I'm constantly seeing both new and experienced investors make this underwriting mistake.
Many investors use a flat (let's say 5%) vacancy rate for every property they analyze, regardless of the market, property class, or unit mix. Let's talk about why this is a mistake...
Why vacancy matters so much: Vacancy equals lost revenue, and revenue changes drop straight to your net operating income. Get this wrong and your entire deal falls apart post-closing.
Most investors don't realize vacancy rates should vary dramatically based on three critical factors:
Factor #1: Property Class & Market
A worn-down C-class property takes longer to renovate and lease than a pristine A-class building. C-class properties typically carry higher vacancy rates because tenant turnover involves more work and attracts a different resident base. Don't use the same assumption for both.
Factor #2: Unit Mix
This is the big one most investors completely ignore.
Studios and one-bedrooms = higher vacancy (8%+) due to transient residents.
Three-bedroom units = lower vacancy (3-4%) because families and roommate situations stay longer.
The difference between using 5% across the board versus 8% for studios could mean thousands in lost revenue annually.
Factor #3: Management Quality
Be brutally honest about your management company's efficiency. If they're slow to turn units or you're managing in-house without proper systems, increase your vacancy assumptions accordingly.
Average management company = higher vacancy rates in your underwriting.
Bottom line: Stop using cookie-cutter vacancy rates. A studio-heavy C-class building managed by an average company might need 8-10% vacancy assumptions, while a B-class property with a larger average unit mix managed by an excellent PM might only need 3-4%.
Get this wrong and you'll face cash flow surprises that can sink your investment.
P.S. - One percentage point in vacancy on a 50-unit property can mean $30,000+ in annual revenue. Worth getting right, don't you think?