Buyers like having a large lot, so value would depend on how much de-value splitting the lot off would cause to the current property. The value is somewhat reduced if you leave it up to the buyer to do all the work of splitting it off.
Your best plan of action is getting an analysis of what your property would be worth without the extra land, and, what the split off property would sell for. If you choose to to sell it that way, it's best that it's done prior to putting it up for sale as there are a lot of actions to go through to actually get it split if you feel the two values are significantly higher than selling it as a package. A buyer will not pay full value for a lot that has not been through the rigorous splitting process, there is just too many unknowns, i.e. will the county/city really allow the split - there are always nuances that show up during the process.