Once the rehab was done I refinanced out of the private loan into a conventional loan as a primary residence.

For the son to qualify for the $250k exclusion, I believe he would most likely need to meet the ownership and use test, meaning he needs to own and live in the home as his primary residence for at least two of the last five years before the sale.

I think we have the option of taking out a heloc  and begin to invest elsewhere.The other idea , that I was thinking is paying off our primary residence which we owe 425k.

In this case my plan is to use a HELOC on my primary residence to purchase the property in Puerto Rico.

***BUT keep it ***B - Buy it F - Fix itLive - Primary Residence itLeave - Keep it, rent it out, and find a new primary residence What made you interested in investing in this type of deal?

But you cannot exchange from an investment property into your primary residence. 

I don't think being a long distance landlord is as easy as some tout but full disclosure I've stayed with 80 miles of my primary residences. 

., the 2006 or 2009 IECC — International Energy Conservation Code).Cost Considerations for a 90-unit or 112-unit Project:The actual cost of qualifying for the 45L tax credit depends on various factors, including:Energy Efficiency Compliance: The primary cost will come from ensuring that each unit meets the required energy efficiency standards, which typically involve energy modeling, certification from third-party energy raters, and potentially upgrading insulation, HVAC systems, windows, and other components of the building to meet the necessary performance levels.Energy Modeling: Typically, you'll need to pay for an energy consultant or engineer to model the building's energy performance and ensure it qualifies for the credit.

I'm looking to use it as an investment, not a primary residence.

I would look at the DSCR loan as the first option.By the way, I suspect you are going to be disappointed by any refi appraisals   ADUs typically appraise poorly   JADUs can lower the property value Vs having the space as part of the primary unit.