I am dealing with this right now, also. See the "Separation Between Garage an Room Above" thread on the Building a Home forum.

From the 2003 IRC...
309.2 Separation required.
The garage shall be separated from the residence and its attic area by not less than 1/2-inch (12.7 mm) gypsum board applied to the garage side. Garages beneath habitable rooms shall be separated from all habitable rooms above by not less than 5/8-inch (15.9 mm) Type X gypsum board or equivalent. Where the separation is a floor-ceiling assembly, the structure supporting the separation shall also be protected by not less than 1/2-inch (12.7 mm) gypsum board or equivalent.

In our locality a garage door would indicate a garage and therefore require the fireproofing. Would a large sliding patio door suffice? I suggest talking to you local inspector and tell him what you plan to use the room for.

Thanks Tom. I checked out the thread you mentioned in Building a Home forum. I see that a suspended ceiling could also be used and meet the fire code. I was thinking of a double door hinged to swing open and allow easy access for the riding mower. But if there is the possibility of parking a car in the basement, the garage door is probably best. We were hoping to avoid the extra expense of fireproofing the unfinished basement area. I guess it will have to be finished to some extent after all. It would be cheaper than building a detached garage.

Also check about the furnace and water heater. You may have to get them installed higher off of the basement floor.

I didn't think about elevating the furnace and water heater. I know that is required in a regular garage. I wonder if enclosing the furnace and water heater on the floor in a separate room would pass inspection. Another question to ask the builder.