If you are familiar with commercial leases, then you probably know that there is a difference between the square feet that you are charged rent on (rentable square feet) and the square feet that are actually contained in your space (usable square feet). There are different ways to calculate rentable square feet which is typically calculated by determining a tenant’s share of the common spaces in the building. This added square footage is called an add on factor. Buildings with a high add on factor, such as 25-30%, have a lot of extra expenses that tenants have to pay for outside of their space. Furthermore if there are areas that are not part of your space, but that you have exclusive rights to, such as balconies, roof decks and similar areas, you may have added expenses which you are 100% responsible for related to the repair and maintenance costs for those areas. It is important when you review a new lease to understand both the space for which you are paying rent and for which you are responsible.