@William Sing Thanks for tagging me in!
@Adam Wayne A few thoughts come to mind when it comes to the mortgage side of things:
FHA's rehab loan is the 203k. Fannie Mae and Freddie Mac also offer rehab loans: HomeStyle for Fannie and CHOICERenovation for Freddie. All three allow "non-occupying coborrowers" (NOCB), which is to say, folks who'll cosign, but not move into the property.
If you want to do the work yourself, however, FHA is your best bet. It's much more friendly to "self-help" renovation loans where you act as your own general contractor. You'll need to sell the underwriter on your ability to do the work in a timely manner (6 months or less), while keeping up your day job ('cause you gotta have income to pay the mortgage, right?). If you go the self-help route, you'll be required to finance the cost to have a contractor do the renovations, but you'll only be able to use the loan to pay for materials (you can't pay yourself for labor). The logic: if you can't (injury?) or don't complete the work in a timely manner, they want to have funds escrowed to pay a GC to step in. Any money you save by not having a GC involved will be paid down on the loan after the work is complete.
Rates on reno loans are higher, so you'll probably want to refi when you finish up, so your final, long-term loan will be smaller and reflect your hard work. But you need to qualify for the bigger payment on the bigger loan.
Very importantly, FHA will allow you to put 3.5% down on the aggregate of the purchase price and all hard and soft renovation costs (the actual materials plus the loan-related costs plus a "contingency" reserve which is usually an extra 10% on top of the estimated costs to do the work) for a one-unit or two-unit finished property. BUT if you have a NOCB who is not family AND/OR your finished product is a two-unit property, your down payment requirement increases to 25%. Notably a house with an ADU is still considered single-family.
With a NOCB, Fannie and Freddie will require 5% down on a single family (with or with out an ADU) or 15% down on a two-unit finished property. Terms won't change if your cosigner is family or not.
I would add a caution about the size of your loan relative to FHA loan limits, which vary a lot by county, but it looks like you're in Seattle. IF that's where the property is too, you're cool. But if not (or for anybody looking at this post later), here's where you can look up the FHA loan limit for any county in the US: https://entp.hud.gov/idapp/htm...
Hope this helps!