Had my sister-in-law come live with is a little over a year in as a house hack.

Over a decade ago a friend and I bought a house together.

I cannot see how build two, say, 1200 sq ft houses with two roofs, two foundations, presumable about 25-30% more windows and siding is going to be cheaper a duplex that might have a little extra fireblocking and soundproofing.Now, maybe your GC is thinking that detached homes tend to get higher rent than comparably sized and finished duplexes.  

We didn't think we would so we bought our "forever" house right away.

He renovated a property for me in Dania Beach last year and is currently doing my friend's house in Fort Lauderdale.

Purchase price: $137,999 Cash invested: $15,000 Sale price: $260,000 Rental House purchased in 2006 and was my primary residence until 2019.

Purchase price: $505,000 Cash invested: $55,000 Sold our first condo and used the equity as down payment and for a couple upgrades on this this single family primary residence / house hack.

And it can work, but like you noted, if you are trying to create a commodity house, then fine.  

They typically do not add value to the house, so when the buyers are looking at comparables, none of them sit there and say...well every other house sold for $200,000, but my gosh this one has solar panels....let's pay $240,000 for this one.Maybe one day, maybe someplace, but NOT today in NJ I expect.Only way I would even look at solar panels is if it is for your forever house, and you have enough land to install them on the ground, not on the roof....so maybe if you have 1-5 acres or more. 

I'm guessing you also have checked for the ARV and what houses in the area are selling for to help determine if it's worthwhile.