Unless the land in your area are very unique and limited. Re. cost and value. You may want to check the National Association of Home Builders website. there are a lot of remodeling info. Very important, you may want to check the zoning by law to see what you can build in a property before making any offer.
Why don't you buy the correct house?
Unless you just want to have a remodeling PROJECT.
You would have to go down to the County Planning/Building to find out if you can do the add-on:
Please don't try to do it without PERMITS.
Good luck and may God bless