A close estimate of the building s air leakage is also necessary. A blower door test is the best way to measure air leakage. A good estimate will also include an inspection of the size, condition of seals on joints and insulaiton, and location of the distribution ducts in forced air systems. The placement of supply and return registers, should be appropriate for the system type and size. The orientation of the house also effects heat gain and heat loss through windows.
Overhangs can reduce solar gain through windows.
Make sure the contractor uses the correct design outdoor temperature and humidity for your area. Using a higher summer design temperature results in over-sizing air conditioners. When the contractor is finished, get a copy of their calculations, assumptions, and the computer printout or finished worksheet. This is your only proof that they did the job right. They are also somewhat useful for very rough sizing.