Yes, you certainly can install solid wood flooring in an unheated cottage. In fact, lack of heat in the winter is actually a benefit when it comes to solid wood. It’s wintertime heating that dries wood out, causing stress, gaps and cracks. No heat means less risk of ugly surprises. How fancy is your cottage? You can use any kind of ordinary flooring woods you like, but a casual style opens other options, too. I’ve seen inexpensive, #3 pine used as cottage flooring. All the knots and swirly grain looks great. I’ve also used #2-grade 1×6 tongue and groove white pine flooring when I built my own place and it works well. Pine is soft, but that’s not a deal breaker as long as you don’t mind dents developing over time. The only conventional wood I wouldn’t use for flooring is cedar. It’s softer than pine and too prone to damage in my book.