Kitchen consultation granite slab and Siemens appliances?

My question is currently we have planned an 80cm induction cooktop, do you think 80cm is enough or should we definitely go for 90cm?

How should we know if 80 is enough? You have to decide that for yourself. I have had both, 90cm doesn't offer much advantage. I would rather look at things like flex zones and similar.

Second question is if we take the 80cm cooktop should we also take an 80cm extractor hood or rather a bit bigger, 90cm?

Exhaust or recirculation? Definitely something larger than 80. I chose 110 for the 80cm cooktop (wall-mounted hood with head clearance, recirculation). In the last house with the 90cm cooktop it was a 100cm island hood.

With the 80 cm hob, you have a usable area of about 45 x 70 cm. Mark this on a board and place your pots (up to 5 pieces) on it. Then you will experience the usable area. Also draw in the control panel, then you will have the exact usable area. We chose the 80 cm one to keep a bit more workspace and the usable area more than suffices for us. Next to the hob, we have other Siemens i700 appliances, the oven with steam burst function, 4D circulation, grill, microwave (up to 600w), pyrolysis... (it is not a steam cooker, but the function is great for baking bread to get a super crispy crust on roasts, etc!) and additionally a steam cooker with full oven and grill function as well as a warming drawer. Having two cooking compartments has proven to be extremely practical for us. The oven with steam function was defective twice and was replaced without any issues; the new device shows no negative abnormalities. You can also take a pragmatic approach with the extractor hood. Will you be cooking greasy steam over the entire width? We have a Falmec Marilyn hood over the 80 cm free surface induction hob, which is theoretically too narrow. In practice, I just slide the area with stronger steam to the middle under the hood. Works perfectly.

Thank you for your messages I have another question about the range hood. How strong does it have to be/should it be? We have an open kitchen and an open living dining room on the ground floor and we have an open staircase to the basement and the other floors We will probably have recirculation because of KFW55 since we have taken advantage of the KFW loans we are not allowed to lose the KFW55 status with a core drilling (we also do not want to have everything measured separately) our builder does not do that Are 409 m³/h or 615.0 m³/h enough or should it be more?

Is 409 m³/h or 615.0 m³/h sufficient, or should it be more?

What at which level? Free-blowing? With activated carbon filter? Which hood manufacturer? Which hood type (island, ceiling-mounted, integrated into cooktop, etc.)? That cannot be generally assessed.

Sorry, I hardly know anything about this

Manufacturer Siemens like all other kitchen appliances
Type: canopy hood slanted on the wall
Recirculation
409 m³/h highest normal level or 615.0 m³/h on intensive level

This hood was recommended to us by the kitchen installer and is also one of the few that is even 80cm wide

My question would be how strong or how much air it must at least pull so that it is sufficient for us and filters enough air in the open house

If you are skeptical about your kitchen builder's recommendation, just choose one size larger in a design you like. I wouldn't make a science out of it.