Buy and renovate a bungalow from 1970?

Wow thank you very much for the numerous responses.
- Yes, the "must" referred much more to whether the state gives certain requirements regarding what needs to be done in case of an ownership change. - My thought regarding moisture, which is always an issue, is that if the basement is damp, it is difficult to "rework" underneath the slab.
- Yes, we will definitely bring an expert on board.
- I forgot to mention that this is a solid construction made of sand-lime bricks. Sorry
- The house is located between Cologne and Bonn
- We would like a controlled residential ventilation system, underfloor heating, RC2/RC3 windows and doors, and electric shutters. Otherwise, we are quite undemanding.
- A rough benchmark such as 100k, 150k, or 200k would already be a sufficient reference point.
For said +200k, one could also get a comparable new build with land, of course under the condition that one can get one... which brings us back to the actual problem.

I advise you not to participate in the whole insulation craze with an older house. This saves a lot of money and keeps you away from the specific moisture problems that occur when making a building envelope airtight that is not designed for it. Instead, invest in a good and environmentally friendly energy and heat supply. If you need a bit more resources, but they have high sustainability, then you are doing everything right. Often, living in highly insulated houses ([Tupperdosen]) is also unpleasantly stuffy.

Yes, now the OP is already planning a controlled residential ventilation system. And of course, it’s great not to have to ventilate. That is then due to old, leaky windows, leaky doors, and roller shutter boxes. So everything but optimal.

So a mandatory replacement could be the heating system. Otherwise, to my knowledge, there are no obligations.

The controlled residential ventilation system easily costs 10-15k€, underfloor heating depends. Maybe it can be milled. Then I’ll put it at around 10k€. Then the basement ceiling urgently needs to be done!

I stick to my 200k€+, as long as you want to achieve a somewhat modern standard. Which I now assume with underfloor heating and controlled residential ventilation.
In the process, electric, water/sewage, and bathrooms will be renovated. Not because “it’s like that,” but because then you have peace for the duration of living there.
Likewise, I would bring the roof up to a reasonable insulation standard. Heat, therefore energy, therefore money, escapes there.
With new windows, it makes sense to go directly to triple glazing.

The facade is debatable. Personally, I would insulate it and price that in.
Economically, this is rather rare. Depending on the current structure, one could consider blow-in insulation. These are inexpensive and effective.

If you like the place, I would definitely have it inspected by an expert at the second viewing.
Maybe something has already been renewed, which would make the renovation cheaper.
Then I would summarize the estimated renovation costs and partially deduct them from the purchase price.

Go to the bank beforehand and get a basic financing confirmation.
You send this together with the offer to the seller or agent.
They either say yes or no. If no, then build new.
If the location is not absolutely perfect, I would rather build new at a similar price. Then you can decide everything and have to make fewer compromises.

We would like a controlled residential ventilation system, underfloor heating, RC2/RC3 windows and doors, and electric shutters. Otherwise, we are quite undemanding.

I am renovating a house from the 60s and can confirm that with these wishes the project very quickly approaches 200k. That would correspond to about €1300/m² for you.

Look, you buy a house and first have to pay money to bring it back to a shell condition. And then the reconstruction. Either you save somewhere and can live with the conditions of the house as they are, or it will be as expensive as a new build.

Often living in highly insulated houses (Tupperware) is also unpleasantly stuffy.

It is not the insulation that causes this, but the airtightness. That applies to all new buildings (OK, unless you build against the law), but insulation does not make a house airtight, not even close.

Therefore, if you upgrade an old building to current standards regarding airtightness, you should also address ventilation. Controlled residential ventilation, central or decentralized, window-frame ventilation, however you prefer. The important thing is just to plan it accordingly.

If you can really get a comparable new building with a similarly sized plot in a comparable location for 200k more, take the new building.
Legally mandatory is the insulation of the roof (or the top floor ceiling, which is the same for a flat-roof bungalow) as well as the replacement of the heating system if it is older than 30 years and is not at least a low-temperature boiler.
If the heating system is from the year of construction, you will have to replace it.

Roof, new heating including conversion to underfloor heating, windows, ventilation, electrical system, bathrooms --> definitely 150k upwards, the 200 from are also not unrealistic.