How to best realize a double garage afterward?

Is the electricity from the photovoltaic system enough to charge an electric car? In any case, you will then also need an electricity storage. Often the car is probably only in the garage at night for charging. And of course for the electric gate drive as well. Are you sure that this works only with photovoltaic without a power connection?

Is the power from the photovoltaic system enough to charge an electric car? In any case, you also need a power storage system. Often, the car is probably only in the garage at night for charging.
And of course, also for the electric gate drive. Are you sure it works that way with just photovoltaic without a power connection?

I think it should be enough. I'm also worried about the storage. However, we don't drive much/often. About 10,000 km per year. Therefore, the car can take its time charging.
But just because of the gate drive, a storage system is necessary. It should then be installed in the garage.

I am a fan of garages, but I view your project critically for the following reasons:

Are you sure the width is really 5.45 meters?
From the site plan, I deduce 7.10 meters wide for the 3 parking spaces, which makes 2.36 meters per parking space. Plus 2.98 meters for the "garage space," which totals only 5.34 meters.
From that, the wall thicknesses still need to be subtracted, e.g., 17.5 cm for a calcium silicate stone.
That results in an internal width of only 5.01 meters. The clear drive-through width of the gate would then only be around 4.60 meters.
That’s quite a tight fit for two cars...

What model of full electric car is it supposed to be?
A VW ID3 has a 48 kWh battery in the base version, a Tesla 3 in the base version 52 kWh. To charge them with a photovoltaic system, that system should — according to what I quickly found on the internet — have at least 10 kWp; a larger system probably wouldn’t even fit on the garage roof. The market price for such a system is around €16,000.

What does your driving profile look like?
Usually, e-vehicles are charged at night when you sleep and don’t drive or when you are at work. Photovoltaic systems don’t deliver much electricity at night, if any at all. Consequently, you need a battery in the garage that is continuously charged as soon as the photovoltaic system delivers power, but which is large enough to charge the car. I found prices on the internet of €800 / kWh (!!!).

Have you thought about these points?
Maybe it is going to be a Renault Twizzy (6.1 kWh)…

Regarding the width: I'm not entirely sure. You are right about the measurements you determined. However, I'm not quite clear on why the space wouldn't be enough. One parking space is especially wider because of the garage walls. If it becomes a double garage instead of a single garage, I wouldn't have more garage walls because of that. Or do the walls have to be twice as thick? By double garage, I don't mean a garage with two single doors, but a garage with a wide door.

It will be a BMW iX3. However, €16,000 for the photovoltaic system is too much for me. So that idea is off the table, or perhaps I would have to inquire with the neighbors if it's possible to lay a cable under the footpath after all. Exactly, a battery will probably be necessary. But maybe it doesn't have to be that big, since the car won't be driven daily. So the requirement is not that the car be charged from 0 to 100 in one night.

Do the other garages standing there have electricity? Is there no possibility to lay a cable from the street? However, I don’t know what it costs if the garage gets an independent power connection with a meter.

"Garage space" altogether is only 5.34 meters.
From that, the wall thicknesses must still be deducted, e.g., 17.5 cm for a sand-lime brick.

Minus assembly clearance for the garage on the right side of the plan.