industry standard question - add on quote to a buyer

[Matthew Hicks]

Hi Everyone,

We have a buyer for a home we are re-developing in a historic area.  The buyer wanted to put a roof top deck onto the house.  We provided them with a quote with a fair amount of cushion on from our estimated cost due to uncertainties surrounding constructing a deck on the top of a sloped roof (the deck will be accessed from the third floor out to the roof of he 2nd floor as the back half of the house is only 2 floors).  

We learned after the fact that we had to go to historic review commission to get the deck approved (despite it not facing the street or being visible from the street).  In any case, it now looks like the deck is going to have to be built into the roof itself, increasing our costs rather substantially.

So is there an industry standard of sorts to consider when discussing this with our buyers?  Should I be eating the additional cost, or would this be considered an unforseen issue that I can/should charge them for?  

Thanks!