Rehabbed and flipped it.Took it down to the studs, and then called in the Building Dept Inspector, and told him our plans.He agreed with everything that we wanted to do.As the work progressed, we had him come over to take a look, and he said, everything that we had done, was exactly the way he would have done it.We had to build a fire wall in the basement, because the garage was part of the basement.We thought that we had to put in a solid metal door, and he told us, that a solid wood door is legal.

There is also a brick parapet running through the middle that acts as a firewall between the four units (it is a quadplex).

I have been told that mice can still come from the other homes connected to mine, via bitting through the firewall, and make entry again. 

That is annoying.We have a great firewall here!

@Megan Clancy The answer is reasonable firewalls between sets of assets, and plenty of insurance. 

Get a new PM who will act as a true firewall and also implement your goals and strategies.

For example: there can be no plastic pipe in plumbing, the walls between units must meet certain specifics to be a firewall, if it is forced air each unit will need its own furnace as units cannot share a return air duct, etc.  

If you get a lot of these or have high net worth you might want to think about a intermediary LLC to act as another firewall.

A lot line will have to be "implied" when figuring distance between existing structure, firewalls and window % of the facade. 

Specifically looking for both the fire suppression system and if you have firewalls with the other tenants.