House Renovation Construction Year 1955/1974 - Estimation and Feasibility

Completely agree. You can't make a statement without experts. And for me, there has to be some sort of extension somehow. Without that, the original poster basically has no own living space or bedroom, and depending on the building fabric, your budget is already used up for that.

What I think you shouldn't completely overlook either: The next things will come up soon as well. In a new building, you ideally have peace for the time being. With an old house, you won't avoid ongoing investments because something can always happen. Unless you finish everything perfectly. Then you might as well build new ;)

An important aspect is indeed the father. With a lot of personal effort, it will be a longer construction site. What does the old man do during this time?

I think that is too general.
Even a basic renovation with water, sewage, roof, windows, electrical work can be significantly cheaper than a new build at an appropriate purchase price. But only as long as the fabric is sound.
If you add dry rot, a dilapidated intermediate ceiling, etc., it becomes massively more difficult.

I think that's too generalizing. Even a basic renovation including water, sewage, roof, windows, electricity can be significantly cheaper than a new build at a reasonable purchase price. But only as long as the structure is sound. If there is dry rot, a rotten intermediate ceiling, etc., it becomes massively more difficult

Exactly my point. It all depends on the structure.

I think that is too generalized. Even a basic renovation including water, sewage, roof, windows, and electricity can be significantly cheaper than new construction at an appropriate purchase price. But only as long as the structure is sound. If you have dry rot, a dilapidated intermediate ceiling, etc., it becomes much more difficult.

I had mentally included the extension in the upper floor. If I'm not mistaken, you have a height of 1.70 m in the middle of the room. And don’t get me wrong. You can renovate a lot with 150k. But I can imagine (and for us it will be like this) that over the years you will have to make bigger investments again and again. If 150k is your maximum budget and then the 20-year-old heating system gives out three years later.

The thing about you can just build new right away might have been a bit too generalized, I agree with you there.

But for example, : You guys have also renovated. Was everything completely renovated right away or are there always things that still have to be done? That’s of course part of the charm of older houses :)

We have not completely renovated everything. The facade and roof from the 90s are still great, heating from 2010 still great. Electrical system, bathrooms, water pipes, doors, ceilings, floors are new, walls repaired. Attic converted, some windows new. Unfortunately, not much original is left. But the attic door still is. Currently, 3 windows are still missing, a new front door, new plaster in some rooms, insulation of the basement ceiling. New canopies. Basement to be replastered sometime. So there is still quite a bit to do, but we like building and in 2017, it was financially and timewise not possible for us to do more. We are doing it step by step :). In spring, the second children's room will be plastered. This year we renewed the shed roof and plastered one children's room plus some small stuff.

First of all, thanks for all the contributions! I really didn't expect such quick and numerous responses.

Welcome to the forum.
You have taken on an exciting big project.
I'll start with a few simple logistical questions:
Will the house still be permanently large enough despite the extension in the roof? Are your girlfriend and father each willing to live together as three?
Where will your father live during the renovation?

I was aware that we would probably have to move out at least for a while. The neighbor has a small holiday apartment where my father could be accommodated practically 30 meters away for that time. This would not be a problem. The biggest problem currently is showering, since only a bathtub is installed and getting in and out is very difficult for him alone. However, I would only do so reluctantly with the current plan to install a new shower in the old house.

Underfloor heating will mean completely new pipes anyway. But that doesn't matter, because the entire floor plus screed will already be torn out and the walls look like Swiss cheese for the new electrical wiring. KNX may not be quite so expensive as a DIY job, but still several thousand euros extra. Which leads to the budget:

150,000€ is a lot of money, but with the amount of changes it will be used up faster than you think. I think you should spend some money and urgently talk to an architect and quickly involve the structural engineer.

The budget is not set in stone and KNX and underfloor heating are not necessarily required. Equity capital of about 40k is currently available as well. But I actually wanted to keep that rather invested in my ETFs etc. Absolute priority number 1 is the expansion of the living space.

[*]I have had bad experiences with energy consultants myself; the ones I had were self-righteous know-it-alls. I kept them very short.
[*]I advise you to inform yourself about the limitations of KNX regarding communication (phone/TV/internet etc. etc). More and more today goes over IP/Ethernet, WiFi/WLAN is almost always just a second-best solution.

Isn't demolition and new construction ultimately significantly cheaper?

Isn't an energy consultant mandatory for claiming subsidies? At least for the replacement of windows, the roof, and the age-appropriate renovation of the bathroom, I would have tried to apply for something.
Basically, I would have decided on KNX TP, especially because the range of options in this area is very diverse, and I expect the longest support. What is standard nowadays in the areas you mentioned? Currently, I see a normal telephone connection for DSL / Fritzbox here. Of course, if everything is torn up anyway, I would lay cat-6 cables in parallel and bundle them into a large VLAN-capable switch. There is no fiber optic available here.

The house is also somewhat adapted to the plot. It is located between two streets and becomes wider towards the back. A possible new build would have to be built further "back" on the property to make it wider as well. Further extension of the house is therefore not an option, as otherwise the 16m privilege in Bavaria would no longer apply. Here the eaves height from the natural ground boundary applies, right?
It is about 6 meters from the outer wall to the middle of the lower street. That would mean the house may be a maximum of 12 meters high to the eaves? The current house is about 4.95m to the eaves height; the natural ground boundary is about 2.5m lower again. So at least for the upper floor there should be no obstacles here?

Of course, everything depends on the existing structure, that is clear at first. Our house, however, is the only house in the entire settlement that has not been extended. But we also have one of the largest plots here.

I had also considered demolition and new construction, but according to my layman estimation, that should be considerably more expensive? Of course, you would have peace of mind then and be up to date. Of course, as already mentioned, it depends on the circumstances. If the structural engineer or architect finds severe defects, the prices could naturally come closer together.

I just looked at the water pipes. The water supply lines were done with the heating in 2003, the wastewater pipes also still look quite new and are made of plastic. However, I have found no invoice or similar for this.

However, our house is the only house in the entire settlement that has not been extended upwards.

But were the other houses once small, plus a chicken coop, then expanded into a larger house and only afterwards extended upwards?

If that is the case, you should talk to the neighbors who have architects and companies experienced in this area. That could significantly speed up the search for suitable partners.