Cost of extn - only the extended area or entire room?

We are about to renovate a two-storey Mid Terrace house with an original narrow 'two storey return' to the rear of the house (measuring 6m x 2.3m on each floor).

We are looking to extend the width of this two storey return by 1 meter from 6m x 2.3m to 6m x 3.3m on both storeys.

When calculating the expected cost (using RIAI recommendations of 2700 euro per square meter), should I calculate using just the extended area (6m x 1m x 2 = 12 sqm, which is 32,400 EUR) or should I calculate using the entire size of the rooms (6m x 3.3m x 2 = 39.6m, which is 106,920 EUR).

Thanks for your help,
C

I do not think you can use this basic rule, e.g. €2700 per sqm, in your case because your extension is too small (extra 1m in width).
There is as much work in this job than there will be if you were to extend by 5m for instance.

the economics of this would shout 'dont do it, its not worth it'.... you are adding on 135 sq ft, and you probably wont have much changed out of €50,000... thats roughly €370 per sq ft......
if you have the space i honestly beleive you could get a two storey extension 3 times that size .. ie 400 sq ft for about €10,000 more...

I agree with bacchus, I maybe cheaper/easer to knock the return and rebuild than add on 1 meter.

Thanks for your replies.

The problem is - 2.3 m is not really wide enough for a kitchen including a table (I had a look using a kitchen planner). And we cant really go any wider than 3.3, as it would infringe on the rest of house (we would need to move a period sash window in a dining room). I know its hard to visualise ..

I will wait and see what the architect suggests next week...