danieloakworth
Stage 1 in the process
At the very early planning stage, you would want at least a clear length of 3100m, being made up of 2500m runway plus 60m strip plus 240m RESAs at each end, plus for the sake of simplicity and erring slightly, a 2% clear takeoff gradient at each end of the 3100m. That will give you slightly constrained widebody operations. In a more perfect world, 3000m of runway is needed. Shoehorning a transatlantic operation into 1800m of runway with, at best, 90m RESAs is going to be a nightmare and shortsighted, but possibly do-able. So check your airport and see if it is really worth proceeding.
Stage 2
Check your existing runway strength. Almost certainly it has to be significantly strengthened (and maybe widened). It is ENORMOUSLY cheaper to close the runway for a few weeks or months and fix the insides of it then to overlay it. The cost of the strengthening and the extension will determine if it is really worth proceeding.
Stage 3 - professional analysis
Follow Mutt's advice, and get the professional performance analysis. This will validate and address the many assumptions and short-cuts in the process above, and it will give you your real options for extension length. The professionals will request the various surveys that the airlines themselves will later request. Then get an engineer to do a prelim design and a QS to cost the work, and the cost/benefit can be worked out to see if the airport can make money.
Stage 4
A visit to the Routes conference anybody?