Short answer: run two separate analyses — one for the day-one scenario with the vacancy, and one for the stabilized scenario with all units rented.

Here's how I approach it:

First, don't panic about the vacancy. A vacant unit at closing is often a feature for house hackers and can be a negotiating tool for investors. The question is: who is the right buyer for this property, and how does the vacancy affect each buyer differently?

For the investor scenario, I model the property day-one with the unit vacant, then again fully rented at 90% of HUD Fair Market Rent for that bedroom count. The gap between those two scenarios tells you how much of the work is already done for you versus what you're taking on.

On a Manchester 4-unit I analyzed recently, the property was losing $584/month with one 3-bed vacant. Fully rented at 90% of HUD FMR ($2,250/month for a 3-bed in Manchester), it flipped to +$1,200/month. That's the real picture.

For the house hacker scenario, it's different math entirely. Living in the vacant unit, you're comparing your carrying cost to what you'd otherwise pay in rent. In Manchester, a 3-bed rents for $1,800–$2,200 right now. If buying the 4-unit costs you $1,000/month to occupy, you're ahead of renting before year two even starts.

The key is: don't evaluate a vacant unit as purely a liability. It's a variable. Model it both ways, then decide which buyer type you actually are before deciding whether the deal makes sense.