First you need to factor in the planning permission time/costs, whether or not you'll have objections from the neighbours (which could potentially increase time/costs) and then it depends on the type of extension you're wanting, how many doors, windows, type of roof, whether need to dig foundations, if pipes need to be moved, etc. and then of course the costs of whatever you're waning to do internally (new kitchen cabinets, new plumbing, new electrics, new flooring, etc.)
Without all of this, I think it would be nigh impossible to give any sort of quote.
All of the above would have it's own costs, but then you need to factor in the architect fees, builder's time (usually charge per day or per job) and then factor in the 'unseen' things like wood, nails, screws, etc. And typically, whatever quote you get, you'll inevitably exceed it because there will be 'hidden' costs or delays for one reason or another, etc. So factor that in as well, just in case! 