Cost of plastering basement interior walls - What costs are involved?

We only realized today that according to the contract the interior walls in the basement (Ytong) are not to be plastered. Somehow we overlooked this, or as naive builders did not assume that one would leave "bare" Ytong blocks as basement walls. Especially since there are still gaps between the Ytong blocks in some places. We are not aesthetes here either, but we would simply like a plastered wall that we can paint if necessary. Question: The basement is currently fully built. Does anyone know if a) the walls will be handed over as they are or if there will be further treatment (for example to close the gaps) and b) what costs (approximately) can I realistically expect if I have a plasterer do it. The area is about 60sqm. Before the electrician installs his surface-mounted wiring, we would like to have this matter resolved.

Please ask the bricklayers with a tip to mix a bag of waterproof slurry and coat the basement walls with it. It looks like a brush finish, which is okay for basements. They will mortar the stone gaps first. K.

We offer plastering at 15€/sqm with lime gypsum plaster. We are talking about wall surface, not floor surface.

Here in Bavaria, 16 EUR/m2 are charged for lime-cement plaster (with gypsum plaster there could quickly be problems with moisture). By the way, for plastering, the m2 of wall surface are calculated, not the m2 of floor area - so be careful, because 60 m2 of basement will probably be the floor area for you.

That's why slurry coatings, which are then whitewashed. For basements, that's enough.

because 60 m2 basement will probably be the floor area for you.

No, it is indeed the wall area. We have a semi-detached house. The majority is exterior wall. We have 3 basement rooms. The 60 sqm is about right.