Beginner seeking advice - when should planning start?

I wouldn't start with gimmicks now that probably aren't in your budget (air conditioning) Also keep in mind that you have to make compromises even with a new build. Due to the plot, development plan and budget. It will never be perfect.

Yes, my question and as I said, we want to do without an air conditioner anyway if a good indoor climate can be achieved another way. Looking at the last summers, I can easily do without 30 degree nights :)

Nevertheless, make a list of what is important to you and also why. Sometimes the why invites reflection. And prioritize the points.

At the same time, consider a life concept. It’s not what you want to live in that counts, but how.

Also consider that you will have to make compromises even when building new.

Compromises are nothing bad; they are made on various levels and here in this country usually at quite high levels by builders. We tend to look at things negatively and say “giving up” – e.g., on an air conditioning system. With a view to what we create for ourselves as homeowners, life becomes much better. An example from the car world as we know it and which is already somewhat dated: I can drive an inline six and enjoy the driving comfort. I can drive the same inline six and, thinking about giving up the second bank of cylinders, can’t shake the feeling of only driving with half an engine. The compromises in downsizing can be really tough, but you can truly learn to drive with half engines without becoming unhappy. Whoever detects irony in this example may take it seriously, whoever can’t recognize any irony, please do not take all this seriously.

Yes, my question and as I said, we want to do without an air conditioner anyway if you can achieve a good indoor climate in another way. Looking at the last summers, I can easily do without 30-degree nights :)

No, you can't achieve it differently. A cooling heat pump might make a difference of 2-3 degrees. Then ideally all the shutters should stay down during the day as well. But it gets dark inside the house ;) Otherwise, like almost everyone, you live with the temperatures. Once they are inside the house, they stay there – that's what good insulation ensures. P.S. What exactly do you understand by indoor climate? Only temperatures?

With this, rooms can probably also be cooled in summer, which I think would be better than an extra air conditioning unit that consumes additional electricity?

Only to a limited extent. The floor can be cooled with it. But in summer, you want the air in the rooms to be cool and you also need to dehumidify, because that is what a person perceives as heat inside a room.

I don't want to spoil the hype about heat pumps for you, but by the way, a heat pump is basically exactly what you don't want. It is an air conditioner that is operated in reverse and then heats the air/water in the house instead of cooling it. The mode of operation and the main components are the same as a "regular air conditioner," only the scale and the medium it acts on/uses are somewhat different.

We want to do without an air conditioner anyway if a good indoor climate can be achieved in another way.

Difficult with today’s construction methods. It is intended that the heat remains inside, and construction is done accordingly. Then in summer, you suddenly come to realize this. At least a well-thought-out shading concept is required.

If you look at the last summers, I can well do without 30-degree nights.

That will most likely not get any better in the future either.

Good indoor climate in the hot summer (up to 120°F dry heat in the shade) without air conditioning I experienced in an Adobe Brick House in New Mexico. Walls up to 80cm thick made of clay. Warm in winter, cool in summer. Here neither typical for the country, nor inexpensive or optimized for square meters. Alternatively, build the bedroom in the basement or provide a summer sleeping area there.

Consider a life concept at the same time. It’s not where you want to live that counts, but how.

So the how definitely takes precedence, the only thing that is fixed in the "where" is "a house". Nothing more, nothing less.

No, you can’t manage it differently. A cooling heat pump maybe makes 2-3 degrees difference. Then ideally all shutters must stay down during the day as well. But it will get dark inside the house ;)
Otherwise, live with the temperatures like almost everyone else. Once they are inside the house, they stay there – good insulation ensures that.

P.S. What exactly do you mean by indoor climate? Only temperatures?

We have already been making sure that all windows are sealed and darkened for some time, and last year we also bought a portable air conditioning unit. Otherwise, thermal blinds on the windows and at most open the windows at night.
Regarding P.S.: So far, apparently I have only considered temperatures, if you ask me like that :D

Only partially. The floor can be cooled with it. But in summer you want to have the air in the rooms cool and also have to dehumidify, because that’s what people feel as warmth inside the rooms.

I don’t want to spoil the hype about heat pumps for you but a heat pump is basically exactly what you don’t want. It is an air conditioner that is switched in reverse and then heats the air/water inside the house instead of cooling it. The principle of operation and the main components are the same as a "usual air conditioner", only the scale and the medium it acts on/uses differ somewhat.

Difficult with today’s construction style. It is intended that the heat stays inside and accordingly it is built. In summer there is suddenly a wake-up call. At least a well-thought-out shading concept is needed.

This will most likely not get better in the future.

Regarding air conditioning and heat pumps I meant that the air conditioning is "on top" of the actual climate solution like a heat pump. Regarding the costs of purchasing an air conditioner I haven’t inquired at all whether it would fit the budget.

Good indoor climate in hot summer (up to 120°F dry heat in the shade) without air conditioning I had in an Adobe Brick House in New Mexico. Walls up to 80 cm thick made of clay. Warm in winter, cool in summer. Neither typical for the country nor affordable or optimized for square meters here. Alternatively, build the bedroom in the basement or provide a summer sleeping place there.

Originally my girlfriend and I were enthusiastic about earth mound houses (actually still are) but apparently there are some insurmountable hurdles to realizing something like that. Especially regarding permits, let alone requirements for the plot itself.

Back to the initial question: I am 30 years old, will turn 31 this year. My girlfriend will be 29 this year. Dream-wise/thought exercise, the idea would be to maybe move into our own home in 3 years, when would be appropriate to start taking care of the first things? A plot is also not yet available.