We would like to run a grinder pump to our septic and also add a new electrical panel to be able to handle more and then rent this back house out to tenants. 

Jonathan will be speaking at BPCON on a panel: How to Flip Properties: The Right Team, Right Price, and Right Operation with High Interest Rates or Out of State if you have any interest seeing him live in action! 

I'm planning a detached garage ADU conversion (tear down) and am currently upgrading my main electrical service/panel on the primary residence.

What about a well and solar system?

. - NOTE: any major renovations to a building may void the grandfathered exception and require upgrades to meet current building codes.3) As @Nathan Gesner pointed out, a single circuit can often be upgraded by running new 3-wire romex line from electrical panel to the plugs on that specific circuit.4) You can also install a GFCI plug (and not ground it) and that will often be acceptable.5) Why should the property owner be required or expected to upgrade the electrical, when it meets the grandfathered building codes of when it was built?

I do have quite a bit of paneling work which was purely for aesthetics, while other jobs were actually necessary to bring the rental into service. 

And they’re telling me that we have to create one main shut off for both panels which would force us to completely redo and upgrade all meters, panel, and go to 400 amp from 200.

We’re building out the building to be nearly net zero, geothermal and solar; it costs about $100K but there are rebates and tax incentives and rebates.

Also have to remember this just prints the walls, you still have all the MEP rough in etc. to be done. 10+ years ago our stick built home was panelized and it took them a day to frame each floor. 

The only real challenge we ran into was the seller originally wanted to set this up as a illegal duplex so there were multiple electric panels and corresponding wiring so that was going to have to be removed and redone in a single family format.