How to best realize a double garage afterward?

Do the other garages standing there have electricity? Isn't there a possibility to lay a cable from the street? However, I don't know the cost if the garage gets an independent power connection with a meter.

The garage to the left of ours and the other parking spaces, I think it has power – at least they retrofitted an operator to the swing gate and I wouldn't know where else it should get its power from. But that is the garage of House 4. They may simply have had the cable laid through their garden. I don't know about the street option, that would be one. I'll ask the municipality about it. Thanks for the input.

Minus construction space for the garage on the right side of the plan.

What do you mean by "construction space"? The garages currently there are also directly next to each other. So no distance was left, if that's what you mean.
That was also planned for the single garage on our spot (which was never actually built) - it was supposed to be attached to the garage located to the right.

An electric car powered only by a photovoltaic system (even with a battery) will not work. Definitely not with the possible installation area of your double garage. You will not get enough power in winter. And the idea of photovoltaic in battery in the car is extremely uneconomical due to charging losses. Definitely better to install an additional power connection!

The garages currently standing there are also directly next to each other. So no space was left, if that's what you mean.

Because they were built together. You cannot subsequently build something with a 0.0 distance. It doesn't work. If you're unlucky, the wall tilts slightly by 1-2 millimeters, then even more distance results.

Because they were constructed together. You can't build something later with a 0.0 distance. It doesn't work. If you're unlucky, the wall tilts slightly by 1-2 millimeters, which results in even more distance.

These are prefabricated garages. I wasn't present when they were set up, but in my understanding, they were placed side by side on the strip foundations. Now we would have a foundation made and have our prefabricated garage placed next to it – what difference does it make whether the two garages are placed there with a gap of 5 hours or 5 years? Or am I overlooking something (which is quite possible – I have never had garages installed before)?

You cannot place or wall anything at a 0.0 distance. Not even a piece of furniture against the wall. You should actually be able to understand that. But do whatever you want. Fortunately, you are not dependent on centimeters.