FInancing for 2 homes on acreage: 1 main home and 1 ADU

[Camhaoir Brecken]

Please bear with me as this may be a little confusing. 

I am a veteran. We own acreage outright. There is no mortgage on our existing home. Having said that, we would like to build on the acreage both a main home and a smaller home for our in-laws. The in-laws want to build the smaller home with the proceeds from the sale of their current home. Then, we would live in the smaller home with them while constructing the larger home for us. Seems easy enough. Well, here is where it gets a little funky. We will have the cash from selling our current home also, but it will not cover the entire cost to build the main home. It will cover the build up to locking up the exterior. To make it even more interesting, my husband is an architectural designer so plans are done, builder is easy to obtain.  We prefer not to take out a huge loan at the onset to build the main home, if possible. What are some financing options? 

If the home is only need the interior completed can that be financed alone? HELOC, Reno Loan, VA, HUD 203(K)? What is the easiest and simplest way to achieve this?

Thank you so much for any ideas that can be offered!