Price difference between ground slab and basement. Can it be estimated?

Hello!
I'm jumping in here with a question and hope it's allowed!

We are currently planning a single-family house (110-120 sqm) with a basement apartment (40-50 sqm) and waiting for an offer.
We were told that the basement apartment is always the cheapest option. Is that really the case? On a flat plot? Even when considering extra costs like light wells, external stairs, excavation (removal), etc.?
Without the basement apartment, the basement wouldn't be an issue for us, even now, with our 70 sqm apartment, the basement is almost empty. The garden will be big enough anyway.

Best regards

Living space in the basement is more expensive than "normal" living space. Therefore, it cannot be different with a granny flat. It is cheaper to build somewhat larger or taller than to build deeper.

Thank you very much for the response! I also can’t quite believe that a basement granny flat with everything that comes with it is the cheaper option... Do you have any suggestions on how to integrate a granny flat most cost-effectively? I just can’t make any progress :-(
120 sqm main apartment and 45 sqm granny flat. Over 3 levels I can imagine the distribution of space, but unfortunately not over 2

-Main apartment on the ground floor and granny flat on the upper floor
then the main apartment becomes much smaller than 120 sqm and the granny flat on the upper floor larger than 45 sqm

-Ground floor divided into granny flat and main apartment and upper floor main apartment
only works if the ground floor part of the main apartment is very, very small and I would like to have the "normal" rooms there (living room, kitchen, WC, utility room?)
or you end up well above 165 sqm total area and then have a huge upper floor (which we don’t need) and have high additional costs due to the many extra sqm

-to take some space from the ground floor of the main apartment for the granny flat and build a part single-story (adjacent) I can well imagine, but our developer says it is significantly more expensive than building a basement for the granny flat. He says that for the single-story part you have almost the same costs per sqm as for two-story (or 1.5) for 2 sqm due to additional roof and floor slab

Granny flat with an outside staircase over the garage also occurs to me. But that probably doesn’t pay off either. Especially since we’re not even sure if we want a garage, a double carport would be enough for us

Why do you absolutely want a granny flat? 120m2 means you don't have unlimited resources, so why?

The basement apartment is for the mother/mother-in-law. It is foreseeable that she will not be able to live alone (far away from us) for much longer. We want to spare her from a nursing home. 120sqm because only a maximum of one child is planned for us, maybe none at all. I find it normal to think about how to do it as cheaply (but of course also sensibly) as possible. We did not save money on the house construction at all costs (we don't have to), but I question every expense for its usefulness. With every hundred euros not unnecessarily spent on the house, I can do something nice.

Under the condition of a flat plot of land, I consider a basement with a granny flat suboptimal. A slab foundation and 2 floors are certainly more sensible. Ground floor and part of the upper floor as the main apartment, second part of the upper floor as the granny flat. Although this has the disadvantage that possibly 2 staircases have to be accommodated: I would still do it this way. As an option - as you mentioned: ground floor with granny flat and part of the main apartment, upper floor for the main apartment. The question is also how you handle the laundry room and the kitchen (kitchen open or closed - question of the adjoining dining room/dining area and the living room). There is also the possibility of providing a smaller area on the upper floor (e.g. granny flat on the upper floor with a small terrace). Furthermore, it also depends on the shape of the roof (roof slopes?).