There are several ways to achieve this but explanations are lengthy. Garage Construction Cost Breakdown. 6 Steps To Adding On Above The Garage Room Above Garage Garage Addition House Cost Get free estimates from garage builders near you or view our cost guide below. Cost of building above a garage . Dispose of soil excavated from foundation trench 500. The final price depends on the size and if its attached or detached. Dig and fill foundations with concrete 2750. On average expect to spend around 27000 to build a garage. It could cost around 20000 to build an extension over a single garage including the roof. Ad Search Cost To Build Over Garage. If adding a built-in garage expect the costs to be that of an attached garage with living space above 60000 to 65000 plus the added expense of creating an access point from the garage to the house and from the house to living space above the garage. The cost to build a 1-car garage is between 7500 to 14200 a 2-car garage ...