Isaiah is condemning the accumulation of land wealth. The problem with one who accumulated land upon land (joining house to house meaning the consolidation of multiple land-holdings into a single land-holding) is that it cannot be done without displacing one who previously lived on the land. This the ultimate owner is "alone in the midst of all the earth" because he has bought up all of the land and evicted the previous tenants. Being the sole owner, he is now alone because he has removed all others.
"This practice violates the spirit of the Law of the Jubilee, the property law of ancient Israel, which states that "the land shall not be sold forever." (Lev. 25:23.) Instead, land was to remain within families and clans as a perpetual inheritance. (See 1 Kgs. 21, in which Neboth refuses to sell his ancestral lands to King Ahab.) The hoarding of land described in verse 8 was in violation of this law, for when all property was purchased by a few wealthy individuals, there was no place for the original families to dwell. Having no homeland, they were forced to move to the cities or live on the property of the owner as indentured servants or slaves." (Ludlow, p. 117.)
This was no idle question, but rather one of social immediacy in Isaiah's time:
"With Damascus and Assyria both weakened in the first half of the eighth century, first Jehoash and then Jeroboam II were able to reign over a peaceful and increasingly prosperous Israel, while a period of similar accomplishment was enjoyed by Judah under Amaziah and Uzziah. Agriculture and trade flourished. From the Book of Amos we learn, however, that the prosperity and national confidence were experienced chiefly at the summit of the society whereas the majority of peasants were in dire straits. No doubt taxation and corvee played a role, but the particular focus of Amos is on the massive shift in land tenure from traditionally guaranteed family holdings to privately amassed estates taken over by debt foreclosures on impoverished farmers. In short, as in Solomon's united kingdom, the "wonders" of eighth-century Israel were concentrated in a privileged class who rose to their advantages by the systematic deprivation and disempowerment of the peasant majority." (Gottwald, Norman K. The Hebrew Bible - A Socio-Literary Introduction. Fortress Press, Philadelphia. 1985, pp. 345-6.)