Mont. Code Ann. § 70-16-203

Each coterminous owner is entitled to the lateral and subjacent support that the owner's land receives from the adjoining land, subject to the right of the owner of the adjoining land to make proper and usual excavations on the adjoining land for the purposes of construction, on using ordinary care and skill and taking reasonable precautions to sustain the land of the other and giving previous reasonable notice to the other of the intention to make the excavations.