Apart from the cost of filling in the valley, I think that the logical way to go would be east. If you extend to the west you run into the St.Peter's village area. Not that the extension would run into the village, but the a/c would be extremely low passing over it. It might well result in St.Peter's church having to be demolished. Also, the land slopes down from the 09 threshold area, so a lot of leveling would have to take place. Perhaps the cost of compulsory purchase, leveling & social disruption would be greater than filling in the valley !
Having said that, the elevation of the 27 threshold is, from memory, considerably higher than the road to the east of it, & even more so than the valley & surrounding land - a pretty daunting challenge to fill it in. I suppose that the water tower would have to go too.
I am pretty sceptical about this project. Some Guernsey politicians seem to think that all they have to do to expand air services & increase visitor numbers, is to extend the r/w. I would suggest that, if the demand for new routes actually exists, then the existing operators would already be operating them with their existing equipment. Why would a longer r/w, of itself, provide the impetus for new routes ?
Then there is the hope that EZY would be encouraged to operate low cost routes (primarily Gatwick). If they were, that would be the end of AUR ; because they couldn't compete with EZY -unless the States were to subsidise them even more ! Why would the States spend tens of millions on the extension, encourage EZY to operate, & then watch their multi million investment in AUR go down the pan ?