Residential property consists of a co-ownership share in commonhold property, (land and all buildings not under special ownership) and special ownership of rooms for residential purposes. Residential property is established upon construction of a building with different residential units in order to create commonhold apartments. The property is divided up into co-ownership shares on the basis of the declaration of division, which also must be notarised, and special ownership in a certain apartment is assigned to these shares. The declaration of division also contains the rules governing the relationship between the apartment owners, the so-called commonhold property regulations, which are supplemented by the German Act on Residential Property (WEG). https://www.vpmk.de/en/en/residential-property
Wohnungseigentum besteht aus einem Miteigentumsanteil am Gemeinschaftseigentum (Grundstück und alle Gebäudeteile, die nicht Sondereigentum sind) und Sondereigentum an Räumen, die Wohnzwecken dienen. Wohnungseigentum wird begründet bei der Errichtung eines Hauses mit verschiedenen Wohneinheiten zur Schaffung von Eigentumswohnungen. Das Grundstück wird hierzu aufgrund der Teilungserklärung (ebenfalls zu beurkunden) in Miteigentumsanteile aufgeteilt und diesen das Sondereigentum an einer bestimmten Wohnung zugeordnet. Die Teilungserklärung enthält auch die Regeln der Wohnungseigentümer untereinander (sog. Gemeinschaftsordnung), die durch das Wohnungseigentumsgesetz (WEG) ergänzt werden.
https://www.vpmk.de/de/immobilienrecht/wohnungseigentum

I´m still looking for a convincing reference

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Note added at 28 Min. (2016-08-23 20:03:31 GMT)
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OR: "shared ownership Apartment"
Should my son buy a shared ownership flat? | Money | The Guardian
https://www.theguardian.com/money/2007/apr/.../personaleffec...
... good salary but can't afford anything here in the south-east of England. He's thinking of buying a shared ownership flat but I have my doubts.

German term (edited): WE-Anteil > Wohnungseigentumsanteil

Compare in AUT: Eigentumswohnung which might tentatively be your actual 'owner-occupied' or, pace Allegro, freehold flat.

Wohnungseigentum: §§ 1,9 WEG = AT vs. DE Wohnungseigentumsgesetz.

Dietl/Lorenz dictionary: flat property (vs. flat beer) or AmE: (cooperative) apartment ownership (mixing up the Swedish scheme of cooperative housing).

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Note added at 42 mins (2016-08-23 20:17:10 GMT)
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BTW, I am both a commonholder in a block of flats in West London and, in Vienna, a Wohnungseigentümer with an Eigentumswohnung. So feel free to claim that my answer is borne of ignorance.

Explanation:

1. "Proportional" and not "percentage" as this is clarly expressed as fraction (see asker's text) and not as a %

2. "Commonhold", whilst it may have similarities, is a UK and/or US etc., creation. I think it is better to use a "neutral" term here which does not suggest that "commonhold" is the identical system as that applicable in Germany. We cannot assume that the English-speaking reader (who might for all we know be from Iceland, or Estonia) will identify with British/common-law terms.