Expense stops split operating cost risk in modified gross leases. See how they work, where base year errors hide, and what a compounding overcharge looks like.
Think your lease might have this issue? Run a free CAM audit to check.
Find My OverchargesCheck your own documents before you keep researching.
Find overcharges in your CAM reconciliation. Most audits complete in under 15 minutes.
Find My OverchargesSee a sample report firstAn expense stop is a lease provision that draws a line between what the landlord pays and what the tenant pays for operating expenses. Below the line: the landlord's problem. Above it: the tenant's problem. In a modified gross lease, that line is almost always defined by reference to a base year or a fixed dollar threshold per square foot.
The concept sounds fair. The tenant only pays increases, not the full cost of running the building. The problem is that the baseline matters enormously. A poorly constructed or deliberately depressed baseline can cost a tenant thousands of dollars per year for the full lease term, and the error compounds invisibly with each passing reconciliation.
For a complete breakdown of how modified gross leases work and where overcharges concentrate, see the Modified Gross Lease Guide.
In a modified gross lease, the landlord sets a rent that includes operating expenses up to a defined threshold. That threshold is the expense stop. Once actual building operating costs exceed the stop, the tenant pays their pro-rata share of the excess.
This structure differs from a pure gross lease, where the landlord absorbs all operating costs regardless of what they are. It also differs from a full NNN lease, where the tenant pays a pro-rata share of all costs from dollar one with no landlord contribution floor.
The Midland Central Appraisal District's 2024 Mass Appraisal Report describes the structure directly: "a general office building is most often leased on a base year expense stop. This lease type stipulates that the owner is responsible for all expenses incurred during the first year of the lease." That first-year cost level becomes the baseline. Everything above it in future years flows to tenants.
The Harris County Appraisal District's 2025 property assessment guidance similarly explains the base year structure for office buildings: the tenant pays base rent plus their proportional share of operating expense increases over the base year amount. The mechanics are the same across most office markets.
There are two common ways to define the stop:
Base year stop: The stop equals the actual operating expenses incurred during a specific calendar year (the "base year"). The tenant pays increases above that actual dollar figure. The stop moves with reality in the first year but is then frozen forever.
Fixed dollar stop: The stop is negotiated as a specific dollar amount per square foot, regardless of what actual costs were in any year. This is simpler but requires the tenant to verify the negotiated rate reflects realistic stabilized costs.
Under a base year stop, the math at reconciliation time works like this:
The tenant pays nothing for the base year costs. They pay only the $25,000 that reflects cost growth above the base year level.
Under a fixed dollar stop at $12.50 per square foot, the calculation is different:
Both structures look straightforward. The risk surfaces when the baseline is wrong.
Variable expenses like janitorial, utilities, and HVAC are not fixed. They scale with occupancy. A building at 35% occupancy has dramatically lower variable costs than the same building at 95% occupancy. If the base year happened to be a low-occupancy year, those variable costs were artificially suppressed. Every future year looks like a cost "increase" even when costs are simply reflecting a fuller building. This is the gross-up problem.
The industry standard for addressing low-occupancy base years is the gross-up: adjusting base year variable expenses to what they would have been at a stabilized occupancy level. Most published practitioner guidance and SEC-filed leases cite 95% as the standard threshold. A publicly filed standard-form modified gross office lease (SEC EDGAR, 2004) uses a 95% trigger concept for variable expense gross-up, confirming this is a documented market standard, not a tenant negotiating position.
BOMA's published guidance on green and standard office leases also identifies 95% as the gross-up target for variable cost categories.
When this adjustment is not made, every year of the lease term carries an embedded overcharge. The formula is straightforward:
If the actual base year variable expense was V₀ but the grossed-up value at 95% occupancy is V*, and the tenant's pro-rata share is s, then the tenant pays s × (V* − V₀) in excess charges every single year.
Over a 10-year lease with a $60,000 annual overcharge:
| Lease year | Annual overcharge | Cumulative |
|---|---|---|
| Year 2 | $60,000 | $60,000 |
| Year 5 | $60,000 | $240,000 |
| Year 10 | $60,000 | $540,000 |
The overcharge does not grow over time in this example, but it does not shrink either. It repeats every year because the base year is fixed and its suppression is permanent.
If the variable expense suppression is larger (common in buildings during construction lease-up or pandemic-era low occupancy), the annual figure scales proportionally. CAMAudit's detection engine flagged a case where a building's base year was established during 42% occupancy. The suppression in variable expenses ran to over $110,000 per year across all tenants at their pro-rata shares. No single tenant's gross-up error was visible, but the aggregate impact over 5 years exceeded $550,000.
Some modified gross leases include a cap on how much operating expenses can increase above the base year in any given year. These caps interact with the expense stop in a specific way that affects the landlord's ability to catch up on prior-year uncaptured increases.
Non-cumulative cap: If costs exceed the cap in a given year, the excess is lost. The landlord cannot roll it forward. In year 3, if costs grew 8% but the cap was 5%, the landlord absorbs the 3% excess. In year 4, the cap resets at 5% above year 3's capped level, not year 3's actual level. This protects tenants in high-inflation environments.
Cumulative cap: The cap applies to cumulative increases from the base year, not year-over-year growth. If costs grew only 2% in years 1 through 3, the landlord has "banked" unused cap headroom. If costs spike in year 4, the landlord can recover more of the spike because the cumulative cap has room. This is more favorable to landlords.
The distinction produces real dollar differences in high-volatility expense environments. From 2022 through 2025, commercial property insurance premiums surged significantly. Buildings with cumulative caps could pass through more of those spikes in the year they occurred. Buildings with non-cumulative caps were limited to the annual ceiling.
When reviewing a modified gross lease, identify which type of cap applies before accepting any reconciliation at face value. The CAM Stop vs. Base Year article covers cap mechanics in additional detail.
Error 1: Un-grossed base year variable expenses
The most frequent and expensive error. The base year was a low-occupancy period and variable expenses were not adjusted. A building at 40% occupancy with $180,000 in janitorial costs should have a grossed-up base year of approximately $450,000 for that category at 95% occupancy. If the base year uses $180,000, the tenant pays their share of the $270,000 "increase" every year going forward, even though it represents normal operating cost, not real growth.
Dollar example: Tenant's pro-rata share is 12%. Annual overcharge from this janitorial gross-up error alone: $270,000 × 12% = $32,400 per year. Over a 7-year lease term: $226,800.
Error 2: Fixed dollar stop set below stabilized costs
When a fixed expense stop is negotiated at $12.00/SF in a market where stabilized operating costs run $14.00/SF, the tenant is bearing $2.00/SF above stop from day one. There is no low-occupancy year to blame. The stop was simply set too low, transferring above-market cost risk to the tenant immediately.
Dollar example: 5,000 SF tenant with $12.00 stop versus $14.00 stabilized costs. Year 1 overcharge: $10,000. Over a 5-year term: $50,000, before any cost growth.
Error 3: Expense category mismatch between base year and reconciliation years
In years 3 through 10, new expense categories appear in the reconciliation that were absent from the base year. The base year balance for those categories is zero. Every dollar of expense in those categories looks like a pure increase above base, even if the expense is ordinary.
This happens when management companies change and repackage line items, when insurance is restructured, or when new service contracts are added. The fix is to restate the base year to include equivalent costs for the new category based on the prior billing methodology. Landlords rarely volunteer this adjustment.
Error 4: Applying the expense stop calculation to the wrong denominator
The tenant's share of expenses above the stop should be calculated based on their pro-rata share of the building's total leasable area. Some landlords calculate the stop threshold based on the tenant's allocated square footage but apply the pass-through to a pro-rata share calculated against a different, smaller denominator. The result is a higher per-tenant charge than the lease authorizes.
Dollar example: A building with 100,000 total SF and a tenant with 10,000 SF should carry a 10% pro-rata share. If the landlord uses a 90,000 SF denominator (excluding vacant space), the share rises to 11.1%. On $200,000 in expenses above stop: the tenant owes $20,000 at 10%, but $22,200 at 11.1%. The $2,200 annual discrepancy compounds over a 10-year lease to $22,000.
Step 1: Locate the expense stop definition in your lease. Find whether the stop is base-year or fixed-dollar. Identify exactly which expense categories are included and which are excluded. Note the gross-up provision, if any, and the target occupancy percentage.
Step 2: Obtain the base year operating expense statement. Request the actual general ledger or expense breakdown for the base year, not a summary. You need line-item detail.
Step 3: Obtain the building's occupancy rate for the base year. If occupancy was below 90%, request the landlord's gross-up calculation for variable expense categories. Compare the grossed-up figure to the actual base year figure. The difference is the potential annual overcharge.
Step 4: Compare expense categories between the base year and the most recent reconciliation. Any category present in the reconciliation but absent from the base year represents a potential phantom increase. Verify whether the category is genuinely new or a rebundling of prior expenses.
Step 5: Recalculate your pro-rata share. Divide your leased square footage by the total leasable area stated in the reconciliation. Confirm the denominator matches the definition in your lease and that it has not changed from prior years without explanation.
Step 6: Verify any cap mechanics. Determine whether your lease has a cumulative or non-cumulative cap on expense increases. Calculate what the landlord would be entitled to pass through under the cap and compare to the amount billed.
I built CAMAudit because tenants receive a one-page reconciliation summary and are expected to accept it. The errors live in the base year calculation and expense category definitions, not in the arithmetic on the summary page. Most tenants never look one level deeper.
The same tenant, the same space, three different lease structures. Numbers are intentionally round to make the logic clear.
Assumptions:
Scenario A: Full gross lease
The landlord absorbs all operating expenses. The tenant pays base rent only. No reconciliation, no exposure to rising costs.
| Year | Building Expenses | Tenant Exposure |
|---|---|---|
| 1 | $600,000 | $0 |
| 2 | $624,000 | $0 |
| 3 | $648,960 | $0 |
| 4 | $674,918 | $0 |
| 5 | $701,915 | $0 |
| Total | $3,249,793 | $0 |
The landlord prices this risk into the base rent. In a rising-expense environment, the tenant benefits if actual expenses outpace what the landlord assumed when setting base rent.
Scenario B: Modified gross lease with expense stop at $8.00/SF
The landlord covers the first $8.00/SF of operating expenses. The tenant pays their 10% pro-rata share of any costs above $8.00/SF building-wide.
The stop: $8.00 x 100,000 SF = $800,000. In this example, Year 1 expenses are $600,000, well below the stop. The tenant pays nothing above base rent until building expenses exceed $800,000.
Year 5 expenses: $701,915. Still below the $800,000 stop. The tenant's expense pass-through for the entire 5-year term is $0 above the stop.
This outcome is tenant-favorable when the stop is set above stabilized costs. The landlord took on most of the expense risk.
Now recalculate with a stop set at $5.00/SF (a stop set below current costs, a common aggressive landlord position):
Stop: $5.00 x 100,000 SF = $500,000. Year 1 building expenses: $600,000. Year 1 excess above stop: $100,000. Tenant's 10% share: $10,000.
| Year | Building Expenses | Above $500K Stop | Tenant's 10% Share |
|---|---|---|---|
| 1 | $600,000 | $100,000 | $10,000 |
| 2 | $624,000 | $124,000 | $12,400 |
| 3 | $648,960 | $148,960 | $14,896 |
| 4 | $674,918 | $174,918 | $17,492 |
| 5 | $701,915 | $201,915 | $20,192 |
| Total | $74,980 |
A stop set $1.00/SF below Year 1 stabilized costs produces $74,980 in tenant expense exposure over 5 years before any other issues arise.
Scenario C: Base-year NNN structure
The tenant pays base rent plus their full 10% pro-rata share of all operating expenses from dollar one, with no landlord contribution floor.
| Year | Building Expenses | Tenant's 10% Share |
|---|---|---|
| 1 | $600,000 | $60,000 |
| 2 | $624,000 | $62,400 |
| 3 | $648,960 | $64,896 |
| 4 | $674,918 | $67,492 |
| 5 | $701,915 | $70,192 |
| Total | $3,249,793 | $324,980 |
The tenant pays $324,980 in operating expenses over 5 years. The full gross lease passes $0. The modified gross with a well-set stop passes $0 in this scenario. The modified gross with a below-market stop passes $74,980.
The gap between these scenarios is entirely determined by where the expense stop is set and whether the base year was properly grossed up. A $1.00/SF difference in the stop level on a 10,000 SF space creates a $50,000 difference in cumulative exposure over a 5-year term.
There is no universally better lease structure. The answer depends on four variables: expense direction, tenant leverage, property type, and term length.
Variable 1: Market direction (rising vs. flat expenses)
In a rising-expense environment, tenants benefit from structures that cap or limit pass-through exposure. A full gross lease or a modified gross with a high stop transfers expense growth risk to the landlord. A full NNN or a modified gross with a below-market stop transfers that risk to the tenant.
After 2022, commercial insurance and maintenance costs rose faster than typical projections. Tenants in full NNN leases absorbed those increases directly. Tenants in modified gross leases with well-set stops did not. When evaluating a lease in a market with above-average cost volatility (coastal markets, properties with aging infrastructure), the value of a landlord-contribution floor is higher.
Variable 2: Tenant leverage at negotiation
If you are signing a short-term lease in a high-vacancy market, you have leverage to push for a higher stop (or a full gross structure). If you are the anchor tenant in a new development, you may have leverage to lock in the denominator and negotiate a base year gross-up provision explicitly.
If you have little leverage, focus less on changing the stop level and more on protecting yourself from bad base year mechanics. A properly grossed-up base year in a rising market is worth more than a slightly higher stop that is still applied against a depressed base.
Variable 3: Property type
Office buildings typically use base-year modified gross structures. This is the market standard for most US office submarkets, and departing from it requires justification.
Industrial properties lean toward NNN. Retail varies: anchor tenants often negotiate modified gross or expense stop structures; smaller inline tenants typically sign NNN leases with the anchor's leverage pulling down the denominator.
For office tenants, the question is not usually which structure to accept but how to negotiate the base year mechanics and gross-up provision within the modified gross framework.
Variable 4: Lease term length
Longer terms increase compounding exposure. On a 10-year lease, a $30,000 annual overcharge from a bad base year totals $300,000. On a 2-year term, the same error produces $60,000 in exposure. The longer the term, the more critical it is to get the base year right from day one.
Short-term tenants have less exposure and less negotiating leverage on economics, but more flexibility to simply not renew if billing practices are adversarial.
The practical framework:
Choose the modified gross structure with a high stop (above stabilized costs by at least $1-2/SF) when you have leverage and the market is in a rising-expense cycle. Confirm the base year gross-up provision is explicit and uses 95% as the occupancy target. Lock the denominator RSF in the lease at a fixed figure.
Accept a base-year NNN only when base rent reflects the NNN structure (i.e., is materially lower than gross lease comps for the same space), and audit every reconciliation year without exception.
An expense stop is a fixed dollar amount per square foot that defines the landlord's annual operating expense contribution. A base year uses actual operating costs from a specific reference year as the threshold. Both structures result in the tenant paying increases above the threshold. The key difference is that an expense stop is negotiated as a static number, while a base year is derived from actual costs incurred, making the base year more sensitive to occupancy conditions during that specific year.
Operating expenses like janitorial, utilities, and HVAC scale with occupancy. A base year with 40% occupancy will show significantly lower variable costs than a stabilized building at 90% or 95% occupancy. When that suppressed figure becomes the baseline, tenants pay their share of the difference as a cost "increase" in every future year, even though the costs simply reflect normal occupancy. The industry standard is to gross up base year variable expenses to 95% occupancy to correct this distortion.
If the actual base year variable expense is V₀ and the correctly grossed-up amount is V*, the understatement is (V* - V₀). Each year, the tenant pays their pro-rata share s of that understatement: s × (V* - V₀). This repeats every year for the full lease term. On a 10-year lease with a $50,000 annual overcharge, the total excess paid is $500,000, all traceable to a single error in year one.
Yes, within your lease's audit window and the applicable statute of limitations for contract claims. Most state statutes of limitations for written contracts range from 4 to 10 years. Illinois allows up to 10 years for written contracts. Course-of-dealing defenses can apply if you paid under the same methodology for many years without objection, so acting promptly after discovering a base year error is important.
The industry standard is 95% occupancy, documented in published practitioner guidance, BOMA materials, and publicly filed office lease forms. A 95% target means variable expense categories are adjusted to reflect what costs would have been with the building at 95% capacity. Some leases specify 90% or 100%. Whatever the lease says controls. If the lease is silent on the gross-up percentage, 95% is the documented market standard.
What is an expense stop in a modified gross lease?
An expense stop in a modified gross lease is a threshold that defines the landlord's maximum annual contribution to operating expenses. Expenses below the stop are the landlord's responsibility. Expenses above the stop are passed through to the tenant at their pro-rata share. The stop can be defined as a fixed dollar amount per square foot or by reference to actual operating costs in a base year.
Why does a low-occupancy base year create an overcharge?
Variable operating expenses like janitorial, utilities, and HVAC scale with building occupancy. A base year with low occupancy produces artificially low variable expense figures. That suppressed figure becomes the permanent baseline for future reconciliations. Every year after the base year, tenants pay their share of the gap between the suppressed baseline and actual stabilized costs, even though the costs reflect normal occupancy rather than real expense growth.
What is the industry standard for base year gross-up occupancy?
The industry standard is 95% occupancy. Published practitioner guidance, BOMA lease materials, and SEC-filed office lease forms consistently reference 95% as the target for grossing up variable base year expenses. This means variable cost categories are adjusted to what they would have been if the building had been operating at 95% capacity during the base year.
How does the compounding formula work for an un-grossed base year overcharge?
If the actual base year variable expense is V₀ and the correctly grossed-up amount is V*, the tenant pays their pro-rata share s multiplied by the understatement (V* - V₀) every year. This repeats for the full lease term. On a 10-year lease with an annual overcharge of s × (V* - V₀), the cumulative excess is that annual figure times ten years, all caused by a single base year calculation error.
What is the difference between cumulative and non-cumulative expense stop caps?
A non-cumulative cap limits year-over-year expense increases independently each year. Any excess above the cap in a given year is absorbed by the landlord and cannot be carried forward. A cumulative cap applies to total increases from the base year, allowing the landlord to recover more in a high-cost year if prior years were below the cap ceiling. Non-cumulative caps provide stronger tenant protection in volatile expense environments.
This article is for informational purposes only and does not constitute legal advice. Lease interpretation, operating expense obligations, and dispute rights vary by specific lease terms and jurisdiction. Consult a licensed commercial real estate attorney for advice specific to your situation.
