Feasibility of financing a single-family house approximately 150 m²

I also lack the context:
Properties purchased and how financed?
Condition of the properties electricity (dis)water, access road, divided and surveyed ....
When is the second property supposed to be sold and for how much?

The earthworks are missing from the incidental construction costs, or have you already included them here as well?

The 350,000 is initially a rough estimate; our architect said it is feasible. We will have to clarify soon how much earthworks are included once concrete costs and a floor plan are available.

Are the €30,000 development costs a concrete offer or already carried out? If 700 sqm are already needed just for green strips and access roads, it could be a bit more expensive.

These are the costs for wastewater (with a lifting station), water (with water meter shaft), electricity and internet + earthworks for these and the road, based on a cost estimate from January with an approximate 10% surcharge.

€15,000 for outdoor facilities... could work if you only calculate material, but it might not. With a long driveway, you spend a lot on gravel and paving, even if you do everything yourself. Above a certain scale, a considerable amount must be spent on rental machines. Since it definitely won't be enough for the double garage with workshop, better plan for a garden shed in advance.

I am aware that this is probably too little, but this is where I would most likely make cutbacks in the next few years and do a lot myself.

The notary will also ask for a significant fee when you register your mortgage. This point has so far been overlooked.
I also am currently not quite clear how much of the land you already own. A variable loan for this is listed in the costs, but no remaining debt is specified.

We have financed the entire plot through a variable interim loan from January this year with a rate of €605. As soon as the loan for the house can be applied for, the interim loan will be paid off so that we only have one large loan.
So to speak, we own the entire plot. The €150,000 includes the land price itself at €105,000, the demolition of the nursery at €21,000, and the rest for initial development work/architect.

Income 5,800 euros Expenses (excluding savings, excluding rent) x.xxx euros

I probably stated that incorrectly. Income 5,800€ Expenses 3,415€ (excluding cold rent) Available income for credit and additional ancillary costs - 2,385€

Hello,

As already written in almost every answer regarding financing, everything depends on the second income. If one income disappears for unknown reasons or becomes significantly smaller, it becomes difficult. But for you, it is already certain that you are doing it. Otherwise, you wouldn’t have bought the plot of land. So it means just close your eyes and push through.

Fingers crossed

Thank you. That’s also how it feels to me. We started the project in February 2020 with the viewing, signed the purchase contract in August 2020, and then unfortunately the establishment of the development plan has taken until now. We did not expect the extreme development of prices.

I also lack the context:
Land purchased and how was it financed?
Condition of the land electricity (dis)water, access, divided and surveyed ....
When is the second plot supposed to be sold and for how much?

The entire plot we bought is 4,500 m² and is developed via a right of way and utilities. We had a development plan drawn up, resulting in 2 building plots. The land is already divided and surveyed. It now consists of one plot with 900 m² which we want to sell later, and our part with forest, a longer access road, and green space with a stork chimney.
In order to avoid paying speculation tax, we can sell the 900 m² at the earliest in August 2030. We expect a selling price of €100,000 (pessimistic, current prices for plots are significantly higher).

I also presented the project here with site plan and development plan:

https://www.hausbau-forum.de/threads/Grundstück-4500-m-gaertnerei-erstellung-b-plan-in-eigenregie.42390/