The cost of building a mudroom addition will vary greatly depending on the size and complexity of the project. Factors such as the size of the mudroom, the materials used, the amount of labor and installation required, and the amount of custom features requested can all impact the total cost. Additionally, any plumbing or electrical work that is required will add to the total cost. On average, the cost to build a mudroom addition is between $15,000 and $60,000, but could be higher or lower depending on the specifics of the project.