Developers typically need to pre-sell a certain percentage units in the building before construction has started to prove to their lenders the project is market viable and can be sold out or near to this target. Once the developer has sold the required percentage of presale units in the building by a certain date, the lenders will release the funds to allow construction to begin.
The vast majority of new market housing in British Columbia is pre-sold rather than sold completed. Certain developers like Aragon Properties do not presell all of their units, but this quite rare.