Build a basement under the garage retroactively

Hello,

I have a few ideas to enlarge the basement/the garage. Since, due to the space conditions, only building a basement is an option, my questions:

1. Building a basement requires a permit - I assume so.
2. Does it even make sense to build the basement afterwards?
3. Technically feasible?
4. How does it look in terms of costs (approximately)? I have no idea how to calculate that. The size is about 100 sqm. I have attached a photo for this. Green: property boundary; Blue: current parking area to be basemented (these are the mentioned 100 sqm), Orange: it would be a dream if the orange area could also be basemented, it would be a nice large basement if blue and orange are combined. I would like additional parking spaces for the cars, but talking about an underground garage seems exaggerated to me.

2. Does it even make sense to build a basement afterwards?
3. Technically feasible?

If you want to tear down the structures anyway and rebuild on top, then yes. Otherwise, you can’t just add a basement to the existing building.

I do not intend to tear down the roof. The residential building, of course, not....

You can generally say "tearing everything down and rebuilding will be cheaper than adding a basement afterwards." You can forget that idea right away.
Foundation for house and garage is about 50-100k€
Shoring to the property boundary another 50-100k€
Earthworks and possibly water control 20-30k€
Basement including everything will cost about 140-200k€
Surface, paving all restored about 15-20k€

Total comes to 300-450k€

1. Building a basement requires a permit

Your project definitely does.

2. Does it even make sense to build the basement afterwards?

No.

3. Technically feasible?

Yes.

4. How does it look cost-wise (approximately)?

Six-figure How does it look legally with the existing building? The boundary construction already seems to be overstretched. How far away is the neighbor? Is there a development plan? My tip: Forget it!

If it becomes six-figures, then the project will be buried....