Surely the first thing to do is find out how much money your lender will forward, it's not like banks are throwing money around these days. What's the point of wasting money on getting plans done etc if they are only going to lend enough money to do half the extension? Once you've found out how much money they will lend you, get a few builders around to give you a rough estimate. If this falls within the amount the bank will lend you, then the next thing to do is get the existing foundations looked at by an engineer. If they are not sufficient to support an additional storey, it will mean the existing foundations/extension will have to be underpinned and reinforced, adding thousands to the costs the builders estimated. The cost of a trial pit or two, and an engineer for an hour would be money well spent, and will cost you a lot less than paying for planning drawings/application, which as I said earlier, will be of no use to you if you can't afford to do the works...