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. 

Contractors are paying their experienced workers $250/day these days in our parts. 

No lending institution or bank would be permitted to operate with those ratios.

There is a provision in all my contracts that permit me to assign the contract to a newly-formed entity that I control.

They couldn't get a construction loan and were constantly blaming the cities being tough on their permit process.

Before you sign anything, definitley pin down the timeline for permitting and building.

Check with the county and make sure it was permitted.