I believe that planning permission is good for 6 months after being granted but after that youd have to apply for an extension (for which the fee is more expensive than the original application fee of €34 I think).
Yes - the 2 storey extension will be significantly more expensive than a single storey even if the square footage is the same. If you are building to the back it will likely be a flat roof that joins the house at the level of the upstairs ceiling. If its at a gable end youve got major extra work as you need an entire new pitched roof for the extension, and assuming you want to capitalise on the much bigger attic space it will give you, you'd need to knock the existing gable wall in the attic, which will require alternative support for the roof.
The flat roof jobs dont look too hot but you probably wont see it from your garden anyway so itll just be an eyesore for your neighbours.
Other factors:
-foundations need to be dug deeper and wider
-you now need joists at first floor level and a flat roof for the extension - more work than just the one roof on a single storey extension, which even though bigger its easier to do one big job than 2 small ones
-scaffolding and the risks associated with 2 storeys make work that bit slower
-a big factor is how will the upper storey tie in with the existing upstairs - you've got to at least knock the back wall which will probably require another RSJ (reinforced steel joist) in addition to the one downstairs, but whats the layout upstairs - do you need to move any other walls. If so are these stud partition or load bearing concrete walls? Moving walls means hassle, which becomes major if theyre load bearing, and think of the associated plastering, skirting, doors etc costs.
-another factor is 2 storey is more hassle for the builder and its regarded as more complex so be sure theyll make you pay for that.
as a ballpark guess for a 200sq ft 2 storey I'd say you'd do well to get it for 70k and the single storey you might get for 50k.
Oh yeh, the post above mentions that you can build an extension to the rear so long as its not bigger than 40 sq metres and not higher than the house. This would imply you can get a 39 sqmetre 2 storey extension without planning permission as long as it is not taller than the house - I would be very very surprised if this is true - I believe any 2 storey extension needs planning permission (give yourself 4 months from date of application to date you're actually allowed build, all going well).