So, we first made a foundation out of concrete for the garden house in our garden, to be sure... so that it doesn't mold, and then built the house out of wood. I would recommend proceeding this way..
So we have now decided on a complete wooden garden house. I also built our foundation out of concrete, simply to guarantee a solid base. We ordered the house itself online from [Gartenhaus GmbH], as building it ourselves was too complicated for us.