Minimum $100,000 rehab based on my 10 minutes of inspection, shingles missing, foundation problems, cracked tile in house, cracked drywall joints everywhere in the house, rotting deck in backyard etc....

Much easier to when you start with people who understand the question.One way a friend does this (not USA based) is to set up joint ventures.

Here are some financing options to consider:HELOC or Cash-Out Refi on Another Property – If you or your mom have equity elsewhere, this could be a more flexible and lower-interest option.Business Line of Credit – Since the property is in an LLC, you might qualify for a business LOC, which could cover construction costs.Private Money Lender – If you’re okay with 12-15% rates, a private lender could fund this as a short-term loan, especially if the guesthouse will generate strong income.Construction Loan – Some lenders offer short-term construction financing, which converts to a longer-term mortgage once completed.Seller/Partner Financing – Bringing in an investor or structuring a joint venture with someone willing to fund the build in exchange for a share of the income.Credit Card Stacking or Unsecured Business Loans – Riskier but could work if you need to bridge a gap until the unit starts cash flowing.Would love to hear more details—especially your timeline and expected rental income.

We didn’t find a joint investment, but I found an off-market deal and invested with my dad separately.

We formed a husband and wife Qualified Joint Venture Series LLC and we are the only Members.

You can structure it as an LLC or joint venture, which offers clearer tax and liability benefits.Your son doesn’t need to be on the loan or title if he's a silent partner, and you can manage the mortgage, credit, and insurance.

Quote from @Havital Miltz: Normally you would set the LLC in the state where the property is.You have no tax benefits for setting up an LLC, it is more a protection from lawsuits and going after your personal assets, however not a guarantee....As a husband and wife you do not need to set up a partnership - IRS has special considerations for married couples for self employment, you could also apply that for a rental...Hope that helpsA joint venture is available to any 2 people co-owning real estate for investment purpose as tenant in common  under Proc. 2002-22 (unrelated to marriage) but it can't be owned in an LLC together. 

My acquaintances are jointly the trust administrators, the property owners are jointly the beneficial owners of the trust.

I have several different loan products but my most popular one is the new joint venture product that hasn't been out very long.

Partnerships & Joint Ventures (a great option for you) Partner with an experienced investor who funds the deal while you handle operations.