Building with a small budget feasible?

I am not sure how seriously I should take it, but judging by the listings on the large real estate portal, my property has already increased in value by 62% since last year.

Moin, man gut wir kommen jetzt endlich aus NRW weg - noch 6 Wochen :D

Elsewhere people only cook with water too ;). By the way, a city from NRW once won the title "most livable city in the world" and is regularly among the top in Germany today.

Elsewhere you also only cook with water ;).

Maybe, but in Bavaria the water tastes better :D

SH, where we are is at the top.

We don’t actually want to be especially big. 100 to 120 sqm is enough for us. We’re currently living in 75 sqm with a 25 sqm garden plus terrace... and annoying neighbors.
We’d like four rooms and an extra guest WC would be great....
Equity is not that significant. We have 20k now. Then there are 12k in the Riester account, which can be used if necessary. We’re currently saving about 1000 a month. A bit more adds up over time.
We also have a large circle of acquaintances whom we could probably enlist for one thing or another. But of course, we would want to pay them for that... so some of the equity would have to be kept back for that.

We had a similar amount of equity, a little more but not much.
The problem with holding something back is that the bank preferably wants to finance only the house, not the additional costs. That means even with 100% financing, ideally, you should have the additional costs saved up. That might just work for you (if you have similarly low costs with SH as we do here in Lower Saxony).
We also struggled with the kitchen. We had to spend most of our equity on the additional costs, not much was left for the kitchen. We saved and scraped it together bit by bit during the construction phase. Everything that was no longer needed was sold off on classifieds. :D
By the way, I can recommend that – people underestimate how much cash you can get that way, plus it reduces your moving volume. Life is easier with lighter luggage. But with 75 sqm, you probably don’t have that many unused treasures anyway. Still worth a look.

If you can get friends and acquaintances involved with the flooring and painting work, that would probably save a lot already. We had quotes from the general contractor for flooring (10,000) and painting (14,000). We had the flooring done by them, it looks great. Painting was too expensive for us, so we did it ourselves with limited requirements given to an inexpensive specialist company and paid around 5,000 for that. That saved us the money we needed for the paving work, but now our walls look like that. :D
It has, uh, country house charm. :)

It may be, but the water tastes better in Bavaria

I prefer to stay where German is spoken. Bavaria is like emigrating...