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From the information contained in your letter, your engineering services are not subject to tax. However, the portion of your design of water and sewer plants which relates to a permanent building required to house the process facilities is subject to tax. If you subcontract the building design services, you may purchase these services on a resale certificate and charge tax to your customer. In this regard, the Department does not consider the design of small, mobile control buildings that can be easily disassembled to be taxable as building engineering services.