Sell the property completely or partial sale with investment?

I'll summarize
Factory ruin with contaminated sites
Contaminated sites are not always just asbestos but soil contaminated by decades of leached operating materials of all kinds.

Market value 1 to 2 million?? That's quite an unrealistic range

Relatives who work in property management but don't know anyone who roughly calculates the project?

If the area were so attractive, one of the developers from next door would have already knocked.
Is it possibly purely an industrial area?

As a project financier, I have to agree with Egon12 (but liabilities?).
My proposed approach is:
1. First clarify what the property is actually worth. If necessary, have the appraiser carry out a residual value analysis.
2. Clarify the divisibility of the property.
3. Clarify building possibilities, submit a preliminary building inquiry - you will probably be able to do this even without a licensed building coordinator.
Only when steps 1-3 are completed can you even start to think about what is possible.
Perhaps it will then turn out, for example, that the property is not worth that much, so a partial sale does not bring much equity and a high financing would actually be necessary. Or vice versa. Or...
If the location is so interesting and the surrounding apartment buildings have really paid off for the project developers/investors, I do wonder why the inquiry for this property has never reached you. And in the East, Dresden is truly one of the few places where supra-regional project developers/investors also invest.

Is something like this conceivable or will you just be ripped off?

Why "or"? Both! The family thinks they are sitting on a "treasure" they don't want to squander lightly. Without outside help, however, it cannot be recovered. The professional treasure hunters will mercilessly exploit every gap in knowledge to secure the largest possible share for themselves. So it is more a question of the individual characters and their willingness to take risks. If the semi-knowledgeable grandchildren are really eager to gain experience and are willing to give it their all, why not. Security-conscious pessimists should rather sell completely and start over at a much smaller scale.

As long as the property is free of encumbrances, it should not be a problem to find a bank that finances such a thing. Now the question is what can be built on it and how the 6 owners/heirs want to and can divide the work. It certainly won't work in the family to divide the income by 6 and have 2 or 3 take care of the apartment building. The issue of division of labor and compensation needs to be discussed. So many apartments don't manage themselves on the side. But it is definitely an opportunity for your family to make a significant step towards a secure future. I would not sell it.

As long as the property is free of encumbrances, it should not be a problem to find a bank that finances something like this. Now the question is what can be built on it and how the 6 owners/heirs want and can divide the work. It definitely won't work within the family to split the income among 6 and have 2 or 3 take care of the apartment building. The topics of division of labor and remuneration need to be discussed. Managing so many apartments is not something you do on the side. But definitely, it is an opportunity for your family to make a significant step toward a secure future. I wouldn’t sell it.

Well, if the land really has a value of 2 million and one wants to build several six-story buildings on it themselves, it won't be easy even with a bank. Land value share of the total project estimated at 20% = total investment costs 10 million = debt capital 8 million. Lending 8 million to real estate amateurs just like that (who takes on a value-retaining cost overrun guarantee?, external project controller by the bank? tax-wise reasonable? project company non-recourse?), few banks do that anymore. Moreover, the family does not want to become highly indebted.

It is roughly about 8000m². If I assumed 200 euros/m², we would be at 1.6 million. Why no one knocks on the door is the question. The old factory building is pretty intimidating, I fear. It is also being used, so maybe no one realizes that it wouldn’t be needed. But you are right, whether it is really worth that much would of course have to be clarified first. Maybe we are totally mistaken - but I wouldn’t call it a "treasure" anyway.

I don’t think there are any significant pollutants in the building. It was neither a chemical factory nor a gas station, nor was asbestos used or anything like that. But it has to go and it is quite large.

If we sell the whole thing, the immediate question is, what to do with the money? Put it in the bank? Ha! Build a house for the grandchildren? Try buying attractive building land in a rural area near Dresden! (It has to be in the countryside) They already tried that - nada. Invest? Yes, exactly! But we are not really into gold and paintings.

Nordanney is certainly right. We will probably have to get informed first and then see further.