Is it considered, Islamically, among the rights of a neighbor that the plot of land adjacent to her dwelling, which customarily has been built upon without transfer of ownership, be built upon?

Since the land is not owned by anyone, and custom dictates that whoever owns a dwelling has the right to build on the adjacent land, that land is considered within the purview of the dwelling’s owner. Therefore, this land is not "dead land" (mawat) that can be acquired by revival (ihya'), because it is not separate from this purview. As long as this plot of land is common property (musha') for the adjacent dwelling, and custom dictates that the owner of the adjacent dwelling is the one who benefits from it, then it is within the purview of that woman, and her right to build on it remains.