Quote from @Sherry McQuage: Make sure to check the zoning to make sure mobile homes are permitted there; check for any hazardous waste designation for the area; will you need to have the parcels surveyed out in order to sell the size you intend? 

Zoning Districts That Allow ADUsADUs are permitted in residential zoning districts and some mixed-use districts.

I'd check local zoning ordinances to see what is permitted. 

Labor, supply chain (we are on an island) and red tape: from offers and closing, to funding to permits.  

You cannot even apply for an STR permit on a single family home until after you've owned it for five years (and there is no guarantee you will get the permit).

If you do not, I would still love to connect sometime when time permits.

My understanding is that the ballpark standard of "unless it would be outrageously tough" (something along those lines) is what applies if you're doing a rehab, seeking new permits, doing structural changes anyways, and things of that nature.For existing pre-ADA structures, my understanding is that the standard is more along the lines of "if it can be easily/cheaply done, you have to do it."

Ensure you understand local regulations regarding permits; it’s usually a good idea for your associates to form an LLC for liability protection, but individuals can do renovations as long as they pull the necessary permits.

Rent to price ratios, # of new building permits vs new residents.

In a city like Philly, you will face many challenges - you need to learn the market and know the neighborhood and the block and make sure all of your numbers are within reason, you need very reliable contractors, you need a to understand the permit process, you need to be familiar with market expectations.