Fence construction - mounting posts on uneven sandstone wall?

A good idea! But up to what height would that work? The elements are 2.50m long and 1m high. The sandstone blocks show differences of sometimes more than 5cm over the entire length, so that in the extreme case such a "mortar base" is quite high

Why do you only consider the appearance of the pure base?
What matters is how the overall fence looks afterwards. It should be straight horizontally. But also from the side, it should not look like a snake shape. For that, it is important that all bases are at the same height and also aligned laterally.
And one more thing:
It doesn’t help to make the base or the entire post deeper than the wall is at its highest point – or do you want to shorten all your welded wire panels?

What really matters is how the entire fence looks afterwards. It should be straight horizontally. But also from the side, it shouldn't look like a snake shape. For that, it's important that all the feet are at the same height and also aligned laterally.
And one more thing:
It doesn't help to make the foot or the entire post deeper than the highest point of the wall – or do you want to shorten all your panel mats?

But as soon as you cut the posts, the spot for rusting is already there. Or if there's a small scratch on the powder coating. or, or, or.
If you haven't bought anything yet, take V2A or V4A. The extra cost evens out with practically eternal durability.

Hm... this is somehow more complicated than I thought

Would it perhaps be an option to sacrifice 20 cm of space and simply pour the fence directly behind the wall or onto the wall? Something like this, from above:


The posts in red, which are almost leaning against the wall and then enclosed with a semicircle of concrete. Then the soil behind would have to be compacted properly so that the fence with the concrete doesn't fall over again... is that possible?

The advantage would be that the posts are cheaper (no base plate) and you don't have to level things complicatedly.
Or does that look very ugly?

Would it perhaps be an option to sacrifice 20cm of space and simply concrete the fence directly behind the wall or attached to the wall? So

Hello changeling Probably the easiest solution. But then I would still leave some space to the wall. 10cm, drill holes or excavate. The posts a bit longer. Align plumb and concrete in. You want to plant a hedge by the fence anyway. Steven

Then I would leave a few centimeters of space next to the wall.

What is the background? I want to "waste" as little space as possible, even if there will be a hedge there (it is not supposed to grow directly under the fence).

What is the background?

Hello Changeling

Dirt will accumulate between the wall and the fence. Moss and plants. At 10 cm you can pull it out from the street side. If you go directly to the wall, you have to work with pliers and tweezers.

Steven

alternatively with slightly more material usage:

according to the method in post #2 align the crossbeam over the entire length (horizontal) on the respective sides and mount the posts on it