So why not set your house as your destination in google maps before you start driving? Then when you emerge from the underground parking, you will have signal again and it can route you correctly. Why do you have to set it once you are already driving? How does that help you?
I have an offline-compatible osm-based map app that can do useful stuff like routing without network connection. Of course it doesn't know about changing conditions ahead, but on the other hand it's nice that it never gives up on me, it can always do its job.
Even when I know exactly where I am going and the route I want to use i'll often map it anyways so I can find out about traffic or accidents ahead.