Behavior in case of imminent delay by the property developer

which is usually also adhered to.

No, the rule is also 6-7 months at Viebrockhaus.

By the way, Viebrockhaus has always advertised a construction period of 3 months,

They do advertise, yes. Then building is done with a tent, which you also have to pay for as an extra, as far as I know. At least the rush surcharge.

Well... I've seen the houses from Viebrockhaus, into which people moved 4 or 5 months after construction started. But I do say, you don't argue in court over 1 or 2 months... especially not with a general contractor who has a much, much longer breath.

But that doesn't change the fact that we're talking about unlaid eggs here. Let the general contractor start construction at the beginning of November and by the end of May he must hand over the house in such a condition that one can move in. That gives him 7 months for the construction. That roughly corresponds to what even conservative general contractors guarantee to the builder when constructing a single-family house. And "being able to move in" does not necessarily mean that everything is finished and free of defects.

We're not talking here about a general contractor who builds a single-family house for the client, but about a property developer who sells the house and land together, and in such large projects (including rows of townhouses), a period of at least 1.5 years, rather two years, must be expected before one can move in after construction begins. But honestly, I believe that given the current interest rate and construction cost situation, this construction project will be put on hold and may not be built for another 5 years or even longer. Unless [Volksbank] steps in as the owner with its own equity and rents out the apartments and houses itself, so as not to incur a loss on the land. That would be your hope if they don't want to buy you out then.

for such large projects (also for rows of terraced houses), a period of at least 1.5 years, rather two years, is to be expected until you can move in after construction start.

I think it depends on what one calls construction start, etc. Our area is larger and the development, for example, was already underway before there were any building permits. But from the start of construction of our row house block to moving in, I expect less time for us and apparently our builder does too, according to the contract. Of course, I can only say for sure when we live in the house, but our builder had written "only" 1.5 years (from contract signing to moving in) into the contract, probably only that long because we signed when there was no building permit yet, so they counted more buffer time. Ultimately, he now has about a year from the building permit to handover with us. (But he also started immediately after the permit.) Whether it will fit in the end, we'll see.

First of all, the start of construction in November is being considered. We have to wait and see. It is not impossible to build a house in 6 months. But even if not, compensation is only due after the guaranteed handover date. It is unfortunate that the financing is already running and one is paying commitment interest... we had to do that too... but it’s not the developer’s fault. And even then, one would have to sue for it, which costs endless nerves...

If almost nothing is sold, the developer will not start. The economic risk is too high. And since it is already foreseeable that people will not be flocking to the developer right after the summer holidays, it makes sense to talk about the case now. The OP may not have any claims yet, but if he waits a year, he will also lose a lot of time to buy a house elsewhere.