An appurtenance is something subordinate to or belonging to another larger, principal entity, that is, an adjunct, satellite, or accessory that generally accompanies something else.
In a legal context, an appurtenance refers to a right, privilege, or improvement belonging to or that accompanies a principal property.
The idea being expressed is that the backyard "belongs" to the house, which is the more significant of the two properties.
In Gestalt theory, appurtenance (or "belongingness") is the relation between two things seen which exert influence on each other.
In the English language, appurtenances are most commonly found in toponyms and demonyms, for example, 'Israeli', 'Bengali' etc.