(Belgium, the country where peopole speak a foreign language to understand each other... )

It's a very strange situation! I guess you should renegociate the contract. Because the owner can't change the object of the contract without your consent.

If the garden is sold separately, I guess the new owner still need to respect the contract... and you should pay a part of the rent to him.

If the expert is a good one, he asked you to sign a "mandat" where you gave him the power to do the "état des lieux"

In this case, the expert did his job for you and for the owner, and the état des lieux is 100% valid.

He is letting you th option to purchase the house, or to leave.  Of course, this is illegal. But what other solution does he have?...

You should ask him, because it is 100% unclear.

To stay legal, ask him a letter giving you the right to leave with a one month notice. He will probably be very happy to sign it. I would. Because he is in trouble with the contract... and in trouble with the sale of the house.

Negociate. The best and first good way is negociation (and you are right to ask the questions: know your rights, and you'll be in better position to negociate...)

We have a 9 years rent and are now in our 10th year.
The owner is selling the property with garden.
1. We are renting the house and a large garden.
In the letter he mentions now that the large garden is a building plot and will be sold separately
What happens if the "garden" is sold? Can the new owners of the "garden" come onto the grounds and start building because we are still renting it together with the house?
Or does the current owner needs to change our contract and rent price because we no longer have access to our garden?

2. When we moved in we received a "description of the property" but unfortunatly this was in French so we did not understand what is all said.
The owner said he paid the expert for us and was going to translate it but we nver got it. We never signed/agreed this "PV etat de lieux" of the property as we could not understand it. So did we rent a propery without a 'etat de lieux' and what does that mean when we leave?

3.We got e letter from the landlord that he is going to sell the property and gave us the option to buy the house or terminate the rent.
This is the text
"....inlicht opdat jullie over
de nodige tijd kunnen beschikken om zelf de nodige maatregelingen te treffen hetzij
voor aankoop, hetzij voor beëindigen van de huur"

This is confusing to us what he means with "hetzij voor beëindigen van de huur"

-Is the owner suggesting he is terminating the rent or that we terminate the rent?
If he terminates it then we have 6 months the time i think and can give a counter notice for  1 month.
If the text says we terminate we have to stay 3 months?

Thank you for your help
Kind regards
Patrick