Property taxes, insurance, and loan payments tend to remain stubborn no matter how many units are occupied. Treat them as the immovable pillars of your model, and measure everything else against their weight so surprises become manageable rather than catastrophic.
Utilities for common areas, turnover labor, cleaning, minor repairs, and management fees often shift with the number of leased units. Track these drivers monthly, then calculate per-unit and per-occupied-unit rates to strengthen forecasts and protect margins when leasing momentum slows.

Estimate gross potential rent, subtract typical concessions, and apply expected vacancy. Then compare remaining income to expenses and debt to reveal the occupancy level required to cover everything. Many owners discover a surprising cushion, or an alarming gap, once numbers are honest.

Identify average revenue per occupied unit, then subtract the variable cost tied to that unit. The remainder is your contribution margin. Divide fixed obligations by this margin to estimate the number of occupied units needed to reach parity without heroic assumptions.

Nudge inputs by small amounts to test resilience: raise insurance five percent, pull rents down ten dollars, add one turn per year. If the model collapses immediately, the plan depends on luck. Iterate until modest shocks feel survivable, not existential.