What it means
Office tenants pay for more than the space behind their own door. Lobbies, corridors, shared washrooms and amenity areas all cost money to build and run, and the add-on factor is how the landlord spreads that cost across the tenants.
The calculation is simple. The shared area is divided by the usable area, and the resulting percentage is added on top of each tenant's usable space to give the rentable area.
A tenant with 1,000 usable square feet in a building with a 10% factor therefore pays rent on 1,100 square feet. This is why two offices quoting the same rent per square foot can cost very different amounts.
A building with a grand atrium usually carries a higher factor, and the tenant helps fund that atrium every month through the rent. The factor has a close cousin, often called the loss factor or load factor, which describes the same shared space in a different way.
The loss factor divides the shared area by the rentable area instead of the usable area, so a 10% add-on factor is the same thing as a loss factor of about 9.1%. Names and bases differ by market and by landlord, so always ask which one is being quoted.
Confusion over the two is where leases go wrong. Some landlords fold space that is not really shared, such as structural columns or plant rooms, into the quoted factor, which inflates the rentable area without giving the tenant any extra amenity.
Measurement standards exist to stop the argument, and the BOMA standards are widely used for office buildings. For a manager signing a lease, the factor is a negotiation point, not a footnote.
Ask what is included, how the building was measured, and which standard was used, then compare buildings on the effective cost per usable square foot. A high factor is not automatically bad if the shared facilities are genuinely useful to your team.
In practice
Real-world examples.
Example
A law firm compares two offices. One quotes $40 per square foot with a 10% add-on factor and the other $37 with a 20% factor, so the effective costs per usable square foot are $40 x 1.10 = $44.00 and $37 x 1.20 = $44.40. The cheaper headline rent is actually the slightly more expensive office.
Example
A design agency discovers that its landlord counted a basement plant room as common area. A remeasurement under a recognised standard cuts its rentable area from 6,000 to 5,640 square feet, a 6% reduction, which at $50 per square foot saves 360 x $50 = $18,000 a year.
Example
A start-up deliberately chooses a building with a 25% add-on factor because the shared floor includes meeting rooms, a gym and a reception service it would otherwise have to pay for separately. For this tenant, the factor buys real services.
Formula
Calculation
Add-on factor = (rentable area - usable area) / usable area, which equals common area / usable area. Rentable area = usable area x (1 + add-on factor).
Worked example. A floor has 20,000 square feet of usable space and 2,000 square feet of common area. The add-on factor is 2,000 / 20,000 = 10%. A tenant leasing 1,000 usable square feet pays rent on 1,000 x (1 + 0.10) = 1,100 rentable square feet. At $40 per rentable square foot per year, the rent is 1,100 x $40 = $44,000, which is an effective $44 per usable square foot rather than the headline $40.Case study
Seen in the real world.
This fictional story follows an invented operations manager, Priya Rao, who is relocating a 30-person team into 4,000 usable square feet. She builds a comparison table that converts every landlord quote into an effective cost per usable square foot. One quote carries a 20% add-on factor, which means 4,800 rentable square feet.
Priya asks how the factor was measured and learns that it includes structural columns and a basement plant room. After she negotiates them out, the factor falls to 15% and the rentable area to 4,600 square feet. At $45 per square foot, the 200 square foot reduction saves $9,000 a year, or $45,000 over a five-year lease, and the measurement clause becomes part of her standard lease checklist. This is an illustrative scenario with invented figures.
Watch out
Common mistakes.
- Comparing headline rents without the factor, because the building with the lower rate per square foot can easily be the more expensive one once the factor is applied.
- Not asking what the factor contains, since some landlords include space that is not truly shared, which makes the quoted factor look like a shared-amenity cost when it is not.
- Mixing up the add-on factor and the loss factor, which use different bases (usable area and rentable area), so a 10% figure under one is not the same as a 10% figure under the other.
Questions
People also ask.
How is the add-on factor calculated?
Divide the common area by the usable area. A floor with 2,000 square feet of common area and 20,000 square feet of usable area has a factor of 10%, which is then added to each tenant's usable space to set the billable area.
Can the add-on factor be negotiated?
Yes. Both the percentage and what it includes are negotiable, and tenants can insist that the building is measured under a recognised standard such as the BOMA standards.
Does a high add-on factor always mean a bad deal?
No. If the shared space contains amenities your team would otherwise pay for, the factor may be good value, so compare the effective cost per usable square foot and what it buys.
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