This probably isn't the answer you want to hear but I personally wouldn't be doing an extension at the moment. Material prices are increasing rapidly, availability/logistics of materials is becoming a problem and the market is extremely busy. Even companies as big as Kingspan have had to delay all their orders and most decent builders have work planned in through to 2022.
You'll end up paying more and risking having a half built extension waiting on materials which could be an absolute nightmare on a two storey extension.
We plan to do an extension but will be waiting at least 12-18 months to see what the market does.
