What changes. The old seat comes off. Depending on the type, the attachment goes under a seat or takes the old seat's place as a complete seat – as ours do. A T-piece goes onto the angle valve, much like connecting a washing machine. No drilling, no new pipe, no wall.
What happens when you move out. T-piece off, hose straight back onto the valve, old seat back on. Hence the single most important tip in this whole article: keep the old seat.
What really matters. If you connect it yourself, you're responsible for it being watertight. After installing, open the valve slowly, check the connections, and leave a dry cloth underneath for a few hours. A dripping T-piece is the only real risk – and you notice it right away.
With an electric seat. It needs an outlet within reach. If there is one, everything above applies. If one has to be installed first, that's an intervention in the apartment's electrics: agree it beforehand and have a professional do it. Extension cords next to the toilet are not a solution.
With a warm-water device. Non-electric devices that mix warm and cold – our New Standard, for example – also connect to the hot-water angle valve under the washbasin. That's just a T-piece too, and it comes off just as quickly when you move out.
With a wall-hung toilet and a concealed tank. There's no reachable connection next to the toilet. The T-piece goes on the angle valve under the washbasin, and the hose runs along the wall to the toilet. That's also undone in minutes.