The reason being, I expect, is that due to the volume of your space you are needing something efficient, quiet and unobtrusive ... I've screen shot two examples to show you - the first extracts 500m3per hour but is only 35 decibels loud . The second is a smeg kitchen specific one - it does go up to 800m3 per hour but at a whopping 71 decibels loud ! If this is a living area too - that's just too loud ! In order to get signed off - most build projects, which include fitting a new kitchen, needs a fan that's as good as or better than the one that was there previously ... so that might be another reason . Lastly - money ... the smaller quieter one is actually quite a bit cheaper ! So this might be another reason ! Hope that helps a bit ! :-)

We have a ceiling mounted extractor in a very large room and it works really well. We ended up with a Brittania BEST extractor (bought from John Lewis) simply because we have a structural steel running above and the position of the internal ducting fit best with that model, though it's a bit bigger than we ideally wanted. The slightly lowered section of ceiling around the extractor is also because of the steel and wouldn't otherwise have been needed. We were actually told that there is a minimum height, rather than maximum, for it to work properly - see http://www.besthoods.co.uk/2011/08/ceiling-extractors-8-things-you-need-to-consider-before-buying-a-ceiling-hood/ It's very powerful and we've had no issues with it. A family member has the pop up option and that has been a bit of a hassle - it's not that powerful and ends up getting dirty from cooking every time.

There is a calculation for how much extraction you need to keep grease and smells out of the air and out of your furniture. Basically you need to filter the total volume of air in the room 10 times per hour which for your room means extracting 1300 square meters per hour. Most ceiling extractors extract 700square meters per hour- also their efficacy is reduced when the builder insists on putting in a vent that is too small (almost always they put in 12cm instead of 15cm so this is 50% smaller). The efficacy is also reduced if the extraction pipe is long or has bends in it. So basically to effectively extract in a large room you either need three overhead or extractors or one downdraft one. I get that on the face of it there seems to be a cost disparity but if you want clean air this is what you have to do.

I'd always look to duct an extractor to get rid of cooking smells and condensation, part F of the building regulations governs this. Part F When inserting or removing an internal wall, care should be taken not to make ventilation worse. Any new kitchen, a toilet with no openable window; a bath/shower room or utility room should be provided with a mechanical extract fan. The type of room will determine how much ventilation is required. Part F says that where a kitchen previously had only a recirculating fan this can be retained/replaced if it is made no worse. • Kitchen: 30l/s if over the hob and 60l/s if placed elsewhere • Bath/shower: 15l/s with overrun • Toilet: 6l/s with overrun • Utility room: 30l/s To calculate the size of extractor you need you should look for a model that is able to make about 10 air exchanges per hour, this is the volume of the room (w*d*h) multiply by 10 to give you the required rate of extraction (ao.com website shows you how to work this out). Looking at your plan i'd either fit a 'flyover' shelf above the extractor between the 2 wall units, run the cornice along the front of the flyover and then use flat ducting to run out to the external wall. Depending on the size of the cornice, the ducting might be seen though so instead of cornice it might be better to make a head infill between the tops of the units and the ceiling. Depending on the height of your ceiling I'd consider using extra tall units so the infill isn't huge (don't know if howdens do these). Also looking at your howdens design, if you have an electric or especially induction hob you should consider an extractor which is wider than your hob due to the way the steam/air currents rise under the extractor.