In a net lease, the tenant pays a share of the building's operating costs on top of base rent. Those costs are billed monthly as an estimate and then compared with actual costs after year-end. If the actual number is higher than the estimate, you may owe the difference. This article explains the pieces and shows the arithmetic.
CAM stands for common area maintenance: costs of keeping shared parts of a property running, such as landscaping, parking-lot upkeep and lighting, snow removal, common-area cleaning, and shared utilities. In many net leases, CAM sits alongside property taxes and insurance, and all three are charged to tenants in proportion to the space they lease. Which items count as CAM, and which are excluded, is defined by the lease, so ask for the list.
Your share is usually your leased square footage divided by the building's total rentable square footage. A tenant with 2,500 square feet in a 40,000-square-foot building has a share of 2,500 / 40,000 = 6.25%. Some leases define the building area differently, for example by counting only leased space, so confirm the denominator in the lease.
At the start of the year the landlord estimates costs and bills the tenant monthly. After the year ends, the landlord totals actual costs and compares them with what you paid. If you paid less than your share of actual costs, you may owe the difference. If you paid more, you may receive a credit.
Suppose a 40,000-square-foot building has a year of actual costs of $240,000 in CAM items, $200,000 in property taxes, and $80,000 in insurance. The total is $520,000, or $13.00 per square foot. A tenant with 2,500 square feet and a 6.25% share owes $13.00 × 2,500 = $32,500 for the year, which is $2,708.33 a month. If the tenant was billed an estimate of $11.00 per square foot ($27,500 for the year, about $2,291.67 a month), the true-up is $32,500 minus $27,500 = $5,000. That is a bill that would not appear in a calculator that used only the estimate. Figures are illustrations.
Some gross or modified gross leases use a base year or expense stop. The landlord covers expenses up to the base amount, and the tenant pays increases above it. If the base-year cost is $9.00 per square foot and a later year is $10.20, the tenant pays the $1.20 difference on its area: $1.20 × 2,500 = $3,000 a year. Ask how the base year is set and whether it is adjusted if the building is not fully occupied.
Enter your best estimate of expenses per square foot per year in the operating-expenses field. To see the effect of a true-up, run the calculator twice: once with the landlord's estimate and once with a higher figure. The difference per year is your budget cushion. Our step-by-step cost guide shows the full workflow.
Estimates can be too low for ordinary reasons. A harsh winter raises snow removal costs. A reassessment raises property taxes. An insurance renewal costs more than the prior year. A major repair that the lease treats as an operating expense lands in a single year. Tenants should also ask how vacancy is handled: if some of the building is empty, the landlord may only be recovering costs for the occupied share, or the lease may allow certain variable costs to be adjusted as if the building were more fully occupied. The lease language decides, so read that clause.
One practical habit is to set aside a monthly amount equal to a modest percentage of your expense estimate, so that a true-up does not arrive as a surprise. In the example above, the $5,000 true-up equals about $417 a month if you had set it aside across the year. You can model the cushion by raising the expense input in the calculator or by adding the amount to "other monthly costs".