look at it this way: if you build it to a garage spec - it will be cold & miserable, and no-one will want to spend any time in it........so effectively you're building a small bungalow, so build it to house spec. At least that way you'll have choices about it's uses.
I would probably go on the basis of (very) approx, €700 per sq m, which means you're looking at spending €37k.
Is that the ball-park of what you'd be happy to spend, that is the question.........