53 Pa. Stat. § 16661
No new dwelling house, or other building within said city, shall front upon any street, alley or court, which shall be of less width than twenty feet, or without being made to recede, so that such street, alley or court shall be of that width, the buildings on each side equally receding; the damages for which widening shall be assessed and paid to the owner in manner provided by law in case of opening new streets; every new dwelling house shall also have an open space attached to it in the rear or at the side, equal to at least twelve feet square, and no building of any kind shall be permitted to be erected on any street, court or alley hereafter to be laid out, or if laid out and wholly unimproved by brick or stone buildings before the passage of this act, of a less width than twenty-five feet; and every builder or owner who shall hereafter build otherwise than as aforesaid, shall pay to the said city one hundred dollars, to be recovered with costs as debts of that amount may by law be recovered, and shall also be restrained by injunction from so building; or if having so built after the passage of this act, from the continuance of such building, contrary to the requirements of this act, and shall pay all the expenses of such alterations which the court may decree to be made. It shall be the duty of the commissioners of highways to give notice to the city solicitor of all violations of this act; and if the building be made to conform thereto on notice without suit, one-half the fine shall be abated.
53 P.S. § 16661