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..

extraction comes under part F of building regs, so your architect should be detailing this in the plans for the extension. As i understand part F the important bit is 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. However, I would always recommend fitting an extractor to deal with condensation. • Kitchen: 30l/s if over the hob and 60l/s if placed elsewhere so you could have an expelair type extractor on an external wall that will comply with regs at a minimum.

Technically, no you don’t actually - you can have an extractor elsewhere in the room - as long as it extracts 60litres per second ( for a uk property)

We had this exact issue. We opted for a pop up re-circulating extractor and a small external extractor on the wall above the cupboards. The pop up extractor doesn't do the job well and is noisy so we rarely use it. We rely on the small external extractor and just switch that on when we're cooking. We had our kitchen done over 3yrs ago so they may have improved the pop up extractors now.