I do have a little, limited experience of Spanish wiring systems in domestic properties, not specifically Tenerife though but I expect it's much the same everywhere.
The points already mentioned are spot-on, namely a lot of cabling in conduit and radials for everything. Lighting conduit tends to be the spiral-wound type and quite small. I don't know if they have prescribed zones or not, but I doubt it.
In some properties you will find plastic cover plates scattered around the walls, and removing these will reveal junctions which can give a clue to how the cables are routed.
Many houses are built of hollow concrete blocks and you may find the cavities house wiring and plumbing. Faced with these walls, solid and tiled floors and solid ceilings it can be very tricky to add new circuits and I've seen a few additions done in white trunking as most of the walls are painted white. If you have vaulted ceilings with exposed beams then you have extra scope for additional circuit placement.
The good news is that the Spanish have no fear about breaking into walls or even lifting tiles and making good afterwards, just as we have no problem with breaking into plasterboard walls and lifting floorboards.
Some very modern properties, especially commercial ones, have stud wals with plasterboard over which makes their wiring much more similar to ours, but these are relatively rare.
Wiring is almost always in singles, and the only T&E you will find will be flex.
Single RCD boards are commonplace in older properties and yes, a washing machine in the bathroom is very common. My neighbour has a socket next to her wash hand basin and the light switch is not only inside the bathroom but actually in the shower enclosure!
Tip: If the kitchen sockets are tripping it's almost always the kettle at fault!
