It has only one water, gas and electric meter (i.e.- one furnace and one water heater). 

Meaning the BP calculators cannot predict the impact of a factory closing, a school being sued for $35 million from a football injury, a huge tax break for small business or a proposed highway exit ramp.It is investing and your numbers will be different from mine for the same house (I have a cheap roof guy or I can get all electrical for cost.... you own a lawn company and can do the landscaping for pennies on the dollar).

At 19 I know entry level plumbing, entry level residential HVAC very well, I've learned a little electrical, how to mud and tape drywall, how to paint, and of course like every other money motivated preteen, how to a yard shine.

They gutted it to the studs and have installed all new electric and plumbing.

Yes but this house is a sfr with I guess an unpermitted basement apartment with own entrance, oil furnace, electric meter, full sized windows.

The after development cost (including road, water, sewer, electric, etc.) is around $18,000-$20,000 per lot, so $684,000-$760,000.

I know here in Oklahoma I can get a $50-100 rebate on residential water heater and by switching from electric to gas heating I can get $500-$1000 rebate.

My contractor gave me a detailed proposal of each item to be completed and the cost (demolition, framing, plumbing, electrical, paint, finish work, etc) As each item is completed, he sends a bill for that on Thursdays.

Your estimates for plumbing and electrical are probably off by at least a factor of 5.

I would also have the tenants pay for Electricity, Water & Sewer, and Garbage.