If you go attached, depending on your location, it means a full below frost depth foundation is required. If you go detached you can likely do a slab. Also, like said before a detached garage may be built larger depending on set back requirements.
When I built the 7-car garage at my previous house, the city would limit me to 900 sq ft for a detached garage. I asked, "would an attached covered walkway from the garage to the house define it as "attached"?" They agreed. I designed a covered walkway from the back door of the house to the side door of the garage, with a connection to the garage wall that allowed for vertical deflection. City approved it and I built a 1350 sq ft "attached" garage. Still wish I was in that house.