I live in WA State. My neighbors are building a deck all the way to our shared fence. Can they legally do this?
In order to answer this question, you will need to contact your local government. You will need to talk to someone in whichever department deals with issues of zoning and building. This department can have different names in different jurisdictions.
The reason that you will have to do this is that issues of setbacks (how far from a lot line a building can extend) is different in different cities and can even differ from zone to zone within a given city. For example, in the city where I live in Washington State (and where my wife is a city planner who is involved in such things as zoning and setbacks), there are four different types of residential zones and they do not all have the same amount of required setback in a side yard (I assume that the deck is coming up to your side fence).
As you can see from this link, the deck that you describe would probably be illegal in Seattle. But if you live in some other city, the law could well be different.
So, please do get in touch with your local city government. Ask them about the rules that govern setbacks. There will be someone in the zoning department who will be able to give you information that is accurate for your specific property.