The sources that I have seen would consider the inheritance as income. For example, someone gives a gift from money that had paid ma'aser on it. This is still considered complete income, because you have not paid any money for it. However, see below that you do not need to pay the ma'aser until you actually liquidate (sell) the property and obtain cash for it. Some people will estimate the value and pay ma'aser on the current value so as to establish a "purchase base" for future sale, but the Business Halacha citation below would say that it is not required.
Some people will pay the ma'aser for the person that has received a gift (such as a gift to children) or cause an inheritance to "skip" a generation (go to grandchildren for example) to prevent the intermediate generation from having to pay ma'aser.
Maaser from Inheritance
The fact that somebody has already taken maaser from particular moneys
does not have any ramifications for somebody who receives the money as
an inheritance, because maaser is a person obligation and doesn’t
apply to the money (but to the person).
Separating Ma’aser from Inheritance and Gifts
With regard to inheritance, the Pischei Teshuvah (249:1) writes in the
name of the Shelah that there is a full obligation of separating
ma’aser. This applies even if the deceased was always particular in
tithing his income. The principle behind the ruling is that the
obligation of ma’aser kesafim, unlike the tithing of produce, is
incumbent on the person (gavra) rather than the money (cheftza). When
a person gains, he becomes obligated in tithing, irrespective of how
many times the money had already been tithed.
The custom is not to separate Maaser from gifts of items or real
estate that a person receives, such as wedding presents or an
inherited home or land. However, if the receiver liquidates these
gifts or inheritance, or returns the gift for a cash refund, Maaser
should be separated from the money that is received.
See Tosafos in Taanis (9a), the Sefer Chassidim (Ch. 144), the
Rabbeinu Yona in Sefer HaYirah, the Teshuvos Yaavetz (Vol. 1 Ch. 8).
This is also the conclusion of the Chazon Ish.