When you say extractor fan do you mean a vent hood? He can't vent through an exterior wall? Or am I misunderstanding?

Yes a vent hood. He can't put the vent where above the stove top where I wanted it, because that part of the wall is a strong re-inforced concrete --supporting-- wall. Meaning he cant drill a hole through that wall for the vent because it will compromise the building. The supporting wall runs on the top wall of the kitchen.

I'm don't agree with your builder. I'm not an engineer but holes are put in supporting walls all of the time (ie. windows and doors). When we vented directly out the back wall, we had to drill through a stud, so it was necessary to build a header to carry the load which was neither difficult nor expensive. People vent through exterior walls all of time.

If I read you correctly, the supporting wall is made of reinforced concrete and thus not drillable.

I'm certainly no expert, but I don't see an aesthetic problem with running the duct above (or inside?) the upper cabinets to the next available wall, if that saves your entire layout. You compromise the efficiency of the fan with every additional length of ducting, but 70cm doesn't sound like too much. You might ask about this over on the HVAC forum, where a bunch of fan/venting guys hang out?

I missed the reinforced concrete bit. Ours is an old stone house and we've had holes drilled through 20" of stone for various things and cut a new doorway through the back wall, which I'd say it definitely a supporting wall for a 3 story house. I don't know anything about building though so my situation could be very different from yours. Maybe it's possible to do what Circuspeanut suggests. I wouldn't give up yet.

Have you thought about installing a downdraft instead of a hood ? Electrolux and Dacor both make nice retracting ones that are quite powerful.

I've never heard of a downdraft vent I'm afraid. I live in the medditterranean so I'm not sure if it would be available, they are a bit behind the fashions and latest techs here lol. I'm going to go to the house tomorrow morning and talk with the electrician and ask him if it is possible to have the plan I want and duct out the vent to the side wall since it will only be about 70cm. I'm still open to any other ideas though! :)

I would get a structural engineer to look into this...not your builder. The engineer will know for certain if it's allowed and how to do it. It may just be that your contractor/builder doesn't want to do it b/c he either doesn't know how or just doesn't want to do it (possibly too much work).

I actually got a similar run around from my contractor about venting outside. We had a stop work order for 9 weeks while we worked out a way to vent outside...actually, while I told them to find a way, don't just stand there wringing their hands giving me one excuse after the other about why it couldn't be done. When they finally started thinking about it they then quoted me a ridiculous amount ($4,800 USD) just to move a shower drain & vent that were in the way...I got an outside plumber do it for $900 USD! They still kept making up excuses but I stood firm and told them to find a way. They eventually did when they realized I was not going to back down or meekly go with whatever they said (including them telling me "I've been doing kitchens for 20 years and there's no way it can be done..." Guess what? It's done.

BTW...downdrafts do not work well at all nor do pop ups.

Thanks for the advice everyone. But the issue is that I live in Greece and they have a specific way of building. In most parts of Greece, houses are built to a uniform style and code, I'm sure you've all seen the postcards. The skeletons of the houses are reinforced concrete and those supporting walls are holding up heavy 1m thick reinforced concrete floors and ceilings. Yes my ceiling/rooftop is a 1m thick reinforced concrete slab ontop of those supporting walls. Even if it was possible to drill a hole in those supporting walls, not only would it be extremely difficult, but it would most likely be against the building code we have here which is very, very strict. You see the external walls that aren't supporting, load bearing walls are simply brick...easy as pie to drill through.
Oh and by the way, if anyone is confused about me using the word 'supporting wall' I mean like a load-bearing wall. I have them at the four corners of my house, and one in the middle. They are only about 2m wide each and the rest of the house's external walls is brick.

Hopefully I haven't confused everybody...

I was going to suggest venting through the roof (preferable to a wall in many cases, btw), but you say you have "...reinforced concrete floors and ceilings..." so your builder may say the same thing about drilling through the ceiling.

I would check your local Code and maybe talk to some inspectors b/f I'd give up. I find it hard to believe that no one in Greece vents outside!

Do some research and talk to a structural engineer about your options...I would be very surprised if there weren't structural engineers in Greece.

Thanks everyone. :) Of course in Greece everybody does vent outside. The problem is I dont think Im explaining myself very well. Of my U-shaped kitchen, two side are walls, and one is simply a pennisula to separate the kitchen from the living room as it is open plan. The top wall is a supporting, load-bearing wall of re-inforced concrete ---so no drilling through that. The right-hand wall is NOT a load-bearing supporting wall and is made of brick...so drilling is ok, so I CAN drill and vent out this wall.
My issue is that my favourite place to put my cooktop was against the load-bearing wall. But I think my best solution is to put it exactly there and put in ducting to go along the top of the cabinets, along to the right and out of the righ-hand, non-load bearing brick wall.

Thanks again for all the advice, I think what is confusing everyone is that we have such diff building styles here in Greece to what everyone is used to in the US.

:)