House construction. Sell or rent out the condominium?

Hello everyone,
I have been reading along diligently for a long time. I have already discovered numerous tips for myself and learned a lot from you. Thanks for that.
Now to the actual "problem."
We live in a nice 4-room apartment [ETW], which we have almost paid off - value of the apartment approx. €380k, current debt still approx. €115k. There is about €90k in the account. After the fixed interest period (beginning of 2022), we could make use of the special termination right and pay off the remaining sum in one go. That is the background story.
Our municipality will sell about 50 plots in a new development area in spring (*the development works are in full swing*). Since we have two children, our chances of getting a building plot are good.
Price: approx. €300/m2
We would like to build. Since I am a teacher, I lack a study room. I am fed up with constantly working at the kitchen table. The school stuff is spread all over the living room.
I assume the dream of owning a home would cost us around €600k.

About our income
Net monthly: he - 2600, I 2500 (75% position, private health insurance already deducted), child benefit €400 = €5500

How should we proceed best now?
a) Pay off the apartment completely, sell it, and finance only the difference?
b) Keep the apartment, rent it out (rent approx. €900), not pay it off, but take on additional debt to finance part of the house?
c) Use the equity for the purchase of the plot (+ a small variable loan), sell the apartment after the fixed interest period, pay off the remaining debt from the proceeds, put the rest of the "profit" into the new house...
d) ???????

I know it was a lot now. I hope you can help me a bit...

Must...some things got mixed up in the post. How can I edit and revise it again?

You can only do this within ten minutes of creation, so not anymore now.

You should tell a little more about you all: Are you planning more children, will you eventually return to full-time work? What are your living expenses like? What monthly rate are you currently paying for the apartment? (Have you paid for it entirely yourselves so far?) Approximately how large are the offered plots? What kind of house do you have in mind? And so on and so forth. It’s best to work your way through the questionnaire:

a) Pay off the apartment completely, sell it, and only finance the difference?

That would be my first choice. Buy the plot with equity / variable loan, calmly plan your house, and when it comes to financing, plan the sale of the apartment (that will probably only be in 2022 anyway).

b) Keep the apartment, rent it out (rent about €900),

Return before taxes under 3%. Existing financing must be paid off, low deductible expenses. A crappy idea if you need money for building a house.

c) Use the equity for the purchase of the plot (+ a small variable loan), sell the apartment after the fixed interest period ends and pay off the remaining debt from the proceeds, put the remaining "profit" into the new house...

That is basically option a). Any bank will finance the sale proceeds for you in advance.

Sell the apartment, otherwise it will get too cramped with the house. 600k will probably not be enough for you, depending on your demands. Expect around 800k.

Thank you for the quick answers.

Sell the apartment, otherwise it will get too tight with the house. 600k will probably not be enough for you, depending on your requirements. Count on 800k.

That was my first thought as well.
The apartment is really nice, large, and high-end furnished. It would be a shame to rent it out to strangers...
On the other hand, I think: You have 2 children (and that’s it). Maybe it wouldn’t be bad to keep the apartment (ground floor with garden, plus elevator) as security for old age. If something goes wrong, I can still sell it... hm...

In the next post I will write a bit more about us and our exact ideas regarding the topic "house".