There are several different ways that commission can be calculated for leasing space in a building. As with any commission, the amount and method of calculation must be agreed upon by the broker and the owner. Typically though, commission is calculated separately for each space leased.
One common way to calculate a commission is to take a flat percentage of the entire value of the lease. So for a five year lease on a 5000 sqft space at $12/sqft the commission would be a percentage of $300,000 (5 years * 5000sqft * $12/sqft). This commission is typically paid in a lump sum at the start of the lease, though other payment arrangements spreading the commission over the life of the lease are not unheard of.
Mar 12, 2012