34 Pa. Code § 14a.21
A flammable or combustible liquid storage tank may be located outside underground, if such installation meets §§ 14a.21-14a.26 (relating to underground storage). The tank shall be so located with respect to existing building foundations and supports that the loads carried by the supports are not transmitted to the tank. The distances from any part of a tank storing class I liquids to the nearest wall of any basement, pit or cellar shall be not less than 1 foot, and from the line of any adjoining property not less than 3 feet. The distance from any part of a tank storing class II or class III liquids to the nearest wall of any basement, pit or cellar, and from the line of any adjoining property shall be not less than 1 foot. When tank excavations are lower than wall or column footings, the edge of the ditch shall be outside a slope of one horizontal to one vertical from the edge of such footing. No tank shall be placed within the legal right-of-way limits of any highway, or any highway under the jurisdiction of DOT, nor within 200 feet of an underground mine opening.
34 Pa. Code § 14a.21
This section cited in 34 Pa. Code § 14a.141 (relating to scope and application); 34 Pa. Code § 14a.151 (relating to scope and application); and 34 Pa. Code § 14a.161 (relating to general provisions).