City impact fees per unit Utility infrastructure fee per unit, fire suppression whole building, closing cost, permit fee, engineering stamp fees, fire walls before you buy a single brick.

@Michael Georgiadis, might check city records for a mechanical permit

Yes, having a non-permitted 3rd unit does restrict the resale value of the property, because you or a future buyer could be required by the City to eliminate your 3rd unit and by extension the income from that unit.

We looked at building a 2nd onto the house but the cost of the permits put it over the cost of her building something separate.

IMHO those few brave "I lost a bundle because I didn't check the (foundation/ sewer/ permitting/etc)" are 10x more valuable to me than "isn't BRRR great" (that's me, personally, because I already know about buying & rehabbing, & I know what debt load I am personally comfortable with at this point, and in this market).Also note that a significant number of BP members (myself included) didn't purchase until AFTER 2008.

The permit will not close until fully inspected.

I verified all of the needed information (taxes, flood zone risk, insurance, permits, septic / well information, tree trimming, budget estimates for a new roof...). 

I have to agree with @Travis Watts that if you like what you do and the increased hours are manageable with other aspects of your life (family, recreation etc) then absolutely go for it. 

My engineer office is a few weeks out on survey, but I have permits in hand and my excavator wants to start ASAP. 

First thing I will do is make sure a dumpster is there day one and also make sure we have any permits ready to submit once it is officially ours but is there anything else we can really do to get a jumpstart and stay ahead of schedule?