Residential Relocating Strategy

[Sam Alpha]

This might require a simple strategy, but I'm struggling with putting it together.

We live in our primary home at the moment. 

Appraisal: $470-475k$

Willing to sell: 450k as-is.  (Nothing major wrong, usual wear, floor plan isn't the greatest). 

Amount left on the mortgage: $250k.

Equity: $425k($450k minus fees) - $250k = $175k

At the other end, we would like to build at acost us about $360k over 7 months.  We are hoping to pull the equity from the other sale and transfer it to the new build and have a new mortgage at $185k. With all done, the new property (on the lake) should appraise at around $575-600k.

This is where I need help.  

The numbers above work great on paper.  However, if the market takes a turn, we cannot sell until the other property is built.  Chances are, all the built-up equity, which I plan to transfer would dry up fast and get us in a bind!.

I do have the option at taking a home equity loan at 5%. This does put the equity in my hand and maybe even allow to put the down on the new build. If the market turns and the equity dries up, can I let the bank take the old property or will they hold me liable for the equity loan as well?.  What other implications do a home equity loan have?.

Is there a smarter way of making this transition, whilst protecting the equity?.

Thank you!