Casita addition before or after refi?

[Clayton Cissell]

I have a house with about 70-90k of equity that I’m looking to refinance out of the property. I’m currently living here with a roommate and have a 3.3% interest rate loan on 300k and still owe about 280k. My monthly payment is 1,314 and the roommate pays 1k. My plan is to take the detached garage and turn it to a casita to also rent out while still living in the main property to offset the increase in mortgage from the refinance. In my market the casita would rent for 1000-1400$ a month. My question is would it be best to build the casita prior to pulling the cash? If so what are the actionable steps I should be taking to get that ball rolling? Look for private money? Pull a heloc and build it out, refi after? What’s the best method here?