Tearing down a garage to make a garden?

I would make a raised bed then you don't have to worry about it. Make it about 12" - 15" high and put permeable weed cloth on the bottom (the kind that lets water thru). This way you have new soil put in and the roots will stay about the old soil with the weed cloth as a barrier. Since chemicals tend to go down rather than up, you shouldn't get any kind of contamination. Do a small one first and you will know as the plants will be gnarly if they encounter any contamination. If you want real piece of mind on it, get a soil analysis down by the State Agricultural Extension Service and they will let you know what the condition of the soil is. But I agree with dan, that unless it was being used a some kind of shop or transfer station for chemicals, I highly doubt the average garage generates that much contaminant.

I would assume if there were enough oil in the soil to be harmful nothing would grow in the first place. I personally wouldn't wory about it