Converting a garage into a room, suite, or office adds living space within the existing footprint, often cheaper than an addition. But a garage isn't built like living space, so several things need addressing to make it comfortable and legal.
Insulation and climate
Garage walls, ceiling, and the big door opening are usually uninsulated. A conversion needs insulation in the walls and ceiling, a plan for the door opening (framed in and insulated), and a heating and cooling solution since garages aren't on the home's system.
Floor and moisture
Garage slabs are often below the home's floor level, uninsulated, and can be damp. Raising and insulating the floor, adding a moisture barrier, and confirming proper drainage make it comfortable and prevent mould.
Permits and trade-offs
A conversion needs permits, and adding a suite triggers egress, fire separation, and other requirements. Consider the trade-off: losing covered parking and storage can affect resale in some areas, so weigh added living space against what you give up.