Is it always possible to conceal wires? We want to get ethernet cable to rooms in a 8 year old house

It all depends on the building and how much damage you are willing to have done before made good but should be possible to chase all cables into the walls/ceilings and then make good. However as I said before all depends on the building and how much your willing to have done.

Not an easy task but cables can be chased in with careful planning of cable routes. It will involve lifting floor boards to check UFH pipe clearance and for any other obstructions.

Without a full survey this answer is very difficult to get spot on. But theoretically all cables can be hidden within the walls, cavities, loft spaces. Cables can be "chased" in to brick walls,by chiseling channels deep enough to insert the cables then cap them with metal or plastic capping lengths,finnally resurfacing the wall to conceal. Cavity or stud walls can have cables fed in out up down (within regulation gudlines ie.. Not diagonally ect) to go room room to room or floor to floor. Cables can be fed up to your loft space and back down into another room.a route comonly used for coax TV cable. Or data cable. There are a few restriction relating to 230v power circuits however I won't go into depth as you are talking about dad's cables. Hope this helps. Any spark should be able to survey a route to conceal. Your specific situation dictates how much work, cost, time is required, and Weather or not it is cost effective.

Depending on the type off house (semi or detached) you could mabee have conduit On the outside off the house branching off to the various rooms and coming trough near sockets and haveing the ethernets in small surface mouted boxes just depends how you would feel about white or black conduit on the outside off your house , this would be the least invasive method and should not take to long to do