depends how you do it, my dad acutally built most of ours himself, got planning permission etc and did it all to spec but cos he did most of the work by himself and with the help of his dad it didnt cost THAT much to do, however it was a long time ago, his dads now been dead for nearly ten years and battled cancer for a while before.
I do think they're worth it if you get them done properly and do what you can yourself. Our one here (we didnt build this one) was done poorly and you can tell when it came to making it half decent, it's less then 2 years old but the walls were cracked, windows in poor condition, none of the walls are flat, we added some extra wall (i wont try and explain why and where) which is all lovely and smooth and the original wall is curved
they also added a plasterboard but to one side of the chimney breast but not the other, i coudlnt understand why tbh, you'd want symetry, and as we'ver just redecorated couldnt find any reason for this.
It's added value to our house and i love the fact i have my own little level, it gives a little added privacy.
