One number that I've seen quoted in a number of places for house extensions is Eur 2,500 per square metre. This would include all building and project management costs. In your case that would amount to approx Eur 60,000.
That said, if you were to do any of the building work yourself, or if you managed the project yourself and employed direct labour you could reduce this significantly.
Also, if the extension is less than 40 square meters, and leaves more than 25 square meters of space in the rear garden, you shouldn't need to get planning permission.