Plot found in new development area - Build now?

Good evening,
I would appreciate your opinion on this and we are wondering if it makes sense.

Girlfriend (26) and I (29) live in Frankfurt am Main.
We earn a total of €3200 (she €1500 and I €1700 + WG, bonuses) and the jobs have been permanent for several years.
We pay €900 warm for rent. Other expenses amount to €1500. This includes everything (vacation, car costs, insurance, internet, phones, electricity, food, credit).
We save €800 every month.

We have found a plot of land (400sqm) in a new development area for €72,000 including the share of development costs for sewer and road construction including final expansion and street lighting, as well as property transfer tax, notary, and land registry fees. The costs for various utility providers (water, electricity, telecommunications, etc.) would still have to be paid.
The plot is located in a small town without school, bakery, supermarkets, etc. The neighboring towns are 1~2 km away and have good infrastructure.
The whole area is about 20 km from Frankfurt am Main. Good connection to the motorway and 18 minutes to the city of Frankfurt.

We do not have a construction company yet.

We would manage with 120 to 140sqm of living space.

Equity of €20,000 is available.

What construction costs could we expect here?

Between 300 k€ and 5,000 k€ - It depends,... What do you want to build, what are you allowed to build, what do you have to build ([Bebauungsplan]). That should already be available - read it through and try to understand it. How do you want to build (own work, solid construction, prefabricated house). How big should it be, with a basement, with luxury? Take a look at the other topics, house sizes and costs are repeatedly presented there. 3,200 € is not particularly much - are children planned? If yes, that will be a real challenge.

I personally don't believe this will work.

Land + incidental costs approx. 80TEUR
Additional construction costs approx. 35TEUR
House 130m x 1600EUR/m^2 = 208TEUR
Sampling costs 10TEUR
Painting work 10TEUR
Garage 10-15TEUR
Outdoor facilities 10-20TEUR
All in all around 370TEUR

Equity is used up for kitchen, moving, furniture, etc.
Financing amount therefore 370TEUR x 5% annuity / 12 = installment of approx. 1540EUR
additional house incidental costs of approx. 300EUR
Total burden = 1840EUR

Savings contribution 800EUR
Rent 900EUR
available capital = 1700EUR

Furthermore, I dare to doubt that any provisions for repairs/new acquisitions are planned in the 1500EUR expenses, otherwise you would probably have more than 20TEUR in equity.
We already discussed the topic of children, etc.

Increase income, make babies and save save save :)

I would also say increase equity. Increasing income would certainly be great too, but also one of the hardest to achieve!

Saving only makes sense here if the two of them could still build under the same conditions in 10+ years.

As Musketier has rightly pointed out, the "actual" savings rate is likely still below 800 euros, and thus in the future not only rising land and property prices and possibly rising rent payments would have to be compensated for, but also very likely a higher loan interest rate from 2026 onwards. In the end, effectively about 300 euros per month flow into equity, and then anyone can consider, based on the above calculation, how long the two would have to save. And the higher mobility costs due to commuting are not yet taken into account.

To be fair, the advice here can only be: "Reduce expectations." There is usually little one can change about income oneself... I would also consider whether I really wanted to "bend over backwards" for about 35 years for a "simple" house with surrounding lawn edging in a place without any infrastructure.