The applicant has solid credit score of 750+ and has a good household income as well.

Local Credit unions and small regional banks.  

My credit score is 640 and DTI is 27%.

Your lender will likely base the loan off just one income/credit though (at least that's how my lenders have done it)I would personally advise against it - life is a windy road and you never know when one of you may want out of the deal even if both have full intent to stay in from the onset.

Based on what you've shared, here are some options to consider:Leverage Home Equity – Use a Home Equity Line of Credit (HELOC) or refinance to access funds for a down payment on an investment property.House Hack with a Duplex – Sell your home and buy a duplex.

Credit score- the higher the best. 760-780+ generally gets best pricing for investment property loans with most lenders.

One objection that is common now a days is the first mortgage rate being low and the home owner not wanting to lose that low first mortgage rate.The other objection is they want to use the Heloc like a credit card only making a payment on the portion that they use rather than the full amount.

., but it does make things much easier AND give you access too a wide range of benefits, not the least of which is obtaining your SSN and being able to build U.S. credit and access financing.

Tax Treaty, which allows you to claim a foreign tax credit on your Canadian return for taxes paid to the U.S.

For your client's cash-out refinance on an adult family home in Washington State, it would be helpful to reach out to lenders who specialize in DSCR loans, especially those experienced with unique property types like adult family homes.Contact Local Lenders: Start with local banks and credit unions in Washington that offer DSCR loans, as they may have specific programs tailored for adult family homes.