I'm not from Seattle, but it sounds like you're talking about the 3 Line https://en.wikipedia.org/wiki/3_Line_(Sound_Transit) which will join the current 1 Line and head north up to Everett, WA. Estimated completion date of either 2037 or 2041 based on funding.
When I visit Seattle I only use public transit or walk to get around. I use the light rail as much as possible, but it only gets you kind of the way to anywhere. Plan on an up to quarter mile walk to a bus stop and then probably an additional bus to actual get to where you want to go. The previous poster is right in that you need to add at least an hour to your transit time to account for waiting for connections.
Also, a large portion of 1 Line's southern section is at-grade with auto traffic.
When I visit Seattle I only use public transit or walk to get around. I use the light rail as much as possible, but it only gets you kind of the way to anywhere. Plan on an up to quarter mile walk to a bus stop and then probably an additional bus to actual get to where you want to go. The previous poster is right in that you need to add at least an hour to your transit time to account for waiting for connections.
Also, a large portion of 1 Line's southern section is at-grade with auto traffic.