Construction costs are currently skyrocketing

What exactly does building obligation mean? On the one hand, it is a matter of interpretation and is handled differently depending on the municipality. Must a completion notification be submitted at the end of the 5-year period, as in this case, must the move into the new house have taken place, or is a building application or the start of construction sufficient. That is crucial. If the start of construction were sufficient, then the construction project could basically be postponed indefinitely. However, if one must be finished after 5 years, then it becomes more difficult to avoid the building obligation.

It depends entirely on the municipality. In very popular areas, they tend to be less accommodating than somewhere in the countryside where young people are welcome. There, one can also politely inquire about an extension well before the deadline expires and has a good chance of getting one.

What exactly does Bauzwang mean? On the one hand, it is a matter of interpretation and is handled differently depending on the municipality. Must a completion notice be submitted at the end of the 5-year period as in this case, must the move into the new house have taken place, or is a building application or start of construction sufficient. That is decisive. If a start of construction were sufficient, then the building project could basically be postponed indefinitely. However, if one must be finished after 5 years, it becomes more difficult to circumvent the Bauzwang.

There are plots here in former new development areas with Bauzwang, on which a garage was built at the time and construction has stood still ever since. So much for Bauzwang...

So for us, the construction obligation was defined as the completed shell of a residential building. But elsewhere it can also be the move-in or readiness for occupancy.

How much do you have to lay out nowadays for a 100sqm house (bungalow) without major special requests?

You should already aim for 300k for the house. With a lot of personal effort you can certainly get below that, but you need to have the time, helpers, and the tools.

If house, incidental costs, garden etc. then 350k€ or more it gets very tight again I guess with just under 5k€ net income.

How much equity do you have? As long as you get a rate under 2000€ with a reasonable term, I would definitely do it.

Meanwhile, the banks assume 920 euros in additional costs for a standard single-family house. However, energy costs are already included in this.

"Those" banks generally do not set guidelines for the additional costs of a single-family house. Most banks calculate with a flat rate for living expenses (including additional costs for the house). This then depends on the income level and the number of people/children. We do it that way too.

There are plots here in former new development areas with building obligations, on which a garage was built at the time and since then construction has been stalled. So much for the building obligation...

Then the person also does not continue building. The building permit also expires at some point – usually if construction is interrupted for a year. Moreover, it also expires if construction has not started after three years. Details vary depending on the federal state.

That does get really interesting when you say interrupted. Theoretically, you could first carry out the foundation. Let it sit for three months. Floor slab – let it sit for six months – after each exterior wall also take a two-month break – load-bearing walls one month break. Zwide – four months break. Then first close it up below with windows and doors. Then a six-month break. In the next four months, floor slab sealing and little by little lay empty conduits, pipes, and slots for electrical and plumbing. If you cover the stairwell with a nice board, even more is possible. That’s already two years. Then the same game in the upper floor, another two years. Then interior finishing, another two years per floor. With the supply bottlenecks for various building technology products, you can easily plausibly extend that by another two years. I think you can stretch a house construction nicely over more than 10 years without a break of more than a year without the substance sustaining significant damage.

Theoretically, you could first do the foundation. Leave it for three months. Concrete slab - leave for six months - after each exterior wall also a two-month break - load-bearing walls one month break. Ceiling - four months break. Then first close the downstairs with windows and doors. Then six months break. In the next four months, then waterproof the concrete slab and gradually install empty conduits, pipes, and chases for electrical and plumbing.
If you cover the stairwell with a nice panel, more is possible. That is already two years. Then the same procedure upstairs, another two years. Then interior finishing another two years per floor. With the supply bottlenecks for various building services products, it is entirely plausible to extend that by another two years. I think you can stretch a house construction nicely over more than 10 years without the structure suffering significant damage if you don't have a break of more than a year.

You can't just do that theoretically, but practically as well.

However, there is one single, not to be underestimated problem. MONEY.
Either you finance it, then the bank is breathing down your neck - both because progress is slow and because the commitment interest weighs on your mind. And if you have financed, then you don't need to stretch out the time. The bank will not only advance the financing. They will want the entire construction project covered.
Or you must/can't finance, then you have to keep saving money (for that, you need a decent income) to continue building. That makes no sense either, especially if you need new craftsmen each time for small tasks.

The procedure you described is typical for self-builders who only ever save up for new material and then spend a week of vacation/overtime to continue building. In that case, the house construction is legal and cost-effective, planned over several years.