For a single-car driveway, the standard length is 18 to 20 feet, and the standard width is 10 to 12 feet. For two cars to be parked side by side, the driveway needs to be 18 to 20 feet wide and 18 to 20 feet long.
The minimum width is 10 feet, and a wider driveway lets people get out of their cars without stepping on the grass or garden beds. Even though these are standard sizes, you should leave more room if there are structures near the driveway, like a retaining wall that runs along the property. Homeowners who have to park their cars back-to-back need an extra 18 to 20 feet of length for each car.