Buy now or wait a little longer?

Most banks do not provide financing for probationary civil servants. You can ask, but I think your question has been answered by that. I think ’s suggestion is good.

Since I am currently employed on a temporary basis and he is only a civil servant in the probationary period

If I were in your position with the job roles, I would start the house construction slowly but from now on. 1. The bank will have no problem with "on probation," as this is common 2. It takes time until you find a plot of land. ==> Then buy the plot, pay proportionally with equity, and finance the rest variably 3. You have already completed the necessary house planning. A usage-oriented 160 sqm for a family of five including a home office is rarely seen here (usually, it's 160 sqm for the couple so that someday a child can fit...). No further planning is needed, as the specific house depends on the plot and is then ideally planned individually (a ready-made house of that size with that number of rooms is rare).

1. The bank will have no problem with "on a trial basis," as this is common

Especially because in two years both will be off trial.

No more planning is needed, as the specific house depends on the plot and will then ideally be planned individually (off-the-shelf houses of this size with that number of rooms are rare).

I see no need for individual planning just because of a third child plus home office. Only the current example project I mentioned does not fit because the attic there is not suitable for residential use. But two years is a long time for a developer (whose market is currently undergoing ongoing consolidation) to uncover newly developable building plots. The classic model – somewhat the GDB bottleneck of the German developer scene – has an attic studio as a buffer to also be suitable for families with three children.

2. Time will pass until you find a plot.

Plots for detached single-family houses are generally at least more expensive by the square meter needed for further construction and harder to find unless they have detrimental defects that increase costs, such as a hillside requiring a basement or the like. Flat plots for semi-detached houses built independently come with the goalkeeper problem of having to arrange yourself with a half-neighbor (or bring one along right away). Therefore, I see the family of the OP landing in a developer semi-detached house. The financial situation is not that of poor people, but not generous either. I do not see a detached single-family house here. What the market will realistically offer here is in the league of Raumwunder / Flair and too small, and in the affordable price range, they are semi-detached houses in at least sufficient (if not comfortable) size. Time will tell; with upper advice comes a new house.

Especially because in two years both will have passed their probation period.

I don't believe that.
She is a tariff employee. It is not legally possible under civil service law to make tariff employees civil servants for life. The probation period can be shortened, but to at most one year.

Also, it does not correspond to reality that civil servants on probation so easily obtain financing. I really know many civil servants, and none have succeeded. It may depend on the specific situation, but the usual case is certainly not creditworthiness in this case.

If I were you, I would go to a financing advisor to see what they say about the civil servant on probation, etc.

Buying a plot of land now to build on later can't be wrong if there is no time limit for building.
However, there is somehow already a 1 before 6 digits, so a million, which I would have more than respect for if I were you.

I have to swallow a bit here.
You generously named the costs for the whole family (fun costs), but where is the rest?
Additional costs, mobility costs... I also find the insurances somewhat sparse.

We have an appointment with a financial advisor on Monday.
We do not have a car, and the insurances are included in the rent (household contents, as well as additional costs). The other insurances are listed. Thanks for the tip though. Sometimes you overlook something.

With two lifetime civil servant statuses and children aged 4, 6, and 9 in 2027, this is your cheapest time to move into a semi-detached house as your first own home. In and around Berlin, the nearest "usual suspect" is Brale with a current project in 15741 Bestensee (Amselweg/Stichweg). Such middle houses (Yvonne calls them "starter properties") are ideal for young families without a lottery win, whose children are well supplied with playmates there. It will fit until child 1 finishes studying. The same in green is also available from Weisenburger / Wengerter / Werner, but in your area Brale is the dominant player.

Thank you very much; we have also considered a semi-detached house but want to first try to buy "properly" right away. If that doesn't work out, I will gladly come back to that. Thanks for the suggestion!

Regarding the civil servant status on probation, it seems to be handled very differently from bank to bank. A friend of ours received civil servant conditions with his civil servant status on probation; others were treated like tariff employees in the public sector.
Regarding my fixed-term contract: no, the civil service career (then first on probation) only follows after that. Since I am a lawyer and these are needed in the public sector, I am not worried about continued employment. Of course, the bank has to see it that way first.

In your position at your jobs, I would start the house construction slowly, but from now on.
1. The bank will have no problem with "on probation," as this is common
2. It takes time until you find a plot of land.
==> Then buy the plot, pay a portion with equity, and finance the rest variably
3. You have already done the necessary house planning. Usage-oriented with 160 sqm for a family of 5 including home office is rarely seen here (usually, it is 160 sqm for the couple so that eventually a child can fit in...). No more planning is needed since the specific house depends on the plot and is then ideally planned individually (off-the-shelf houses of this size with this number of rooms are rare).

We also noticed that our idea (5-6 rooms, without elaborate features like a dressing room, large hallways, or master bathroom) is rarely found in 160 sqm. We are therefore open to individual planning as well as prefabricated houses.
What you suggested was also our plan.
We will see how the meeting on Monday goes.

Thank you all very much already for the many good thoughts.

We are therefore open to individual planning as well as to a prefabricated house.

Here, there has not yet been any talk of a "prefabricated" house. I suggested a semi-detached house from a developer project. These are usually and here serially built "brick by brick" and still (to a limited extent) influenceable in the planning. An individual planning would require a developer-free plot.