Calculation and planning, your opinion

Hello everyone.
My wife (41) and I (47) want to build.

Region:
District Kaiserslautern (20km), RLP

House:
Solid bungalow, approx. 125 sqm with partial basement (incl. garage), underfloor heating, air heat pump with controlled living space ventilation.
Simple building design. Turnkey.

Plot:
800 sqm, on a south-facing slope, road parallel to the slope.
Slope: approx. 19% gradient. Over 35m it drops 7m to the south.
Plot is already owned and paid for (30,000.-). Soil survey is available.

Development:
Building gap, supply and disposal lines are located in the street next to the plot boundary (approx. 2-5m).

Development plan:
Building gap, no development plan. Preliminary building inquiry showed: development according to §34.
"Construction is permitted if it fits into the surrounding development in terms of type and extent of development, construction method and built-up land area."

Finances:
Joint monthly net income: 4000.-
Children: 1, already 17 years old (no more planned).
Equity: 50,000.- available, another 100,000.- fixed investment until the end of 2021, building plot (see above).

We live rent-free in our own 3-family house in the city.
Rental income: Ground floor: none (my mother lives there), 1st floor: 600.- cold rent, 2nd floor: we live there ourselves (will be rented out upon moving out, approx. 650.- cold rent).

Joint monthly expenses (total): approx. 2900.-
Monthly savings therefore 1100.- plus rental income as above.

So approx. 1100.- plus rental income would be available for financing.

We have now made a rough cost calculation.
Please correct any misestimations on my part!
Plot costs incl. ancillary costs 30,000.- (already paid)
House costs (bungalow): 125 sqm * 2000.- = 250,000.-
Partial basement with integrated double garage: 50,000.-
Additional building costs: 80,000.-
Outdoor facilities: 20,000.-
Buffer: 20,000.-
Total budget therefore 450,000.- gross
Is this enough?

Attached are a cadastral map and (intended) rough exterior view seen from the street.
(the current course of the slope in green)
I am open to any suggestions. Thanks.

Yes, it fits at first. Please now calculate the project with a BU concretely. It should fit this way.

The total buffer for all unforeseen costs and upgrades amounts to approximately 10% of the project volume. €20,000 may not necessarily be enough for you, it depends on how you want to equip and upgrade your house. However, your incidental construction costs item is relatively generous and will cover some unforeseen expenses.

Partial basement with integrated double garage: 50,000.-

I'm not an expert on basements, but that seems a bit too low to me. Well then, the surplus secondary construction costs will cover the rest.

Terrace facing the driveway? Is that supposed to be like that?

Thanks in advance for your answers.


The ancillary construction costs item is substantial, yes. But it is due to the hillside property, which for me represents the biggest uncertainty at the moment.
Earthworks and foundation... hmm
The buffer of 20,000.- is therefore somewhat smaller, but hopefully will grow once I can better estimate the costs of the earthworks.


I currently can't really estimate the cellar costs either. However, it won't be that large since we want to build staggered to the bungalow "with the slope." To dig/strip as little as possible.
Cellar size together with the double garage is just under 60 sqm.

Terrace or balcony facing the driveway, yes because that's the view down into the valley (south side). Another terrace towards the garden is planned southwest.

Attached for better understanding are a few screenshots.

Please take a look and give criticism...

Huh? Does the cellar stairs come out or go up right in the middle of the living area?