You should also understand the concept of risk-adjusted returns (which is why coastal markets, often beat non-coastal markets). 

You have multiple incomes so in a way you are reducing your risks. 

Is it worth the risk? 

I live in Asheville, nc and our current market appreciation is ridiculous.....so there is the risk that it slows down.

Overlays are the extra rules that lenders put on top of Fannie/Freddie guidelines to limit their exposure to risk. 

Is it worth the risk?

Talk to an attorney, but I wouldn't just because the process involves risk if the deal falls through.

I've not done it and my one criticism is the risk of interest rate increases, but if you're contemplating a HELOC anyway, you should probably read up on this a little bit as it may save you even more money.  

Can my sons carry on? 

I run the risk of having to do some pretty expensive electrical updates (the structure seems to be built to code and no outstanding issues that are obvious) as the house was built in 1978 and the addition doesn't meet current code as far as number of outlets, separate circuits, and arc-fault circuits installed.In the current market I'm thinking I could sell it as is and probably offload it with a $10-$20k reduction in market value.