The best thing I did on a $131k prefab cottage was wait eighteen months
Detached 480 sf one bedroom, prefab shell set on a slab in my backyard, finished last fall. Renting at $1,595. Total spend $131,400 including site work, utility runs, permits, and the pad.
The part worth telling isn't the build. The build was fine, the company set the box in a day and it looked like a house. The part worth telling is that I sat on this for a year and a half before I signed anything, and everyone I know found that annoying.
What I did in those eighteen months: read my city's ADU ordinance twice, then went to the counter and asked a plans reviewer to walk me through it, which cost nothing and corrected two things I had wrong. Found out my rear setback was 5 feet, not 10, which changed which part of the yard could hold the unit and let me keep the mature tree. Got three utility scoping quotes because I'd read that sewer connection is where these blow up. Mine came in at $9,200, the worst quote was $23,000 from a company that wanted to trench the long way around the house.
What nearly broke it: the electrical service. My main panel was 100 amp and the utility wanted a full service upgrade to feed a second dwelling. That was $6,800 I hadn't budgeted and it showed up eight weeks before the set date. I had it because I'd padded 15% and hadn't spent the pad on finishes.
What I'd keep: the padding, and going to the counter in person. Both of those were free or close to it.
Rent covers about 62% of my primary mortgage payment. I'm not trying to make that sound like more than it is.