Hi Anneette,  overwintering geraniums in Western Washington is very dependent on how severe of winter we experience. Rather than use a cold frame try moving the plants roots and all into a cold garage or garden shed. They do not need much light as you want the cold and darkness to put them into a dormant state. Then in April repot the plants and bring them indoors for a few weeks until the weather warms. I have also had luck overwintering potted geraniums by storing them under a picnic table shoved up close to the house. Cold, but not freezing conditions and dry soil seem to be the key. Good luck and hope your spring is filled with geraniums. Keep growing, Marianne Binetti