Frist lets talk about the concept of externalty:
i found this somewhere time ago, alrready translated so it will alleviate my poor english skills and illustrates the point fairly well

Let us assume that you own a home in a sub-division. You spend thousands of dollars landscaping and maintaining your home site. You maintain your house very well. Your neighbor is one of those folks who just does the minimum. His/her place looks O.K., but it is not on the Parade of Homes list.
Do your actions increase the value of your neighbors home?
If he/she were to place their home on the market, would the potential buyer look around at neighbors homes and the neighborhood in general?

I think you would agree that you have added value to your neighbors home. Now how will you get that value added to your neighbor’s home when he/she sells it? Will you politely ask for your share at the closing? What is your neighbor likely to tell you? Thus we have what is known as a positive externality.

(edit: and how can you measure the exact value of that externalty? on this example would be by comparing the price of the house of your neighbor, the one with great sights to your masterwork with another, beyond sigh in the same partition, that, with everything else the same, hasnt those sights, more precisely with one that has the average neighborhood sights (oh if all the externalties were so easy to measure...)

Now assume you have a neighbor that does not maintain their home site well at all. Your neighbor has junk cars in the back yard on cinder blocks, a goat running around grazing the over grown crabgrass in the yard, a cat crapping on your yard marking it as his, and paint peeling off the siding and trim. If you were to sell your house this would probably diminish the value of your well kept home (negative externality). How would you recover this loss?