The father is married and has another child, and a few years ago he mentioned that he and his wife's mortgage is covered by an AirBnB rental guesthouse in their backyard. 

I think, though, that it's important to look past it and actually analyze expected rent vs expenses and mortgage payments.

Monthly mortgage amount is $780/month (PITI).  

Can i purchase land and offset the mortgage payment against it as passive loss and wait for real estate prices to go up and hold the property.

Off market - Online How did you finance this deal?

Only problem is with a short term my monthly mortgage payment will be 2K, and will cut deep into my cashflow during that time.With a little work I can get my COC to 3.1% per month for that 12 years.Appreciation is about average in this area and I expect to get about 4% annually maybe 5%. 

We have a mortgage payment and pay for the electric bill.

I have a mortgage on that property.In about 6 months, a family member is going to sell me their house for an extremely reasonable price.

And the answer is always the same :-) There is no way to accurately do this...too many variables and moving parts, plus it varies too much from one locale to another.You need to find a really good General Contractor and pay him to walk the property with you....over and over....until you feel really comfortable estimating yourself, and that should take years.Alternatively there are online calculators (even one here in the tools section) but I would not rely on them for the reasons listed above.

Some grants are deferred payments, or will be an additional amount added on to your mortgage.