How is that possible? Build-in the garage, attic or that unfinished basement, to add space without increasing the size of the home.
In all cases, the walls and roof already exist, so money is saved from an actual addition to your home. The biggest expense would be extending plumbing piping to the space, if that is on the agenda. In the case of a garage build-in the concrete floor will need to be removed where the waste piping will be located.
Existing plumbing and electrical lines should be located before any work is begun. An electrical contractor should be hired to add any additional outlets and wiring, and a plumber should be consulted for extending plumbing piping for the room.
How will the room be heated/cooled? It may be expensive to reconfigure the duct system; if so, you may choose to use a space heater and/or small split system heat pump.
The garage door and all its mechanical components have to be removed. You must decide whether to fill in the remaining opening with a wall and attempt to match the existing exterior, which will be difficult to do if the home’s exterior has aged some, or a glass system could be installed within the opening, which would provide plenty of light. If this sounds like too much exposure, the opening could be broken up with a panel infill between two windows.
The walls should be checked for adequate insulation. Plan to add insulation if needed; blow-in if the walls are already finished with gypsum board, or batt insulation between the suds, if unfinished – then install gypsum board to complete the wall. Walls can also be finished with paneling or insulated board.
Decide what type of flooring you want. The concrete should be sealed before installing any flooring. You can also build a traditional joist floor over the garage slab to match the floor level in the rest of the home, that is, if the garage floor is lower than the rest of the house, which is often, but not always the case.
Depending on the garage’s existing roof/ceiling structure and the intended use of the space, you could go with a vaulted, exposed beam ceiling, a pitched ceiling, a tray ceiling, a drywall ceiling, or a drop ceiling.
Final comments: This is a lot of work, especially for a non-professional carpenter, or a person with a full-time job! You might seriously consider hiring a contractor to do some, or all of the work. Design, permitting, and materials selection can take considerable time, and a reliable contractor will be able to advise you in making selections that will work for the look and budget that you have in mind.