Hi again. In connection with what you can or can't do with regard to extending, the following link might be of benefit to you.
http://www.environ.ie/en/Publications/DevelopmentandHousing/Planning/FileDownLoad,1586,en.pdf
With regard to doing the work itself, going on personal experience, I wouldn't bother with a builder. I'd go with a good carpenter or a company that specialises in attic conversions. Having said that, a conversion company mightn't be the best people to employ if you're taking off, and raising, the roof. If I were having that done, I'd probably get a good carpenter to do the roof and conversion job and would get an electrician to do the electrics, a plumber to do the plumbing etc. And it should certainly make the job cheaper.
We live in an estate also. I'll PM you tomorrow and tell you what we did when we converted. It may be of benefit to you.