Generally the best way to value things is by looking at what similar things sell for. There is not a lot of buying and selling of facade easements. So what you do is value the property as it is and then value it as encumbered by the easement. If you can’t find sales of similar properties both with and without the easement, it all becomes rather hypothetical.
Proving the value of something is not easy. It's hard enough when you are trying to value tangible items, but what about those intangibles? A particular intangible, non-market benefit of great importance has become all the harder to prove: the easement valuation.
The developing trend against easement valuations was pointed out in a recent Forbes article titled “Easement Valuations Not So Easy Anymore” If you have any possible interest in leaving an easement then it’s a problem well worth your planning attention.