I have heard the term “declaration of the building owner” (in czech: prohlášení vlastníka budovy). What is that?

The declaration of a building owner (Section 1166 of the Civil Code) is a legal action of a property owner (usually of a building) that defines individual residential and non-residential units, their accessories and parts, as well as common parts and rights and obligations related to their use. An owner’s declaration is subject to registration in the Land Register.