Is preparing for an air conditioner worth it?

What I couldn't find was the maximum length of the pipes. Does anyone here maybe have a rough number?

For my preparation, the two indoor units on the upper floor are about 12m pipe length away from the indoor unit.

I can only imagine that it's easier to lay these two "little pipes" directly along with a power cable, somehow you can probably always manage that.

That was my thought too. It’s just a bit flatter together and can be easily routed through one or another cavity. Also, although the pipes are flexible with respect to the property, later 90-degree bends in the conduit could still be difficult.

I looked up the weight of the outdoor unit, it’s under 50kg, isn’t that a load that should easily be supported by a roof? What I couldn’t find was the maximum length of the pipes. Does anyone have a rough number? It would make it easier to estimate where I could alternatively place the outdoor unit besides the roof...

50kg should be fine but definitely clarify something like that because the 50kg also need to be mounted in such a way that any wind loads that might occur are withstood by the substructure. Regarding maximum pipe lengths and dimensions, you have to choose a supplier and then look into the documentation. For my 9kw system, the maximum single length was 25m and the total length was 75m (mine ended up being 73m). I placed my outdoor unit on the north side so that on extremely hot days of 30-40 degrees you don’t have the problem of the sun shining directly on the unit. Otherwise, the outdoor unit can sometimes go into fault because the condenser can no longer handle it.

Thank you, thank you, thank you!

Finally a topic that doesn't seem to have any major problems!

For me, it won't even come close to 25m, I estimate more like 12m like Zubi123 and with the 75m I'm also still well within range even with installation in the basement :D.

The 50kg will definitely be clarified, of course someone knowledgeable has to do and approve that, clearly.

The point about the sun is a good one, which speaks in favor of my imagined alternative location on the carport on the north side behind the house. Does anyone here have any idea how that looks in terms of neighborhood law? I am lower than the neighbors to the north, roughly the top edge of the carport is at the height of their ground floor. I would place the outdoor unit close to the house, so there would still be roughly 4 meters straight line distance to the neighbors' garden and maybe 8-10m to the house.
Is the outdoor unit very noticeable?
The problem would also exist on the roof, here we are talking about a height that corresponds roughly to the balcony height on the neighbors' upper floor.

- probably two pipes per indoor unit (one supply and one return?)

Yes, you need two pipelines. Usually, everything needed to connect indoor and outdoor units has a diameter of about 50mm, and I would at least have the copper pipes and power/communication installed during construction; afterwards, it only becomes more expensive and complicated.

I wouldn’t lay refrigerant lines in already, they just get contaminated,

How should sealed lines get contaminated?

Seals become porous, and if a new refrigerant comes in a few years, it won’t fit.

Um, no, that’s not how it’s done. Especially not with “new refrigerant” and such. Nothing can become porous because there are no seals present. This isn’t a car air conditioner. The pipes are installed and sealed. There is nothing inside except air until the system is eventually added.

What I couldn’t find was the maximum length of the pipes. Does anyone here perhaps have a ballpark figure? It would be easier to estimate where I could alternatively place the outdoor unit instead of the roof...

As others have said, it strongly depends on the manufacturer and type of system. Simply because different compressors are used. For an outdoor unit placed above the indoor units, you also need to consider that an oil trap must be installed in the lines, but the executing HVAC technician will know that.

I have my outdoor unit on the north side, so on extremely hot days with 30-40 degrees Celsius you don’t have the problem of the sun shining directly on the unit. The outdoor unit can sometimes fail because the condenser can no longer manage.

That shouldn’t really be a problem. Yes, efficiency suffers, but the same devices as here are also used in Texas and other places worldwide, and yes, not infrequently directly on the roof.

Is the outdoor unit very noticeable? The problem would also exist on the roof; here we are talking about a height that approximately corresponds to the height of the balcony on the upper floor for the neighbors.

Do you know the issue with heat pumps and the neighbors? It’s nothing different. Of course, it’s best not to have it installed close to the neighbors, but rather facing the street or somewhere else.

could strap the outdoor unit to the roof (is there any objection here?). Can someone give an estimate on this?

Absolutely no problem at all. The bracket (adjustable at an angle) and everything else you need is just a bit more expensive than the usual two brackets for the wall.
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I would place the outdoor unit close to the house, so there would be roughly 4 meters straight-line distance to the neighbors' garden and maybe about 8-10 meters to the house. Is the outdoor unit very noticeable? The problem would also exist on the roof, here we are talking about a height that corresponds roughly to the neighbors' balcony on the upper floor.

Noise is always a tricky subject, as everyone perceives it differently. Here you can only orient yourself to the dB(A) values to be observed. So you need to know what noise your outdoor unit "produces". But it does help if the fan is not facing the neighbors and if there is no large obstacle in the direction the fan is facing, because sound is reflected there.

That shouldn't really be the problem. Yes, efficiency suffers, but the same devices as here are also used in Texas and elsewhere in the world, and yes, quite often directly on the roof.

Usually not, I agree with you, but it can happen if the system is running at its absolute limit and the day is very, very hot. Then it can just happen.
The devices in Texas and Saudi Arabia usually have a larger condenser or are quite oversized, so the problem is minimized.
Otherwise, there's also the garden hose you can attach to the condenser to prevent this ;)