My second house was under contract before it was even listed, but the combination of my GC disappearing and having to restart the permitting process for the carport in March right at the start of Corona lock downs dragged that out and eventually feel out of contract.

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).

Just make sure the town is ok with the land being zoned for multi-units, and look into construction costs, financing options, and permits ahead of time.

There are plenty of deals in Philly on and off-market from wholesalers, who are pretty decent in Philly compared to other places.You will need to get your GC license here and learn the permitting process and necessity here as well.

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

Simplified ManagementFor those managing multiple properties, AB 2622 streamlines operations by expanding your hiring pool and removing the red tape often associated with minor repairs.Why It Matters for TenantsTenants also benefit from AB 2622:Faster Maintenance: Minor issues like plumbing or lighting can now be resolved promptly.Fairer Move-Out Costs: Affordable repairs mean tenants aren’t hit with inflated charges for damage beyond normal wear and tear.Things to Keep in MindWhile AB 2622 makes maintenance easier, landlords should still vet unlicensed workers carefully, ensure proper insurance coverage, and remember that jobs requiring permits must involve licensed contractors.AB 2622: A Win-Win for California’s Rental MarketAB 2622 strikes a balance between flexibility and accountability, making it easier for landlords to maintain high-quality rentals while fostering better tenant relationships.

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

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. 

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

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.