Almost all new build shopping centers are mixed use - they include some combination of office and residential in addition to the retail space. Elizabeth Hamilton, in house Real Estate Counsel at Office Depot, recently reminded me of the special problem this presents in allocating CAM, taxes and insurance. Some portion of each must be allocated to the office and residential components, but should it be on a strict per square foot basis for all users? Taxes and insurance should be allocated among all users equally on a per square foot basis. This means the dominator of the fraction defining a tenant's pro rata share should include all retail, office and residential space. (Of course, creating separate parcels eliminates or reduces the problem.)
CAM may be more complicated. The operating expenses attributable solely to the office component (such as the maintenance of an elevator or lobby area) should be allocated only to the office tenants, meaning that those costs should be deducted from the CAM allocated to the retail tenants. But then should the balance be spread over all tenants, retail and office? Retail tenants use more CAM than office tenants so that may not really be fair. Some landlords analyze it item by item to allocate between office and retail tenants. Some simply figure out what the market rate for office is and deduct that off the top. Others deduct based on a per square foot or percentage reduction and a general application of how they think CAM should be allocated. In any of these methods, the denominator of the fraction is just the retail area (because the aggregate CAM is reduced before the fraction is applied.)
The key here is to recognize the issue and have the Landlord explain how it allocates each item and then to make sure the Lease reflects this methodology. There is definite room for disagreement as to how to allocate, but the actual cost difference is probably not material. However, is this not another reason why fixed CAM is better?
Landlord's have gone to fixed CAM to reduce administrative expenses and disputes with their tenants. The government could accomplish the same by going to a flat tax - no need for complicated tax regulations that create unintended consequences; no need for intrusive audits where the government is at odds with its constituents; in fact maybe no need for the IRS. Take for example the so called "self-rental rule." The Code ( Reg. 1. 469-2(f)(6) if your are keeping score at home) provides (unfairly) that if a taxpayer owns property that it rents to a company in which it has an ownership interest, then any rental loss is passive, but any rental income is active. Really? This means the loss in the first year cannot offset against income in the second year. In fact the loss can never be used at all unless the taxpayer has some passive income from an unrelated deal not involving the rental of property to any entity in which he or she has an interest. Classic case of tails, the taxpayer loses and heads, the IRS wins. 
