Hi Victorius1995 - Welcome to the forums!
I have edited your post to have an English translation and title. As all our staff are English speakers, we ask that players post in English as per the Rules and Guidelines
As our Staff and Moderators are all English Speakers, please avoid using other languages. We need to be able to read everything posted in order to effectively run the forums. If you are unable to or do not feel confident writing in English, please use an online translator. If you are concerned about your post being mistranslated you may post in your preferred language on the condition that a 1:1 translation accompanies it.
Thank you for your understanding.
As to your question, currently the only items that can be built on foundations and floors are furniture. The [Known Issue] post on this contains a tip from another player that allowed them to work around this issue and place items inside, this may hopefully be of some help to you and your cabin - [Known Issue] Difficulty placing crafting stations inside buildings