What should I do with my house and land

A I roughly estimate 550,000 for sale

You do realize that the theoretically sold house with the right of residence means that the buyer cannot live in it as they wish because your mother will still be living there?!

and 450,000 for building a small house

A house with four residential units will not be small. It will be about 400 sqm, 400,000€ for each unit. How long are you willing to give yourself to find four willing individuals or families interested in such a project?

Everyone finances the construction of their condominium themselves.

And as already asks: what is the floor area ratio? Is it still allowed to build a house on it, and then also a multi-family house? I guess you need to come up with Plan C.

You do realize that the house sold in theory with the right of residence means that the buyer cannot live in it as he wishes because your mother will still be living there?!

Of course I am aware of that

A house with four residential units will not be small. It will be about 400 sqm, €400,000 for each unit. How long are you willing to give yourself to find four willing individuals or families interested in such a project?

I am simply looking for solutions. I have a plot of land and don’t want to leave it fallow. I am aware it won’t be easy

And as already asks: what is the floor area ratio? Is it allowed to build another house on it, and then even a multi-family house?

No, of course not one house and then another house. I would then build a multi-family house on the 800 sqm. The floor area ratio is according to the law.

Where do you get the money to finance the construction of the apartment building or a part of it? The amount you need to borrow there will be significantly higher than 100k. Can't you pawn the 100k again on [house/land]?

Is the equalization of accrued gains already determined or have you already consulted a lawyer yourself? The accrued gains are very complicated to calculate, especially when there is a right of residence, etc.

Not final yet, but I know the approximate amount. We have been disputing this for 5 years and it is now also in court.

If I understand correctly, you already have a loan and can’t afford any more. How is the new construction supposed to work then, if you

Not final yet, but I know the approximate amount. We've been arguing about it for 5 years and it’s now also in court

Then your ex won’t wait years for you to receive proceeds from any arrangements. He will expect his share "now" after the judgment (which he is entitled to promptly).
Therefore, I only see the option of selling the land. Or also selling the house "now" and then initially moving into a rental apartment or leaving the mother in need of care alone with the new owners.