Hi, excess land in a buildable lot or existing residential lot usually runs about 50 to 80% per sq. ft. of the necessary or comparable lot values. It is really in the eye of the beholder and depending on the amenities or features, the additional land could be much more valuable. I have two properties that fall in the situation. One is a lake property that has additional land at the top of the hill that is seperated from the main area by a private roadway. It's not big enough to build on and ends up just being a storage area, so it runs about half the value of the other buildable area.
The other, is a larger lot that has a stream and woods at the rear, a great retreat and private area. The value of that, is viewed as a very marketable ammenity to the property and is worth about 150% for that portion over the comparable buildable lots, everyone one wants live water on their property...
The best way to view it is to look to the comparable properties and lot values, then add value for any special feature that can generlly be proven in the market. Some additional property can be a liability to some, like a swimming pool, more space to care for, more area to be responsible for and not much intrinsic value added. What can the area be used for? What additional amenoities can be added to increase value due to having the additional land? What valuable purpose will it serve? If you really can't justify a usable alternative, the additional value deminsihes as the exess land increases. It's like an opportunity cost, if I'm very thirsty I'll pay five dollars for a drink, if I'm full, I might only be willing to pay 50 cents for another drink, maybe nothing at all. If I have a valuable use for the excess land, I'll pay for it, if not, I may not be willing to pay anything for it, and I have turned down free land before! Value is in the eye of the beholder on this. Good luck, Bill