Responsa for Bava Metzia 234:14
<big><strong>מתני׳</strong></big> וכן בית הבד שהוא בנוי בסלע וגינה אחת על גביו ונפחת הרי בעל הגינה יורד וזורע למטה עד שיעשה לבית בדו כיפין
Whereas if the owner of the upper storey wishes to alter to hewn stones, he is permitted; to unhewn stones, he is not permitted; to half-bricks, he is not permitted; to whole bricks, he is permitted; to ceil it with cedars, he is not permitted; with sycamores, he is permitted; to increase the number of windows, he is permitted; to diminish them, he is not permitted; to elevate the [upper storey], he is not permitted; to decrease its height, he is permitted.<span class="x" onmousemove="('comment',' He may weaken the upper portion, thereby giving the lower a lesser burden, but not strengthen it through increasing the burden. ');"><sup>14</sup></span>
Teshuvot Maharam
A. Those parts of the beams which extended into A's portion belonged to him exclusively, and he had a right to cut them off, especially since the intervening wall belonged to him. Although the act of sawing and chopping weakened B's and C's structures, since the extension of the beams into A's room served no useful purpose to B and C, and because of their unnecessary weight were even detrimental, A had a right to cut them off. In any event, B and C are not entitled to any compensation for their loss, now that the act has been done.
SOURCES: Am II, 198.