How much would an extension/attic conversion add to my property?

We live in a 2 bed detatched house in North Co.Dublin and are considering adding an extension and/or an attic conversion. The house is currently in negative equity but I'm not sure by how much. We cannot afford to move to a bigger house at present so the extension/conversion are an option we are looking into. My question is, on average, if we spent say 40k getting the above work done, how much of a return could we expect to be added to the value of our home?

You might be better off asking a local estate agent who knows the area you live in.
The attic room won't be classed a 3rd bedroom unless it fulfils some planning regulations about ceiling height etc. And if it doesn't have proper stairs etc then it may not add much value.
My uneducated guess is that an extension that provides more living space or another bedroom downstairs would add more value than a converted attic space.
I'd also guess with the market in a bit of flux at the moment, it'd be very hard to put an exact figure on value this work might have. Because as you haven't done the work yet, a figure based on today's prices mean nothing.

currently, it will not make as much as you will spend on it.
but in the future, who knows.
if you are extending to suit you needs, then the return is surely not a factor.

My sister got her attic converted last year. She had great plans for it and spent 20k on it including decorating etc. Everyone wanted to be in the attic, playing games, watching tv etc. 1 year on no-one uses it, she wishes she had extended instead. I know that if i was buying a house again an extension would appeal more to me than an attic conversion. Especially if there were only 2 bedrooms in the house as is.