Thanks for that inputA:  You don't carry the HML. 

The question here is since their is a loan for $300k and both of my parents are retired (the only income they have is 1 rental, pension and 401k), I don't believe they could qualify to carry a loan onto new properties. 

They had securities carrying guns question even if you live there.

We finance for  real estate investors with a resonable  seller carry backs for 2nds 

Does it carry greater weight?

Remember, you have to pay for land, const. costs, soft costs, interest carry on your loan, permit and development impact fees, your profit, and fit that all into 120k per door on the unit sale (BTW, 120k per unit walking distance from Chesapeake Bay sounds cheap, but I'm not familiar with the market).2.

Adding a couple of those could be cheap compare to an extra month of carrying costs.

They came back with $145K, though they'd be willing to carry the extra $15K at 5-6% themselves "for a few years until it's paid off or until I refi."

It is very costly ($500/month to get started in my area) and the ROI is around 50-150% in my area which may sound good until you consider the time and effort it takes to convert those leads, maintain/nurture those relationships, and close the deal(s). 

@Carrie Hallensleben Sometimes to get around what the lender was saying, you could try accessing the line and keep it in a bank account for a few months before trying to obtain your next property.