Is financing with land and equity possible like this?

This is the plan, plus about €500 rent from my girlfriend, but that shouldn't be counted as fixed, you never know how long that will last.

The plan is that I pay for the development and demolition work out of my own money.

Then I would have €40,000 in equity + €500 rent, and €2,100 income.

I am currently still in touch with other companies to get an offer that is lower. Although I think it will settle around €250,000-280,000.

The €310,000 was the most expensive company.

That would mean if the house costs €260,000, + additional building costs like disposal of excavation, so I estimate €280,000-290,000 - my €40,000.

I would have a loan of €240,000-250,000.

How old are you? I would wait, save more equity, and in a few years tackle the matter together with your wife so that the financing can be managed by two, or your income has increased. And the property is secure for you, no need to rush.

Is the rent officially paid? If yes, it can be included. However, keep V+V in mind. Then it should work.

What about the costs for the kitchen, furniture, lamps, etc., which are all not included in the house price.

As well as the floor coverings or painting/wallpapering the rooms.

Here, several thousand euros are added.

And for the outdoor area/terrace, at least around 10k should also be considered.

That would mean if the house costs 260,000, + additional construction costs like removal of the excavation, so I calculate with 280,000-290,000 - my 40,000.

I would have a loan of 240,000-250,000

Something is not right. On the one hand, or rather independently, I would state the 40,000* as equity, so please do not burn it already before the bank meeting and loan approval.
House 260,000
Additional construction costs 30,000
Demolition 30,000? If someone knows better, please post!
That would be 320,000

- *25,000 equity, rest in the account for kitchen, lamps and furniture, so equity actually less...
If you don't even need 20,000 for your new household, it reads as if you have nothing except a youth room???

Living expenses that the bank mentions have nothing to do with current expenses, but with the expenses you will have.
And without a rental contract it may be that the rent is not recognized. After all, you have no official rental income ;)

Demolition 30000? If anyone knows better, please post!

We recently paid 16,000 EUR in Berlin for the demolition/disposal of a 90 sqm bungalow built in solid construction from the 1950s with partial basement and subsequent insertion and compaction of material to create a buildable substructure.