If the ground is frozen hard and deep, I would expect him to wait awhile before starting work.
The building process is one that normally has allowances considered for minor problems that may be encountered along the way. A delay of a few days probably will not make much difference in your completion date.
Eventhough there may be snow accumulations, there may be things the builders can do that will not cause a loss in time. Clearing trees and brush, bringing in equipment, preparing the building site etc.
Additionally, if motivated, they WILL be able to move the snow necessary to begin work.........
Best wishes with your new home.
The Eckler Team