?

Knowing PA you can get yourself in some trouble if you change the lock before the tenant moves out even if it is written in the lease.

Not may people know this is a requirement and certification has to be obtained by Landlords doing their own management to management companies as they believe we are acting as contractors. 

It comes down to knowing how motivated the seller is, and getting "permission" from the broker to put in a really low offer.

Keep a close eye on local markets in emerging areas around Los Angeles, where property values are likely to appreciate making future investments easier as you build equity.Let me know if you’d like financing help or need more tips along the way.Best,Drago

If so, how are they appraised?

#2) Why is everyone I know buying  STRs and doing accelerated and bonus depreciation?

Though out my younger teen years real estate investing was always something I was interested in and I have done quite a bit of research on the topic, but am nowhere near being well versed in all there is to know.