Need "outside of the box" financing advice

[Vanessa Pruett-Urie]

I'm going to try to be as brief as possible while explaining everything. My husband and I own our home in Baltimore County, MD it is a large rancher on over 3.5 acres. We have about $180k in equity but have a great loan and don't want to refi it if at all possible. We looked into a HELOC in the past but because of our land they didn't find comps to justify the true value of the house including the land so we didn't pursue it. We also own a rental townhouse (with positive cash flow, but no substantial equity) it was my pre-marriage home. My mother in law owns a house with no equity in another area not far from us.

Here is my current situation.  We want to move my mother in law into a house (a new pre-fabricated house on a permanent foundation) on about 3/4 of an acre of our land.  We are aware of the permits, utilities, and drawings and such that needs to be done to make this happen and the approximate cost to make these improvements.  The land has had proposed lots approved by the county before(prior owner).

We had twins a few years ago and our living expenses skyrocketed an put us in a less than wonderful financial situation.  Credit scores in low 600's now but on their way back up after regrouping.

We just don't have the savings/capital to make all of this to happen, personally. I work for a hard money lender, that may be open to some kind of arrangement if I can propose it correctly. The cost will be around $150k-$170k and the end value for the new house should be around $220k-$230k. If we mortgage (no more than 80%) this house it will have a PITI payment of less than what she pays on her current home, that we will be selling.

Is there any option that I haven't considered that anyone can think of that may work to make this happen.  I am leery about using my boss as a lender just in case there is any refi issues and we can't get him paid off.. That would be a nightmare for my good paying day job possibly!  

Any input would be greatly appreciated!