Adding bathroom plumbing to a detached garage starts with the basic line installation cost of $900 to $1,300. On top of this base expense, you'll need to budget for each fixture. A toilet costs $250 to $550, sinks cost $150 to $800, and a shower or bathtub costs $400 to $1,900. The project will also require ventilation, moisture-resistant materials, and possibly electrical work. Remember that these costs reflect only the plumbing aspects—additional construction, finishing, and permitting expenses will apply to complete a functional bathroom space.