Cost estimation for construction with own labor

Good morning,

We are currently planning an extension to a single-family house, but so far we have no idea how expensive the whole thing might be.

The extension is supposed to become its own residential unit, so 3 rooms, kitchen, and bathroom. We estimate about 80 sqm, single-story and without a basement in a rural area. Heating and electricity can, according to initial statements, be used from the existing house.

How expensive can something like this generally be?

Does anyone have experience with how much can be saved through self-performance? So if we only need the shell construction and do everything else ourselves?

I hope someone can help me so that we at least have an approximate guideline of what such a thing can cost.
If further information is needed, just ask.

Thank you very much and best regards

Quite generally, I would estimate 150-200k. If everything is outsourced. Many costs are fixed costs that occur regardless of size. Heating and electricity also do not offer much potential for savings. Because regardless of the use of the central heat supply, lines etc. have to be installed. To save from the shell construction phase, you have to be able to do a lot yourself or know trusted people who can really help. There are also certain things that one should possibly not handle oneself. Like electricity and water. Besides the potential danger to oneself and others, there can also be insurance-related problems in the future if in doubt. Just as a note. I am a strong advocate of performing work yourself! If it is really meant to be a separate residential unit (maybe also to rent out in the future?), electricity, water, and heating should also be separate. That makes the question of billing much easier.

2000€/sqm
The prerequisite is, of course, that according to the development plan, an additional 80 sqm net may be built over and that a second residential unit is also permitted.

Go to the local construction company and get an offer,
Shell construction
Roof
Drywall
Plaster
Electrical
Heating/Sanitary.
Maybe he is glad not to have to do everything due to overload, and you can finish with EL and subcontracting.