There are not many appliance brands that sell a venting / downdraft hob with a gas cooktop as to do it right needs alot of expertise.. ours works & works really well. The burner are sunk down low in the cooktop..giving it protection from the extractor..

Hmm, I might be wrong but my understanding is that a recirculating extractor doesn't get rid of steam, only grease and smells. It's confusing - I read lots about it and didn't find many definitive answers - but my understanding was that if we had a recirculating-only cooktop extractor, we would also benefit from a wall mounted expelair type extractor too, which we didn't want. Hence we went for an exhaust air model as we wanted ours to do the whole job. The bit where it ducts out has a system whereby it only opens when its extracting - and the ducting is several metres long so there's no sense of cold air coming up through the hob top, even on these recent very cold, windy and frosty days. I can only say that in my experience, if you want an island hob exhaust air model (as opposed to recirculating), there are costs which are additional to the cost of the hob itself - ie the ducting pipes and the work that goes into planning and executing the ducting under the floor and out to the open air. The position of the ducting has to be millimetre perfect at quite an early stage of the build so we had a lot of head scratching and very careful work to make sure that our kitchen plan and the position of the ducting were exactly aligned. We've paid our builder a day rate so there's definitely been an extra, if slightly undefinable, cost there. Perhaps in some cases it's very straightforward but in ours it wasn't, and as I said the ducting itself isn't cheap (around £100/m) - so in a direct comparison between a recirculating and a ducting version of the same cooktop (where the cooktops themselves cost exactly the same) the ducting one was a more expensive option for us, but one we're glad we went for. I should say we're in a 1930s house, which, although we're doing our best to make it energy efficient, is never going to be airtight like a passiv haus. So for us the benefit of getting rid of steam outweighs any potential small energy loss through the ducting to the outside. Of course everything I've said above may be rubbish... I'm just a girl trying to do a renovation with conflicting advice from all sides... :)

I agree with Daisy- you should be thinking about a venting cooktop. If you are worried about noise you can add extra insulation to the wall between the two rooms. separately i don’t understand what you are trying to acheive with this layout and it seems like much space is redundant and it can be improved.

Its often not possible to place a 515mm deep hob in a 600 deep worktop with anything other than a small gap behind, especially if the splashback takes up 20mm too. Hob manufacturers suggest a 50mm gap- its not always possible. Bare in mind that extreme heat could potentially crack the splashback but many people have the same set up as you without any problem