I live in Snohomish County, and can't think of any real issues that would be a concern other than where the water table is in your area. In my subdivision that is was a strawberry field in a marsh, our water table is literally 3" below the surface of the soil! Still, people have patios and are able to successfully pour concrete....

You might want to think of doing a covered patio so you can BBQ year round.

Hi, Danielle - I live in Seattle, too! All the rain means things have
no trouble growing - especially weeds. So, be sure you put a good weed
barrier down once you've excavated the site. Otherwise, you'll be
fighting an assortment of weeds in no time.

Hello Danielle, the biggest key in our environment with a patio is to be sure it drains properly and is laid at the proper slopes or sloped to an internal drain accurately (and does not cause drainage issues back towards the house from runoff). There are also a number or paver options that are permeable and infiltrate water directly into the subgrade and clean free draining rock, these options only continue to expand.

Obviously another key point would be to pick a material and color choice you like and correlates well with your current landscape and house textures, colors and finishes assuming it will be near the house, but even if it is not it is a good idea to create some synastry between the two. You will also want to identify what the main use of your patio is, is it for leisure or sitting only, for meal gatherings, children's play or other use...all of these ideas go into planning the size, shape and finished product. We have designed and installed many non-pervious and pervious patio types with each project being a little different depending on many of the factors mentioned. Feel free to reach out if you are looking for some more ideas or direction. True Scape Design.