What a weird term.
The OED of course has much to say on this matter.
6. Law. (Opposed to personal.)
a. Of actions, causes, etc.: Relating to things, or spec. to real property (see c).
b. Connected in some way with things or real property: (see quots. and Wharton's Law Lexicon).
c. Consisting of immovable property, as lands and houses; esp.
real estate (see estate sb. 11); also attrib.
a. The interest which any one has in lands, tenements, or any other effects; often with qualifying words or phrases, as an estate upon condition, in fee, for life, of inheritance, tail, from year to year, at will, etc.,
real estate, an interest in landed property;
personal estate, an interest in movables;
but the phrases are often regarded as signifying the respective kinds of property. See also fee, tail, etc.
Interesting. The use of "real" as an adjectival qualifier is not a surprise. Real property is land, which cannot just vanish or be destroyed without considerable effort; it is solid as opposed to ephemeral. The contrast between real and personal estate is interesting: real estate is immovable, while personal estate is movable, but both are considered to be property; both are considered to be sources or indications of wealth. They contribute to the owner's net worth. It is a fairly simple distinction. It is the specific legal meaning of the term "estate" that interests me.
The entire concept of the estate, to me, is antiquated. It refers more to the lands of a noble, over which that noble had jurisdiction, than to any little house and plot of land readily available for contemporary purchase. On the noble's estate, no land was ever available for purchase, as the king owned it all anyway, while the noble merely managed it and charged rent. The arable land was available for rent, but as nobles realized they could make more money on animals such as sheep and the consequent wool industry (at least in England, and by the early modern period), houses and farmland were getting more and more difficult to come by.
The management of the estate had nothing to do with ownership; it did have to do with the worth of a noble, or, more accurately, of that specific noble's seat and title. The king gave more lands to those whom he favored; he gave higher rank to those who he thought deserved it. By the early modern period, these estates had been passed down through several generations, such that the noble who then held an estate and its title might not be held in such good graces as were his ancestors.
So what you had was a bunch of nobles, slowly growing out of the feudal system, who were expected to properly manage their estate's lands: to produce as much money or food (preferably both) as possible, while keeping order among tenant farmers, protecting the king's hunting preserves, and implementing tax law. An estate was a privilege, and with that privilege came specific obligations. Yay!
With this in mind, the movement toward real estate as an owned thing is pretty interesting. It makes sense, as the idea of the individual (as opposed to the community; thanks a lot, age of reason) and his private property (as opposed to communal grazing ground) became more important. It makes sense that the language changed with the concept of property: the term "estate" eventually just came to mean what the OED says, immovable property, consisting of land and any structures thereof.
But I think it is strange that the class barriers involved in such language were also broken down so thoroughly. The non-noble individual could become more important as an individual without also using the noble-oriented word: for hundreds of years, estates had been associated only with nobles. It is bizarre to me that a commoner, even one who considered himself equal to any lily-handed noble, would take on noble terminology as well. I mean, class is an awfully loaded issue even now; god knows it was more so then. A commoner -- a merchant of the growing middle class, let's say, who could easily have had more money than many a struggling noble -- who decided that the house he bought was his estate might have ended up with some explaining to do. It would not have been the same thing as declaring oneself a noble, no, but it would still have been a fairly daring move; it would have been like surrounding oneself in the trappings of a noble.
On the other hand, it does make sense that the common people would start to claim the language of the nobles. Such a thing would give them power, and the right to such power; any individual could own property, and why shouldn't they co-opt the term, if just for emphasis? If they were all just as good as the nobles, why shouldn't they use their language? Taking over the term would have been daring, but the merchant would have had a point. It would have emphasized the common people's right to property. So perhaps this was done intentionally. Then again, perhaps it was a natural development in reponse to the sociopolitical issues of the time.