Responsa for Bava Batra 12:8
אמר ר"נ אמר שמואל גג הסמוך לחצר חבירו עושה לו מעקה גבוה ד' אמות אבל בין גג לגג לא ור"נ דידיה אמר אינו זקוק לארבע אמות אבל זקוק למחיצת עשרה
The Master has just said: 'The one should make a parapet for half his roof and the other should make a parapet for half his roof, In such a way that the parapets do not face one another, though each should extend [his parapet a little beyond the middle].' Surely this rule is obvious? — We require it for the case where one of the owners builds a parapet first [without consulting the other]. You might think that in that case the other is' entitled to say to him: Complete the parapet and I will reimburse you.<span class="x" onmousemove="('comment',' Lit., 'take the expense.' [Heb. uzinka, a Persian word meaning 'cost', v. Krauss, TA. I, 20.] ');"><sup>12</sup></span> We, are therefore told [that he cannot insist upon this], since the other can say to him: Why don't you want to build? Because it might weaken your wall. I too [don't want] my wall to be weakened.
Teshuvot Maharam
A. If B is a resident of A's town,*The author uses the term Bar-meboo, a resident of A’s alley, but he is only using the talmudic expression. The decision, however, is that in his own house B may pursue any craft he wants; while A may restrain B from settling next to A and pursuing A’s craft. A can not restrain him from pursuing his craft. The secular government, according to Jewish law, has no jursidiction over trade competition; therefore, the Jews are not bound to abide by their decree in such matters.
SOURCES: Pr. 677.