I've just finished getting an extension of 44 sq m (plus about 15 sq m of very minor internal reconfiguration), GF only, DLR area. Including professional fees (for drawings, engineer, planning costs, cost management, compliance sign off etc), utility connections, floor finishes, basic shower room fit out, but excluding kitchen it was about €2,500 psm (inc VAT based on the 44 sq m). This also excluded decorating (which I did myself) and any landscaping (which will be needed as building is very messy!). I project managed myself, except for cost management - basically valuation of the builders works to confirm it was as proposed in the tender. Final cost ended up being about 7% higher than tender price, mainly due to issues with foundations.
I was shocked too! This was tendered out to 5 or 6 builders, all initial quotes were within about 3% of each other. Eventually we pared back the work to be able to afford it, original tenders would have been closer to €3,000 psm. Our spec isn't anything particularly fancy - UPVC windows, 1x velux, doors, soffits, gutters and drainpipes, laminate floors, €20psyd tiles, flat roof etc. kitchen was biggest extravagance but this isn't included in price quoted here.
I'm sure it would be cheaper down the country, but €800-1,200 is, in my view, unrealistic, at least for good quality. Prices have increased not only because builders are getting busy again but because of increased regulation/compliance, this has added an extra layer of cost.
I would say for a project of your size more than 2 quotes are needed, but I would expect them to be similar. Also, we were on a 12-16 program, eventually ran to 21 weeks.
Good luck with it