Reference 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>