The Seattle Land Use Code is now online!
To find out about building projects try
this excellent database: search by project number, address,
DCLU has rather cumbersome online maps of Seattle.They're large PDF files that have zoning info, plot dimensions and all sorts of other good stuff, but you have to download a whole map section (3000k download: don't try this at home!) before you can zoom in to the bit you want.
Try the City of Seattle Property Finder. Here it is, zoomed in to our neighborhood.
The King County online
maps are much easier (as they have search capabilities) ,
and give access to data about the property owner, etc, but don't
have zoning info for Seattle. Here are two examples: