Commercial Pilot students are commonly required to fly additional hours as Pilot-in-command (PIC) in order to meet the minimum time required by Transport Canada to qualify for the Commercial Pilot Licence (CPL)—in addition to the 30 hours solo CPL training, and the 35 hours dual CPL training, CAR 421.30(4)(a)(i) requires students combined experience levels of 100 hours PIC time and 200 hours total time. The "experience gap" between the CPL training hours and the total time requirements is referred to as "time-building."
Note that the addition of time-building requirements only applies to the accelerated Commercial Pilot Program; time-building costs are included in the estimated costs for the Airline Transport Pilot Program.
Here is how it works: If 23 hours PIC are completed during Private Pilot Licence (PPL) training, and the 30 hours PIC are completed during CPL training, an additional 47 hours PIC would be required to meet the minimum 100-hour PIC experience requirements for the CPL.
Similarly, assuming the student has acquired 100 hours PIC flying experience, and has additionally received 22 hours dual during PPL training and 30 hours dual during CPL training, an additional 31 hours PIC would be required to meet the minimum 200-hour total time experience requirements for the CPL.
When you do the math, this is a considerable add-on to the cost of qualifying as a Commercial Pilot—some $15,000 worth of flying—but there is some consolation in the fact that some time-building is also required for the Instrument Rating, where students must complete 50 hours of cross-country flying experience. This will give some added value to the money spent on time-building.