The book takes place in the U.S. northwest (Washington or Oregon). Young people find a portal to another place / time and begin creating a base there. Eventually they bring across parts for a small plane. As I remember, there are no people on the other side and it was a version of their current location but pristine.
I believe this is Wildside by Steven Gould (who is more known for the Jumper series).
From this review, we see some of the key points, young people, a portal to an unspoiled Earth, and planes:
Eighteen-year-old Charlie Newell has inherited a farm from his uncle Max, and has plenty of ideas how to make money for college from it: In the barn, you see, is a tunnel with a gateway to an alternate world, the wildside, teeming with wildlife and bulging with mineral deposits, where humans never evolved. Charlie brings in his closest friends, Marie and Joey (an item) and Clare and Rick (another item), and insists that they all learn to fly: They'll need planes on the wildside to reach the gold deposits they hope to exploit.
Although it's not specifically mentioned in the review, from my own recollections, they do indeed create a base on the other side first (for taking off and landing planes).
About the only detail that's off is that I believe it takes place in Texas, but (despite the author having gone to university in Texas and setting a lot of his writing there) I also feel like it had something of a Northwest US 'vibe' to it, perhaps from some of the descriptions of the parallel world (which has slightly different climate due to no human intervention)... in fact I thought that part fit perfectly with my own memory until I went to look it up.