Why is this important? What are space measurement systems, and how can it possibly matter to my bottom line? This article explains these important concepts as used in commercial real estate.
If you're a tenant, rent, common area maintenance (CAM) costs, taxes, and sometimes the cost of outsourced services are based on the amount of space you rent.
But which space? How can you know you are being charged a fair price based on the space you use? If your building doesn't have utility submetering, are you being charged a share of utility costs based on the space the landlord believes you occupy? What if another tenant has a data center that uses a lot of power? You could be supplementing their energy costs.
Landlords, you also need to know these terms and standards.
Tenants are becoming more and more aware of costs and how they are assessed.
If you're paying for outsourced services, it would help you to know what standard the service provider uses in figuring out how much space they have under contract with you because in most cases, their services are on a cost per square foot basis.
Definitions first:

• Rentable area: Gross area less exterior walls, major vertical penetrations, and interior parking space.
  It's important to note that rentable area as used here is not necessarily the basis for lease agreements.
  
• Useable area: Rentable area less building core and service areas and primary circulation area.
  This measurement is useful in planning and allocating space.
  
• Assignable area: Useable area less secondary circulation.
  This measurement is used for detailed space management and planning purposes.
  
• Cleanable area: Widely used by janitorial services companies in determining their fees.
  Cleanable area can change drastically (with accompanying changes in fees) based on renovations, changes in tenants, and other factors.
  The process used for cleanable space may appear to be a guesstimate, but when you consider the amount and type of surfaces to be cleaned, amount of trash created, furniture, type of floor plan and use of systems or traditional furniture, it can be surprisingly accurate.
  

To make matters more interesting, there are a number of space management systems, each developed for different purposes and by different organizations.
Here are the most common: 1) Building Owners and Managers Association (BOMA) "New Standard"; revised from an earlier standard in 1996.
For those of you in Federal buildings, this is the standard used by the GSA.
What you need to know:

• Space is measured on a building-wide, rather than floor-by-floor basis.
  
• Vertical ducting for HVAC and services, columns, and interior building projections are included in the measurements and tenants absorb these costs.
  
• Useable area for tenants is measured from the office, or tenant side of walls separating rented space from common areas.
  
• Floor useable area includes building-wide common areas, which are pro-rated by the tenants on each floor.
  Common areas include halls, restrooms, entry foyers, stairs, lounges, and electrical and janitorial closets.
  

2) Building Owners and Managers Association "old standard".
This is the most commonly used system and is the most generic.

• Space is measured on a floor-by-floor basis.
  
• Rentable area is the tenant's pro-rated portion of the entire office floor, including lobbies and atriums and is constant for the life of the building.
  
• Amenities shared by all the tenants, such as break areas, lounges, basement parking or storage, etc.
  , are not considered rentable but landlords will factor costs of these areas into their base rental fees.
  
• Other conditions are as listed in the 1996 standard.
  

3) International Facilities Management Association (IFMA) Standard, also known as ASTM E-1836-98 is used for space programming and forecasting and internal cost accounting.
So which system is best? It really depends on your needs and purposes, and should be discussed with your staff and specialists who are knowledgeable and experienced in performing measurements based on these systems.
Each can have a different effect on your bottom line, and in the case of tenants, you want to be sure to be in agreement with the landlord or property manager about which system is used.