One of the developers I work close with is getting ready to build 4 spec houses.

I have answers to a few of your questions while I’m sure spec builders here can chime in on their end for you. 

The best setups have:5-11 members (ideally 6-8).1 bedroom per member.1 bathroom for every 3 members.Look for properties with these specs:6-8 bedrooms.2-4 bathrooms.Ample parking (street or otherwise).Detached houses fit this formula beautifully, but other property types can work too.

Prices are up in our market but the discrepancy between prices, similar specs, and actively listed is weird.

New builds-broke even in the '08 market on two spec homes, built my own home from the ground up twelve years ago(400k in equity). 

Spec cabins are few and far between. 

Quote from @Eduardo Perez-Borroto: Since the HELOC was not used on improving the home you cannot deduct the interest, nor use that to offset your capital gain.Would adding a battery bank system to my solar panels count as home improvement? 

Off grid ground mount solar, battery generator, propane n septic.

The mega site in Richmond, as well as Mt Vernon, will really change the landscape in these areas as they did in E-town when the battery plant came in play. 

the electronic keypad locks so don't have to deal with keysbut keep in mind batteries need to be changed like once a year