A 21-year-old buying a triplex is not a fantasy scenario, it is a math problem with specific solvable parts

The math you laid out is solid. The assumption doing the most work is that appraiser-scheduled rent holds against actual lease execution, and you named it correctly. That is the variable that matters most once the loan closes.

One risk you did not mention: the 75 percent vacancy factor the lender applies to the appraiser's schedule is not the same as the vacancy factor the market applies to your specific units. A lender using 75 percent of a $950 schedule is modeling $712 per unit in qualifying income. If your units actually lease at $780 but take six weeks to fill, your effective monthly receipt in year one is lower than either figure, and a 21-year-old with a lean personal income buffer feels that gap immediately. The question is not whether the units rent; it is how fast and at what price in the first cycle, before you have a track record to attract tenants quickly.

The Fannie Mae ADU income policy worth confirming with your lender: on a one-unit property it now allows projected ADU income up to 30 percent of qualifying income toward DTI, but a triplex is a three-unit property and sits under standard FHA multifamily treatment, not that ADU provision. Confirm in writing which income calculation framework your lender is applying to the specific property type, because mixing up those two mechanisms is a common error in the research phase.

Employment history point is right. Continuous W-2 from 18 or 19 satisfies the two-year requirement; a gap or a switch from 1099 to W-2 partway through creates documentation friction that slows underwriting even if it does not kill the file.

What does the actual rent comp set look like for this market, and are the comparables for that unit size pulling from the same street radius or from a broader area that might be masking a local softness?

The misconception worth correcting: the 75 percent figure applied to rental income in FHA underwriting comes from the appraiser's rent schedule on Form 1007, not from the leases the buyer signs, which means a buyer who negotiates strong leases before closing can still qualify at a lower rental income figure if the appraiser's market rent conclusion comes in below the actual lease amounts.

@reba_finch09 What I would add to what is already in this thread is the self-sufficiency test, which FHA applies specifically to three and four-unit properties and which nobody has named yet. The lender calculates whether the projected rental income from all units, including the one the buyer occupies, covers the full PITI payment. If it does not, the buyer has to qualify on W-2 income alone without the rental credit, which completely changes the DTI picture and eliminates the main affordability argument for buying a triplex over a single-family home at that age. On a 400k triplex with a 3.5 percent FHA loan, you are carrying mortgage insurance and a rate environment that pushes PITI close to 2,800 to 3,000 a month depending on taxes and insurance in the specific market. Two units at the appraiser's 950 each gets you 1,900 gross, 1,425 at the 75 percent haircut, and the self-sufficiency test counts all three units, so you would need the appraiser to schedule the owner-occupied unit rent high enough that the full three-unit gross clears PITI. Whether that pencils depends entirely on what market the triplex is in. What state is this property in, and what did the list price reflect in terms of current rent rolls versus asking price?

What's the actual lease-up timeline you're planning for between closing and having both units occupied, because that gap is where the DTI math that got you approved stops matching the real cash position, and a 30-day vacancy on each unit at 780 is a different problem than a 90-day one.