In the city limits you do not need a building permit for a "garden shed" that is under 120 sq ft. You do however need a land use permit (
http://www.cityofvancouver.us/upload/contents/439/Land_Use_EDIT.pdf)with a scaled drawing of all buildings on your property (funny thing is you also are supposed to have this land use permit for decks, fences and play structures as well), this is to make sure that you do not have more than 45% of your land covered by buildings (at least this is what I was able to discern). My thoughts on my chicken coop was that it is better to ask forgivness than to be denied and build it anyway after the denial! If you are out of the city limits in clark county then you do not need a building permit or a land use permit.
here is some other info from the clark county site(
http://www.co.clark.wa.us/commdev/code-enforcement/faqN.html):
A permit is not required for:
A structure when it is used 100 percent for agricultural purposes per CCC14.05.101.2. By definition, according to the 2003 International Building Code, Chapter 2, Section 202, Definitions, an "Agricultural Building is a structure designed and constructed to house farm implements, hay, grain, poultry, livestock or other horticultural products. This structure shall not be a place of human habitation or a place of employment where agricultural products are processed, treated or packaged, nor shall it be a place used by the public".
A one (1) story detached accessory building used as a tool shed, storage shed, playhouse or other similar use, provided:
The floor area does not exceed one hundred twenty (120) square feet.
The height does not exceed eighteen (18) feet from grade.
However, the structure(s) noted above must:
Conform to the current county adopted International Building Code.
Meet the setbacks established for the zone in which it is being constructed
Meet all environmental requirements. For environmental inquiries, contact Permit Services at 1300 Franklin Street, Vancouver, WA or call (360) 397-2375 ext. 4489.
edited for grammar!