Is one router enough for the whole house?

In our new build, we have a completely separate basement apartment for our son upstairs.

Now the question arises whether one router downstairs with us is enough and our son can still use Wi-Fi upstairs; if necessary with a repeater. Our installer says that will be difficult because so much concrete and iron is used in new houses. Better would be a separate router upstairs.

Other "smart" people from the family think that is all nonsense. The new good routers could handle that easily. Even without a repeater. How can you recognize a good router?

Does anyone have experience in this area?

That will most likely not work due to reinforced concrete in the intermediate ceiling. If it works, it will not make him happy in terms of quality. The required device is an Accesspoint that is connected by cable to the same network as the router on the ground floor.

Have you had network cables installed, at least one from downstairs (where the router will be) to upstairs (where the access point can go)? The classic repeater that is often sold together rarely makes a young person happy, too many losses. It would absolutely make sense here to install an access point via a cable connection. There are great systems that are easy to set up even for laypeople ([Linksys], [eero], etc.). Depending on size/layout/building materials, it might also make sense to invest directly in a mesh with 3-4 access points. That costs a few hundred euros and you are well equipped.

Repeaters are generally nonsense. As my predecessor already said: install an access point on a network cable and everyone is happy.

In our new building, we have a completely separate granny flat upstairs for our son. Now the question arises whether one router downstairs with us is enough and our son can still use Wi-Fi upstairs

Exactly these two statements contradict each other. If your son uses the internet from you, he is in your network. By default, there is no separation, unless you give him the usually restricted guest access. If you don't care about the lack of separation, then the standard solution in a new building is a LAN cable connection from your network distributor to his. Repeaters are an emergency solution and in a new building this can easily be avoided. Concrete ceilings and underfloor heating are usually the death of Wi-Fi.

The transmit power of WLAN devices is legally limited, by the way. Buying a "good" router therefore only helps range to a limited extent. Especially with 5ghz, it's probably over at the concrete ceiling anyway...