Buying a single-family home with a detached garage and converting it to an ADU is much simpler.

Very little too almost nothing because look at there life.

My personal example is a one bedroom condo I’ve held my entire adult life - and this is in a top tier market - Dupont Circle neighborhood in Washington DC.

Life is not static for most people. 

From what I’ve read in The Economist, demand for “off-the-beaten-path” destinations has increased post-pandemic, as travelers seek quieter, less crowded areas.In my backyard, investors and homeowners alike are increasingly drawn to areas like Gilbert and Chandler due to their exceptional growth potential and quality of life.

I've house hacked twice in Reno, NV and it's changed my life. 

Same answer whether you borrowed against your residence or against your rental A or against your life insurance or from your family or from any other source. 

Per this NY Times article, the average American knows around 600 people.https://www.nytimes.com/2013/02/19/science/the-average-american-knows-how-many-people.html#:~:text=The%20average%20American%20knows%20about,do%20you%20know%20named%20Kevin%3FPer the US Census Bureau, the average American moves 11.7 time in their life, which based upon an approximate lifespan of 84 years, works out to be about every 7 years.https://www.census.gov/topics/population/migration/guidance/calculating-migration-expectancy.html#:~:text=Using%202007%20ACS%20data%2C%20it,one%20move%20per%20single%20yearSo, if the average American knows 600 people and they each move about every 7 years, that means that the average American knows around 85 people that move in any given year.How many of those moves do you want to be involved in?

Sub 30k houses are an absolute disaster with a tenant pool that will destroy your properties and make your life miserable.

A reserve is unnecessary, but I still keep around $15,000 - $20,000 in my account.The point is, that you should sit down and assess your finances to determine what the worst-case scenario may look like, how much you would need to cover it without impacting your life, and whether you will need to build a reserve.