We do it by the square foot method of valuation.
First, find the asessed value of the land and then the asessed value of the actual buildings. Determine what the percentage breakdown is between them.
For example purposes, let's say the land is asessed at 20,000 and the buildings are asessed at 80,000 - 20% of your value is in the land, in that particular neighborhood.
Then do your regular CMA lookups, and take 20% of the sold or active price and then divide that by the number of square feet in the property- you now have the value per square foot of your comparison property.
Do your regular CMA as though the lots were the same size, and then, for example, if your comparison property had 10,000 square feet, and your listing property has 15,000 square feet, ADD 5,000 times your square foot price to the value of your listing property.
It's a lot of math, but if I've described it right, you should be in the ballpark. I've had success doing this in the past. It also works if you have a larger than normal house.