Dividing land for a duplex, how to best sell the second half

To really achieve that, you would have to "play" the developer and sell half a plot with a finished house. However, you bear all the risk during construction.

- You need a bank that goes along with it (or the creditworthiness is so good that it's not a problem) - Construction cost risk - Buyer reservations when a private person acts as a developer - Real developer business is practically not feasible for a private person (compliance with broker and developer regulations, two-account model at the bank, separation of service contracts/general contractor contract) - Real estate transfer tax payable by the buyer on everything - Possibly taxation of a profit as well (if it occurs) ... Actually, as a private person, you can only complete the semi-detached house entirely and then sell one half. Interior finishing then by the buyer according to their own wishes.

However, of course it would be easier and cheaper if we could build the house at the same time with the same construction company.

No. Easier maybe, not cheaper (if you consider €3,000 insignificant).

1. I plan my house and build it alone, then sell the second half with certain conditions. What I don't like about that is that the advantages of a semi-detached house don’t really apply. For example, if I finish my house and then half a year later the buyer of the other half starts building a basement. Or he doesn’t build for 2 years because he lacks the money.

Very common situation. Basement yes/no can be clarified beforehand. No, it must be clarified beforehand. It actually shouldn't matter to you when and how he builds.

2. I play the property developer, with correspondingly high risk.

But also chances to make money. Although as a private person I assess the risk as significantly higher.

In principle, the house does not have to be a mirrored twin. However, of course, it would be easier and cheaper if we could build the house with the same construction company at the same time.

It is more relaxed and sensible for the house building if both are built in parallel.
Usually not cheaper, at least not significantly.

Am I overlooking something or does anyone think of something else?

Certainly: we responded once upon a time to an ad. Back then there were still classifieds in the free paper. They still exist today as well. Private selling platforms are also suitable.
A couple was looking for a semi-detached house partner for a plot they had at hand. We met twice privately to check whether we had the same house interests and could like each other as neighbors.
The latter was consistent, but for us the northern orientation was out of the question. They themselves wanted the southern orientation. And they had the plot at hand.
In the end, they found a construction company that built their semi-detached house for them (they as builders) – the second was then sold by the construction company (as a developer then).
This model would also be an option for you.

What exactly is the reason that you absolutely want a semi-detached house? Is a detached house otherwise not financially feasible?

Thank you for your answers. We want to build a semi-detached house because the budget for a single-family house is rather tight.

Having a plot developed by a developer who then sells the other half definitely sounds interesting.

What we also considered is building the semi-detached house and renting out one half. Since land and rents are very expensive here, that would probably be an option as well.

because the budget for a single-family house is rather tight.

What we also considered is building a semi-detached house and renting out one half.

That is contradictory. For building to rent, you calculate for 20 years. If the budget for a single-family house is already rather tight, then you probably won’t get financing for a semi-detached house with 300 sqm either. Get moving and advertise as soon as possible. I wouldn’t waste any time. At the same time, you can deal with the general contractors (GUs), How long is the seller willing to wait?