How is the terrace area calculated?

One understands under this a ground-level space that is "exclusively assigned to an adjacent living area, provided with a solid floor covering, and suitable for placing tables and chairs."

The "problem" here is the definition of the solid floor covering. All other criteria are met. Just dumping gravel on the topsoil of course is not acceptable. At least it is not really nicely designed here.

Stupid question: if we complain about this now, we won't get the house ... What is the best way to proceed if the square meters are clearly incorrect?

This is set at 20sqm in the floor plan.
When is a terrace a terrace?

What is the best approach when the sqm clearly do not match?

That doesn’t matter at all when buying a house. What counts is "as seen". You buy a plot with xy sqm with a residential building approximately 150 sqm in size (as an example) and auxiliary structures.

Areas are only interesting when it comes to rental properties or the achieved floor area ratio.

You don't have to specifically criticize it, but surely there are other arguments to lower the price a bit if that is the point you are concerned about.

From my point of view, the surface is obviously still missing if you look at the height difference to the air shaft. In that respect, it probably shouldn't really count as a terrace. On the other hand, he could quickly slap on 30x30cm free concrete slabs from ebay-Kleinanzeigen from the neighboring town and legally it would be a terrace. You would probably redo it anyway in both cases. Without slabs, however, it would be significantly easier.

The space for the terrace is there, presumably already specified in the building permit and maybe the substrate is also prepared. From the buyer's perspective, this rather represents an opportunity to reduce the price. On the other hand, the absence of the terrace surface is also obvious and thus already factored into the price from the seller’s point of view.

It doesn't matter at all when buying a house. What counts is the "as seen" condition. You buy a plot of land with xy sqm with a residential house about 150 sqm in size (as an example) and ancillary facilities.

Areas are only interesting when it comes to rental properties or the achieved floor area ratio.

No, that's not quite correct—namely when the exposé states 170 sqm and this is included in the purchase contract.