Building a house compared to buying a house

Hello everyone,

we are a young couple from Bavaria facing a promising decision for the future.
We currently have two options to make our dream of owning a home come true.
A little about us, we currently live in a small apartment with a warm rent of 600€ and are both permanently employed.
My girlfriend, 27 years old, earns 2400 € net and I, 29 years old, earn 3200 € net, meaning together 5600 € and 100 thousand € in equity.
We currently have no children, but they are planned.

Our 1st option would be a demolition with a new build on a family plot.
The plot is approximately 420sqm and is located on a street with a bus line. It is right next to the family, and the gardens could be connected.
The current house dates from 1900 and is half-basement with a vaulted cellar; renovation is definitely not an option for us.
Regarding the new build, we would have to build two full floors and a 45-degree pitched roof.
We would like about 150sqm of living space. We are open to both solid construction or prefab houses.
We definitely want to keep the cellar and would possibly fully cellar the new build.
Unfortunately, we would have to plan a fire protection wall due to the distance to the other houses.
We do not need a balcony or anything else.
Garages are available and we would continue to use them as is; they have been renovated over the years.

Financially, our calculation was as follows:
Paying out family for plot/house: 30 thousand €
Demolition costs: 25 thousand €
Notary fees: 5 thousand €
House 150sqm * 3000€/sqm = 450 thousand €
Additional construction costs 10% = 45 thousand €
Cellar (hard to estimate) 70sqm * 500€/sqm = 35 thousand €
Kitchen: 15 thousand €
Outdoor area (terrace, driveway, gate): 20 thousand €
Buffer for special requests: 20 thousand €

Total sum: 645 thousand €
At 3.5% interest, 2% repayment and 100 thousand € equity, we would currently be at 2681€ monthly without KFW funding.
However, we would like to build KFW 40.

The 2nd option would be to buy a semi-detached house.
It is located in a somewhat quieter traffic area, about 300m further away from the family.
However, the plot is only 304sqm.
The semi-detached house is 20 years old but in very well-maintained condition, is fully basemented and has 140sqm living space excluding the cellar.
The kitchen is included and was recently renewed. Energy efficiency class C with average consumption.
Two children's rooms are available, as well as a converted standing attic.
The garden is currently mostly paved or covered with stones.
In theory, one could move in immediately.

Total sum: 475 thousand € including ancillary purchase costs and without renovations
At 3.5% interest, 2% repayment and 100 thousand € equity, we would currently be at 1851€ monthly.

Since we have never built a house or lived in a semi-detached house before, we would appreciate your input.

Is both even doable monthly and have we possibly forgotten something?
We are very frugal and currently have incidental costs without rent of 1000€ monthly. The bank has also approved a financing. From the household calculation, we could theoretically manage both.

How much effort and cost is it to dry out a damp cellar during a demolition with a new build?

Does anyone possibly have experience with the demolition of a house and can share it with us?

Do you hear anything from neighbors in a semi-detached house?
We asked the current owners and they denied it, but we had the feeling we heard something. Since we have problems with our subtenants here, we would like to be able to exclude this.

We would be very happy to receive feedback and your help!

Best regards

Just quickly between the door and the hinge to your questions:

Is both even doable monthly and have we possibly forgotten something?

Doable? Sure. Sensible? That question you have to answer yourself, since you can assume that the new build will be more expensive than in your planning. Just the 70sqm basement will be roughly twice as expensive as you estimated.
And if you want to pay a good €3k every month just for the house, you really have to want that. Having children doesn’t exactly make financial planning easier...

Apart from that, I keep asking myself these days why it always has to be such big fat houses. 150sqm + full basement + attic spaces.
Sure, that costs a lot of money – well-planned 140 sqm without a basement is financially much more relaxed and not worse in terms of living quality. But well, that comes from the time of free money.
Being economical with money, but not with the spaces in the house...

How much effort and cost is it to dry out a damp basement during a demolition with new construction?

A lot of effort. You have to dig out the entire basement down to the ground. Whether that’s enough, or whether the water comes from below, is another question.

Do you hear anything from the neighbors in a semi-detached house?

Could be. Could not be. Back then, we heard nothing. 125 sqm with four bedrooms (one in the attic under the roof – no official living area). Worked great with two children + home office. Without disturbing neighbor noises. House from 2001.

The plot is only 304sqm in size.
The semi-detached house is 20 years old, but in a very well-kept condition, is fully basemented, and has 140sqm living space without basement.
Kitchen is included and was recently renewed. Energy efficiency C with average consumption.

... at exactly the age where the first aches and pains start (can happen). Only 304sqm plot as a semi-detached house might even offer you more garden than the 420sqm on which a detached single-family house is located.

For me personally, a new build with these parameters would definitely not be an option. Damn, wrong statement. ...a new build in such a lavish size should rather be said “not an option.”

20 years old should be good enough. With a 200k difference, I would tend to choose the existing house here and possibly repaint once.

Why is a renovation not an option? 1900 is a good year of construction.

20 years old should be good enough. With a 200k difference, I would tend to go for the existing house here and possibly repaint once.

... and with almost €1,000 more per month in your wallet, you can really make things nice if something needs to be done/changed after all.

Why is a renovation not an option? 1900 is a good year of construction.

Good year of construction? Houses of that age do not have modern layouts; ceiling heights that almost make tall people bump their heads; damp walls (as with the OP). You have to spend a lot of money only to end up living with compromises.