small new building + expansion of existing house - assessment

There are certain regulations to implement energy-related changes when ownership changes: replacement of heating systems installed before year xx (I believe older than 30 years), or certain insulation measures (top floor ceiling or roof from what I recall), something along those lines. You can find specifics by using a search engine.

At the latest two years after moving into an older property, a renovation obligation under the EnergieEinsparVerordnung (Energy Saving Ordinance) may apply. Then, the insulation of the roof or attic, the replacement of the old heating system, as well as the insulation of pipes must be completed.

There are certain regulations to carry out energy-related changes when ownership changes: replacement of heating systems built before year xx (I believe older than 30 years), or certain insulation measures (top floor ceiling or roof if I remember correctly), something like that. You can find this specifically by using a search engine.

Yes, in the meantime I have used the search engine and read up on it – but I thank you for the tip to look more closely in certain places. I was not aware of that, especially that there is a deadline for compliance. Apart from the relatively old heating boiler (from 1993), which already has to be dealt with promptly anyway – we might get away with a blue eye here – for example, the roof extension with insulation is half from 2004, the other half from 2015. In principle, something has been renovated on the house every year, and this applies to the past two generations. For the things I remember, all pipes were always insulated – probably there are occasional spots where they are not, but I will wait for our appointment with the architect now.

The small large city in Thuringia at the highway interchange is probably my hometown

Only a city of light with many opticians comes into question for me!

There are certain regulations requiring energy-related changes when ownership changes: replacement of heating systems built before year xx (I believe older than 30 years), or certain insulation measures (top floor ceiling or roof according to my memory), something along those lines. You can find specifics by using a search engine.

Multi-family houses and single- and two-family houses in Germany must generally meet certain minimum requirements of the Energy Saving Ordinance. Exempt are only single- and two-family houses that were already owner-occupied as of 1.2.2002. In all other cases of ownership change – whether by house purchase, inheritance, or gift – the following obligations for renovation or retrofit according to the Energy Saving Ordinance apply:

[*]Replace old heating boilers: Heating boilers older than 30 years must be replaced if they use liquid or gaseous fuels (e.g., gas or oil). Exempt are low-temperature or condensing boilers, boilers with a nominal output under 4 kW or over 400 kW, systems used only for hot water preparation, kitchen stoves, or heating devices designed mainly for a single room that also provide hot water. Central heating systems must be retrofitted with controls. If water is the heat transfer medium, thermostats for room-by-room temperature control must be installed. Here you can find more about the different types of heating systems.
[*]Insulate hot water carrying pipes: Uninsulated heating and hot water pipes or fittings must be insulated in unheated rooms.
[*]Insulate top floor ceilings or roofs: The top floor ceiling to an unfinished and unheated attic must, for example, be insulated. At least this applies if the ceiling is accessible and does not meet the minimum thermal insulation requirements.

At the latest two years after moving into an older property, there may be a renovation obligation within the framework of the Energy Saving Ordinance (Energieeinsparverordnung). Then the insulation of the roof or attic, the replacement of the old heating system, as well as the insulation of pipework must be completed.

Is this seriously checked?

Therefore, our plan would be to build a second small house of about 70sqm (bungalow) for my mother – the plot should be large enough for this. She can then peacefully move out of the big house and we take over the large one and remodel or convert it according to our ideas.

What happens to the bungalow if the mother no longer wants or is able to live there?

The question is stupid! I don't know either! At some point someone wakes up and says: oops now I really have to! Your objection is absolutely justified! But I am not the authority!