Establishment of Horizontal Ownership is the act by which the owner of a property divides it and creates more self-sufficient and independent properties, each of which has a certain co-ownership percentage on the plot and on the common parts of the building (foundation, roof, yards and so on). A separate horizontal ownership may be constituted by one or more entire floors that are organically connected between them or a condominium.
It was legally constituted with the law (L. 3471) of 1929 on ownership by floor. It recognizes the per-floor or part –of-floor ownership in the same building, implying the potential ownership of one building by more than one legal person. This marked a crucial change in the relationship of habitants with their built environment. 
The law had as a direct aim to control the extension of the city through prioritizing density and high buildings in the center. The construction of high buildings (polykatoikia), was enabled through this sharing of the cost between more owners. It was also an answer to the housing deficit occurred after the massive influx of refugees from Minor Asia. Above all horizontal ownership was a measure to boost the economy and productivity. The new housing/ownership unit of the apartment introduced the concept of housing as a real-estate product linked to the market fluctuations.
 Marmaras, Manolis (1991), The urban polykatoikia of the interwar-era Athens and the beginning of the intensive exploitation of urban land. (Η αστική πολυκατοικία της μεσοπολεμικής Αθήνας η αρχή της εντατικής εκμετάλλευσης του αστικού εδάφους), Cultural Technical Foundation ETVA (ΕΤΒΑ) p.16