You need to make sure there is a common wall agreement filed on the properties and that there is a fire wall that runs all the way up to the roof for that common wall.

With these items in mind the following would be accomplished by room.Complete Unit:All mechanical and building permits pulledInsulation in walls and ceiling (If applicable)Draft stops in ceiling attic (If required)Install new 1/2" drywall. 5/8" on fire walls (Seperation walls) (local AHJ may require two 1/2" sheets of Type X drywall.

Neat conflicts with any security software so you'll have to disable the security firewall every time you want to use it.

GC walked off the job after collecting the 90% draw on a high end gut job flip (much more costly experience than it sounds) (he also purchased a new truck at that time)Resident cut through the firewall in the attic of a duplex and broke into the unit next door through the locked attic stairwell (went to jail)Resident's guest passed out with their pants down on the sidewalk and the historic preservation district's attorney showed up in my driveway one morning when I was leaving for work to discuss the situation (not a dark experience...just interesting)Cycling through two bad 3rd party management companies in the same year on 34 units was no picnic due to (1) concern over getting paid that amount of rent and matching security deposits with a lame duck manager and (2) resident satisfaction with having 3 management companies in such a short period (3) lots of extra work picking up their slack and (4) concern over ever being able to find a viable management solution.Historic rain came through the day before I was to close on my first real estate investment, contractor had left the sump pump unplugged, and the basement had 4 feet of water in it at the time of closing (HVAC and water heater were in the basement)

They provide the firewall but take a hefty fee.

The office is firewalled off from the residence, and owner got a variance when he sold the practice to allow a non-resident to continue using the space as an office.  

Getting through planning is tough, it's more straightforward to determine firewall requirements etc..  

Most likely you'll need a fire wall between the units, kinda hard to do in the footprint of an existing SFD, if you have room to add on in back for example, or with a breezeway to attach the other unit, might as well build a separate building.

Second is no current fire walls and contractor thinks city will require adding this to propety. 

If you're turning a commercial space into a duplex, I assume that's going to involve moving plumbing, moving electrical, moving HVAC, adding walls, building firewalls, etc.Assuming this is the case, so it's literally a full gut and reconfiguration, and if you're managing it yourself, I would guess you're looking at $60-80/sf for mid-level finishes.