Model numbers and serial numbers for washing machines are almost always behind the door, or on the control panel. However, sometimes the number written on the control panel or dispenser drawer isn’t a full model number but a model range (Buying spares – what you need to know).
Some built in washing machines like to hide their model numbers. The one featured below shows an Ariston machine> There is nothing printed on the control panel. The owner couldn’t find the model number anywhere so presumably it wasn’t behind the door.
On built in appliances the model and serial number are sometimes only shown behind the plastic kick-strip at the base of the machine. It’s worth checking behind the pump filter door first (if one is fitted) as there could be a rating plate sticker there. If there is no serial number to be found anywhere the only option would be to get the appliance out. That is really tricky though and you risk damaging flooring. The serial number should be visible without taking out the appliance. They just like to hide them.