Obviously every market is different, but in my opinion spray foam insulation for roofs is going to be the best ROI for reducing your expenses (especially in Texas). 

If the garage is not built to support the weight of additional floors or is not adequately reinforced, it may not be suitable for conversion.Insulation: Another important consideration is insulation.

@Angeline Rebottaro Just make sure whatever hot tub you buy has enough insulation for the area of your rental.

AC included fully insulated garage and sheet rocked along with man door and way more flat work than any other production builder and of course real Hardi plank  NO plastic siding :).. etc etc. so you pick off your buyer that wants quality or price within reason.

Especially around older communities like Cicero or any older homes, you want to look for what's known as "knob and tube" wiring - individual conductors, possibly uninsulated, strung inside walls and other spaces using ceramic "knobs" as insulating suspension.

Insulate Your Home Insulation Investment: - Ensure proper insulation in walls, floors, and attics

But mainly to me, direct booking is an insulator.  

Actually, I spent a month there cleaning it out and spent $4,200 on dumpsters tossing drywall and wool ceiling insulation.

Horrific designs, cheap windows, cheap insulation fixtures, roof, etc. 

Also, the Milwaukee market tends to be insulated from the vast ups and downs that other markets in thr country might experience.