The work of a construction engineer may be permissible or not if he designs the house for them or draws up plans for them so that they can obtain a Riba-based loan, this action is not permissible. This is helping people in sin and transgression, because this action in this case is connected to the Riba-based loan. But if the designing and drawing of plans came after they obtained the Riba-based loan, then there is nothing wrong with the engineer designing the house, drawing the plans and supervising the construction.