On average, the basement is 1/3 of the entire house, and it doesn't must be a storage room with bare walls and a place for appliances. It can be the same part of the house like the other floors, where you can live or give it in rent to someone, especially if you have a separate entrance, which can also be built. And, of course, it is often used as an investment before selling the house, because it's playing one of the key roles in calculating the home's selling price