predlog
www.gov.me/files/1270034212.doc

As I lawyer I might shed some light to your legal-language dilemma. The thing is that literal translation of the term you posted is closest to the one that Daryo proposed – unregistered land/real property ownership. But your description of the term and the way you understand it refers to something different. It seems to me like a situation where a certain building has been erected on the land that does not belong to the owner of a building. Although it may seem as very unusual, trust me that this sort of legal situation is rather regular than exceptional especially because it originates from the communist era when private ownership was not cherished that much. Anyway, according to our laws, there is possibility that people acquire ownership over such real property/land by way of peaceful, continuous and undisturbed possession over certain period of time. In Serbian it is called održaj and English term for that is adverse possesion. So, I propose the following translation: ownership acquired by way of adverse possession or eventually actual/factual ownership. Please consider my proposals as an alternative to what Daryo proposed as its suitability depends on the context in which the term is used.

ownership (of land in the sense of "immovable property") that hasn't been registered in official registers (Land registry, Cadastres etc...), or in some cases, more precisely "still registered to some previous owner".

In Serbia, it's an ownership on land that hasn't been recorded in official registers but is provable only by a chain of contracts starting with the registered owner.

as in
"Kako kupiti garažu čiji vlasnik po knjigama ne postoji ..."
[http://www.kupovinanekretnine.info/saveti/2012/04/25/kako-ku...]


The rough equivalent in UK would an "unregistered land"

"Unregistered land in English law

Unregistered land in English law is land that has not been registered with HM Land Registry. In unregistered land proof of title is based upon historical title deeds.
..."

[http://en.wikipedia.org/wiki/Unregistered_land_in_English_la...]

--------------------------------------------------
Note added at 3 days5 hrs (2013-07-04 17:52:54 GMT)
--------------------------------------------------

another term commonly used, in Serbia as much as in Croatia: "vanknjižno vlasništvo"

"Vanknjižno vlasništvo je najnormalnija stvar. Kupio neko zemljište, sagradio kuću, ali nije otišao u katastar da je uknjiži. Njegova je definitivno ..."
[http://arhiva.elitesecurity.org/t255394-Kupovina-nadzidjivan...]

"Ukoliko je u pitanju vanknjižno vlasništvo , onda je jako bitno da utvrdite da li je kod predhodnog sticanja regulisan porez ( da li je plaćen, da li to lice ..."
[http://www.besplatnapravnapomoc.rs/2012/05/molim-vas-da-me-s...]
and in fact there was a question about vanknjižno vlasništvo 7 years ago:

Serbian term or phrase: vanknjižni vlasnik
English translation: unregistered (property) owner
[http://www.proz.com/kudoz/serbian_to_english/real_estate/142...]