I would need to know more specs on the house but with material shortages It's difficult to say how much to budget. 80K-120K for full gut perhaps but unsure with lumber beings so expensive and shortages.I would plan 9 months of vacancy between the pain you will endure pulling permits & having your contractors show up everyday to complete the work & then final inspections to get a CO. 

Quote from @John Williams: I recently bought a home that has an attached dwelling unit where Im planning to move in and rent out the main unit.

Root intrusion is rare with plastic pipe because that means a crack that's going to admit soil as well and eventually bust completely, so if you have roots in PVC you might as well plan to excavate.

Definitely not, IMO, Been there and its a waste of time and money and risk unless you have dedicated plan to take advantage of having it.

It seems like what you are planning would be in their best interests so I would discuss it with them first and then make that the impetus to sign a new lease and up the rent.

if not, how would you plan to pay that bridge loan off before the balloon comes due?

I know of a market that I believe will blow up in the next 10 years because of its plans.

Starting small close to home is a great plan.

Please come down and see me and let talk about real estate.My plan now is looking for buy and hold and flips.If your 100% motivated for REI just like i am, come down and meet me.

Meanwhile the  neighbors quality of life will improve once the new wall is constructed 2025 Q1 and the denser use is consistent with the Philadelphia planning commission's 2035 plan.