Value of an extension

I built an extension 2 years ago onto my end of terrace 3 bed house in Fingal. Its ground floor only and we use it as a living room, old living room is now effectively a playroom/2nd living room so we havent gained any extra bedrooms etc. When doing our research before building one thing I read often was that what you spend on it (in our case €30k) is not what it will add to the value of your house which will often be considerably lower. I was OK with that as the extension serves the purpose we want and we intend to live here for a while but am wondering what is a fair amount to add to the house value for property tax valuation for it? It adds 20-25% by area to the house.

We built a similar ground floor extension last year onto our end-terrace in Fingal.
Included new kitchen/dining/WC/Utility plus some internal decorative works
Spent circa €65k and the revised valuation (for top-up mortgage purposes) came in at +40K added to the original value.

Hope it helps...